R/C Soaring Digest - Nov 2002

Page 1November 2002

November, 2002Vol. 19, No. 11

U.S.A. $3.50

R/C Soaring DigestPage 2

ABOUT RCSD

R/C Soaring Digest (RCSD) is a reader-written monthly publication for the R/C

sailplane enthusiast and has been publishedsince January, 1984. It is dedicated to sharingtechnical and educational information. Allmaterial contributed must be exclusive andoriginal and not infringe upon the copyrightsof others. It is the policy of RCSD to provideaccurate information. Please let us know ofany error that significantly affects themeaning of a story. Because we encouragenew ideas, the content of all articles, modeldesigns, press & news releases, etc., are theopinion of the author and may notnecessarily reflect those of RCSD. Weencourage anyone who wishes to obtainadditional information to contact the author.RCSD was founded by Jim Gray, lecturerand technical consultant.

RCSD should not be considered to endorseany advertised products or messagespertaining hereto. An advertising rate card isavailable for businesses, and clubs.

Feature ColumnistsBill & Bunny Kuhlman (B2),

Lee Murray, Tom Nagel, Mark Nankivil,Dave Register, Steve Savoie, Jerry Slates,

Greg Smith, Gordy Stahl

ArtworkGene Zika is the graphic artist

who designs the unique ZIKA clip art.

RCSD StaffJerry Slates - Editor/Technical EditorJudy Slates - Managing Editor, SubscriptionsLee Murray - RCSD Index/Database

(available on-line)Bill & Bunny Kuhlman - RCSD Web Masters

PayPal Coordinators

Copyright © 2002 R/C Soaring Digest.All rights reserved.

R/C Soaring Digest556 Funston Drive

Santa Rosa, CA 95407phone: (707) 578-7871

e-mail: [email protected]://www.b2streamlines.com/RCSD.html

OTHER GOOD STUFF

19 Classified Ads17 New Products 3 Schedule of Special Events

Monthly Feature Photography & Web Version of the Printed Article (where appropriate)Highlights & Mailing Status of the Current IssueAbout RCSD..................................................................................... Subscription Information...................................................Advertising Rate Card (Adobe Acrobat PDF format).............................................RCSD Feature Columnists, Reporters, and Editors....................... (E-mail/web addresses, plus general information about their areas of interest)"Getting Started in RC Soaring" ........ Getting started guide - Adobe Acrobat PDF formatLinks to Organizations, Special Interest Groups & ClubsOn-Line Articles - Great articles originally written for the printed version of RCSD......................................... "Trimming Your Sailplane for Optimum Performance" by Brian Agnew.................................................................................................. "Flys Faster" by Dr. Michael Selig.............................. "The Square-Cube Law and Scaling for RC Sailplanes" by Dr. Michael Selig.................................. "Modifying & Building the MB Raven (Parts 1-4)" by Bill & Bunny Kuhlman............................................................. "Butterfly and Moth Airbrushing Tutorial" by Joedy DruliaBookshelf Listings - A listing of recently published books of interest to aeromodelers.Complete RCSD Index, 1984-2001

RCSD ON THE WEBhttp://www.b2streamlines.com/RCSD.html

TABLE OF CONTENTS

3 "Soaring Site" ....................................................................................... Judy SlatesEditorial .................................................................................................. Special Event Schedules................................................................................................................................ B2Streamlines

4 "Tech Topics" .................................................................................... Dave RegisterTechnical Analysis & Design ..................................................................................... The Problem............................................................................................................................Planform Results

8 "Gordy's Travels" .................................................................................. Gordy StahlSailplane Design ............................................................. A Return to the Land of Trim & Balance

10 "Have Sailplane Will Travel!" ................................................................ Tom NagelTravel Saga ................................................................................................................ Hawaii 2002............................................................................................................................... by Kurt Dumas

13 "Gordy's Travels" .................................................................................. Gordy StahlWhy Are Sailplanes So Addicting? ......................................... RC Soaring... It's Not About Flying

14 Aerodynamics ................................................................................. Gregory Ciurpita..............................................................................................................Center of Pressure, Again

12 Ackerman Aircraft12 Aerospace Composite Products3 B2 Streamlines7 Cavazos Sailplane Design9 R/C Soaring Digest

Special Interest Groups19 Eastern Soaring League (ESL)19 International Scale Soaring Assoc.19 League of Silent Flight19 Sailplane Homebuilders Association19 T.W.I.T.T.19 Vintage Sailplane Association

Advertiser Index

Page 3November 2002

TheSoaring

Site

Happy Flying!Judy Slates

SCHEDULE OF SPECIALEVENTS

February 1-2, 2003Southwest Classic Phoenix, AZ

March 15-16, 2003The Classic Mid-Winter Southern CaliforniaTorrey Pines Vintage Sailplane Regattahttp://www.agcsc.org

October 10-11, 2003Texas National Tournament (TNT) Dallas, TXwww.SLNT.org

Please send in yourscheduled 2003 events

as they become available!

Special Event Schedules

For those of you that have clubschedules in hand for 2003, we’d

sure like to hear from you so that wecan add them to the listing in RCSD.

B2Streamlines

Bill & Bunny Kuhlman have beenproviding fellow modellers withinformation and the results of their in-depth research for many years, now,and we really appreciate their efforts!Regretfully, as I’m sure most of youhave noted with their new ad, theywill no longer be offering their special-ity books commencing with the NewYear.

The good news is that they willcontinue to write their “On the‘wing...” column for us and plan to

continue the web site work that theystarted several years ago, whichincludes the RCSD pages.

Our thanks to them for all they do!And, for all of you, Happy New Year,2003!

Lunch or Launch?

The cover of this issue wascreated by Phil Bauer,

Fremont, California.


TECH TOPICSDave Register

Bartlesville, [email protected]

It’s funny how a random conversation cantrigger a ‘Tech Topics’ column. Especiallyone that didn’t really sink in at the time.

THE PROBLEM

While wrapping up the HLG eventat our Last Fling of Summer this

year, a couple of the contestantscommented that the planes they wereflying ‘just didn’t carry enough wingarea to be able to compete success-fully’. Those comments were made bygood pilots who are capable of win-ning on any given day. Based on theevolution of the airfoils and structuresneeded for DLG, do these fellows havea point we might have missed?

About two years back, we ran a seriesof articles looking at optimizing theplanform for a 2 channel HLG usingthe SD7080 as the reference airfoil. TheMH82 and several other sections werealso evaluated. These sections are allaround 8.5% to 9.5% thick with camberin the range of 2% or so. This wasfairly typical of the airfoils being usedat that time for HLG (the S6063 wasidentified in that analysis as a sectionof potential interest and had beenflown successfully at the IHLGF aboutthat same time).

The overall conclusion was that anaspect ratio in the 8 to 10 range lookedadvantageous. Based on those results,a 2 channel polyhedral HLG (laterconverted to DLG) was developed.This plane, called the Tahlequah,pushed the AR to the high end of theproposed range (~10). I’ve had a lot offun with this design for the pastseveral years. 50 second dead airflights with DLGs are pretty commonand it’s not at all difficult to ride evenvery light thermals to fairly decenttimes. I’m not sure this ship would behighly competitive but some of that islimited by the age and vigor of thepilot. Not too long after that work wascomplete, I noticed several commercialHLG designs had moved to the higherAR range. So I think the trend wasvalid.

What’s changed lately that could affectthe planform optimization? Well, withDr. Drela’s X-Foil code we now havehigher performance airfoils that arethinner with lower camber. Thestrength requirement for DLG hasadded structure (and weight) to thewings. The larger vertical stabilizer/sub-rudder requirement has alsoadded mass. And even if you’re flyinga two channel radio, you’re probablynow using a piezogyro. Bottom line -weight is up and low speed capability(due to the lower Cl-max of theairfoils) is not as favorable. So a lowerwing loading (more area for a givenflying weight) seems like a reasonableoutcome.

With that in mind, let’s re-optimize aDLG planform taking into account thestructural changes and additionalweight requirements of today’sequipment. Let’s also try and see howairfoil evolution may have affected theoptimized planform results.

X-FOIL CALCULATIONS

To carry out this analysis, we’ll usesynthetic airfoil data. For the mostrecent Drela airfoils, wind tunnel datais not yet available. We’d also like tohave lower Reynold’s number datathan is currently available experimen-tally. Since X-Foil will be the informa-tion source for the Drela airfoils, we’llput the older section through the sameanalysis.

Once those analyses are complete, wecan use the planform model developedin this column to compare planformchanges with these airfoils. Given thedemands of modern DLG structures,let’s see if a suggested optimal configu-ration is different from what welearned a few years ago.

The assumptions for the planformanalysis will be provided in nextmonth’s column. For now, we’llcalculate the required coefficient data

Page 5November 2002

and see what it tells us about theevolution of airfoil performance overthe past several years. Recall that wecompared X-foil output against windtunnel data in this column earlier inthe year and the agreement wasremarkably good.

Our first question is the Rn range we’llneed for the analysis. Assuming alimiting average chord ~ 5" (12:1 AR)at a low speed of ~ 15 ft/sec, our lowerRn limit will be ~ 35,000. For the highspeed end, we’ll allow a maximumlaunch velocity of 90 ft/sec for anupper Rn limit of around 200,000. Thepolar analysis code requires a range ofRn curves so we’ll calculate results inX-Foil for Rn of 35k, 50k, 100k, 150k,200k and 300k.

The sections chosen for comparison arethe SA7035 (a re-optimized version ofthe original SD7080) and Prof Drela’sAG12. These represent the ‘big air-plane’ airfoils originally applied toHLG and then the general class of lowRn optimized sections that Prof Drela(and others) have developed for DLG.

For general considerations, the AG12 is6.2% thick with 1.84% camber. Themaximum thickness occurs at 21.4% ofthe chord while the maximum camberis found at 43.1%. The same numbersfor the SA7035 are 9.2% thick at 29% ofchord and 2.55% camber at 40.4% ofchord.

For the X-foil users, the AG12 was runwith default parameters throughout.Ncrit was set to 9 (default). No addi-tional paneling of the section wasmade from the original coordinates.The SA7035 coordinates were toocoarse for reliable convergence so itwas run through the GDES/CADD

routines and then re-paneled withdefault values (140 surface points). Inboth sections stable solutions werefound throughout the Rn and angle ofattack (AoA) range of interest. Incertain regimes (near transitions) smallstep sizes in the solution were requiredbut no other problems were encoun-tered. For the SA7035 at 35k, a slightnumerical instability is found in thesolution. Consequently, this one dataset uses a 5 point smoothing averagefor the final results.

It’s important to note that the dataranges of keen interest for this type ofanalysis are high AoA for low Rn(flying near minimum sink) and lowAoA for high Rn (launch and highspeed cruise). The latter case does notpresent a problem but the former does.

At high AoA and low Rn, the AG12solutions are noticeably more difficultto converge than the SA7035. Whenconverged above stall, the drop off inCl is noticeable. Since the low Rnsections are optimized for extendedlaminar flow, a sharp onset of stall isthe price you pay for laminar separa-tion at high AoA. The older sections(SA7035) are aimed at turbulentseparation and the stall characteristicsare less abrupt.

This tells us that the overall wingplanform (not just the aspect ratio butthe lift distribution) is critical for agood DLG using modern airfoils. Thelast thing you want is a tip stall due topoor load distribution. A root stall isfar more desirable. Tools such as theLift-Roll spreadsheet and the


Schuemann-Ellipse planform fittingroutine developed in this column arenecessary to optimize the spanwisewing load profile. Airfoil blending isanother approach, which is why yousee the most recent designs usingsections like the AG12 but blending tothe AG13 and AG14 for appropriatespanwise load distribution.

A particularly graphic example of atransition effect was reported at aconference at the former Solar EnergyResearch Institute (SERI) I attendedabout 7 years ago. A wind powergenerator airfoil had been developedfor low drag at high AoA and hadbeen installed in the Golden, CO area.To achieve low drag, the section wasdesigned for extended laminar flow.After being in service for ~ 2 weeks,the generator efficiency dropped off ~30%. Being a model airplane guy, itwas immediately obvious what hadhappened. Bug splats on the LE of theairfoil caused an early trip of theboundary layer which quickly pushedthe blades to turbulent flow. In thismode the airfoil had much higherdrag. I can’t recall the appropriateacademic term used to describe ‘bug-splats’ but that was acknowledged tobe the problem.

The solution (short term) was to send agraduate student out to clean theblades every few days. The long termsolution was to redesign for a low dragturbulent flow section. This was not asefficient overall as the laminar flowdesign but almost no change in effi-ciency was noted with time and thegraduate students went on to lessmundane tasks. Those are the trade-offs the real world sometimes forces usto deal with.

So much for serendipity. Let’s take alook at our X-Foil results.

LIFT AND DRAG COEFFICIENTS

The calculated lift coefficients for bothsections are shown in Figures 1 and 2.As expected, the SA7035 has a highermaximum lift coefficient throughoutthe Rn range of this study. The curvesalso suggest a somewhat more gentlestall than the AG12. However, theresponse to Rn change of the slope andoffset - Cl(Alpha=0) - is much moresensitive with the SA7035, especially atlow Rn. This is where the optimizationfor low Rn using X-Foil has created a

significant advantage with the newlydesigned sections.

When we look at the drag coefficients(Figures 3 and 4), Comparable conclu-sions can be made. The low Rn drag islower but the width of the drag bucketis narrower with the AG12 as com-pared to the SA7035. The SA7035 alsoshows a pronounced excursion in themiddle of the drag range at low Rn

presumably due to poor transitionstability and re-attachment. At 100kand above, the differences drag valuesare not significantly different.

This suggests that the SA7035 has anadvantage with a larger planform (3meter range) while the AG12 carriesthe advantage for smaller airframessuch as DLG. Obviously these are thedesign principles behind both of these

Page 7November 2002

sections but it’s reassuring to inferthese results from the calculated data.

In Figures 5 and 6 we plot the moreconventional Cd vs Cl plot and againthe overall drag bucket for the SA7035is typically wider but the AG12 has asignificant drag advantage at low Rnvalues.

As originally suggested, the AG12 will

be a much cleaner airfoil at low speedsbut not as capable of generating highCl under AoA conditions. Which isprecisely where we need to be for lowspeed (minimum sink) flight. The highdrag of the SA7035 under theseconditions suggests a somewhatpoorer minimum sink even though theCl capability is a bit higher.

For the launch phase of the flight

envelope, (low Cl at high Rn) the Cd ofthe AG12 may be slightly lower but theadvantage here seems pretty subtle.

Without running the polar calculation,my suspicion would be that launchheights would be quite comparablewith these two airfoils but the AG12,with an optimized planform (probablylower AR), should have better mini-mum sink. Thus the complete flightenvelope (launch and glide) should bemore favorable. Next time, we’ll runthe planform and polar calculations indetail and see if the ‘gut feel’ pans out.

n

ZIKA


GORDY’S TRAVELS

Gordy StahlLouisville, [email protected]

In my previous trip to the land oftrim and balance, I talked about the

fact that terms like ‘CG’ are the archenemy to getting our planes opti-mized.

As I have said many times in mycolumn, I am fortunate because I get tomeet and discuss stuff like this withguys who really know things. Re-cently, I chatted with one of theyoungest, foremost brilliant free flightmodelers (and RC Sailplaner) RussWhitford of the Slopeflyer.comWhitfords in Milwaukee, Wisconsin.

Russ and I go way back, and while weseldom see each other or even talk,when we do, it’s like we were justtogether yesterday.

Russ had read my article on trim andbalance, and pointed out that where Istated that Rudder Elevator ships ofthe past were always balanced at 30%with lots of up decalage in the horizon-tal stabilizer, because the kits weredesigned by ex-free flight guys whohad added radio control to theirplanes, in fact, the opposite was true -that free flight planes were actuallybalanced way back to sometimes 50%of root chord.

So as we discussed that line, anepiphany came to me as to why polyships had evolved in such a goofymanner.

It was actually something Russ said ashe was discussing the rationale of freeflight trim and balance, where he used

the phrase, “trimmed for a particularspeed.”

And THAT was it, whether con-sciously or unconsciously, or maybe asa result of ‘task’. Kits prior to fullhouse planes (let’s say pre-1980ish)were (for the most part) all set up withsome decalage and lead in the nose to‘balance’ the crookedness of the wingto stab alignment.

Setting those planes up like that madethem very difficult to fly fast for onething, plus pilots learned which speedwas optimum for their setup. Divingthem down from a thermal causedtheir destruction. Often, when at cloudbase, it’s near impossible to tell if yourplane is flying across the sky orspeeding down from the sky, downbeing the bad thing.

Keep in mind that very few modelersalive actually ‘know’ why they didwhat they did in designing a model forThermal Duration flying, mostly theydid what was the norm for the day. Weactually still see it in many of the Euro-Moldy V-tails (fixed stabs) which, ifbalanced like a full flying stab plane,have to be either flown with 1/16”down elevator trim or have the stabassemblies leading edge shimmed upthat amount.

Try to forget applying judgment termslike ‘good thing or bad thing’. Thereare lots of ways to skin a cat but thereare some that are faster than others...The cat is skinned in the end in eithercase.

Now here’s the reason’s it’s been adumb idea for years!

With mass quantities of lead in thenose of a sailplane, once airspeed islost, so is the stab’s ability to supportthe nose against gravity. That’s why,on a hand toss, sailplanes that are setup crooked first balloon, then droptheir noses like javelins... With a reallynice glide in the middle.

It’s why so many newbies end up intrees or watch their planes heal over onlaunch, crashing into ground and

shredding their way to the turnaround.It explains the REALITY of what somecall the ‘deadly down wind turn’.

A sailplane balanced at 30% of rootchord has lots of lead in the nose. Onlythe elevator can make that nose lift.The more lead, the more strength (orauthority) the elevator needs to getthat nose to come up in a turn.

So, when a newbie gets down low andslow, let’s say he decides to try tobring his plane around in that downwind turn; but, since his airspeed isreduced, his sailplane’s elevator haslost its strength to pull the nosethrough the turn. Centrifugal force,mass and inertial come into play,forcing the nose to drop or fade down,away from the turn. Same as a piece oflead swung at the end of a string wantsto fly away from the center, the lead ina sailplanes nose wants to fly out andaway from the direction of the turn!

When a newbie sees his plane is notresponding (coming around), he pullsmore elevator. Since there was barelyenough air on the elevator as it was,raising it more causes the air, that waspassing over it, to ‘baffle’ against it,rendering the elevator ‘gone’... And aSNAP ROLL results — NOT A TIPSTALL (Don’t get me started on thattopic!!) that lead to the nose rushingtoward the earth... And we haveplanes in trees, or with a crumbledwing tip first into Terra-firma.

So, why don’t poly ship guys ‘get it’?

Because they don’t understand what‘IT’ is. When they see a newbie’s planeporpoising, they advise him to, “Addsome lead to the nose!”…Not takinginto account that it takes real thumbexperience to ‘learn to fly’ a crookedairplane. If the newbie had masteredflying his crooked airplane at itsspecific speed, it wouldn’t beporpoising in the first place.

AND THAT’S WHY poly flyers haveso much trouble transitioning to fullhouse planes. Anyone who flys fullhouse will always argue that they aresooo much easier to fly, ‘fly’ being the

A Return to the Land of Trim and Balance

Page 9November 2002

operative word. As in, the pilotcontrols every movement of the ship,every attitude change, where thecrooked planes resist most inputcontrol and hate, to the point of gettingcrazy, speed request changes.

Poly guys are told, by other poly guys,that full house ships are ‘for expertsthumbs’. So, they ‘learn’ to fly theirparticular crooked-unbalanced polyships. Since no other model flies liketheir personalized speed-specific polyships, they have lots of trouble flyinganyone else’s sailplanes, crooked polyor full house balanced ships.

When asked to help a newbie with hispoly ship, that guy will always startadding lead to get the newbie’s ship tofly like the plane he is used to… hisown.

Now the same is true for a full housesailplaner who understands trim andbalance. He will always start pullinglead and adjusting the full flying stabuntil all the decalage is out, so that atany speed the plane flies the sameattitude. That way it allows the pilot tobe able to range further away and notwonder what his plane is doing. It

allows him to spend more time read-ing air, and less correcting attitude forspeed.

Too often, those club experts flyingcrooked-unbalanced planes will holdtheir trophies up, or proclaim theirhours long flights, as a justification oftheir set up. In spite of the fact thathouses fly in the right air.

That they have mastered their modelwell enough to beat guys with fullhouse ships is a testament to theirskills, but also a condemnation to onlybeing able to enjoy that particularmodel.

A balanced plane doesn’t need lots ofelevator in that low, slow downwindturn, to get its nose to come around, soyou seldom see them tip first into theground (don’t confuse balanced andtrimmed with full house, a crooked-unbalanced model still has to fight thesame physical forces). It’s why somany of the ‘pros’ are seen handtossing 120”+ full house compositemodels, circling and catching, orputting then up into the clouds fromthe same toss. A trimmed and bal-anced plane doesn’t waste what little

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A MONTHLY LOOK INTO THE WORLD OF SAILPLANE ENTHUSIASTS EVERYWHERE

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energy is available. They land slower,not because of flaps, or computerradios, but rather because they are notfollowing the masses of lead found inunbalanced models noses. They needless of all control surfaces to makethem do what we want and are ‘set-up’to fly the same at any speed, under anypilot’s thumb.

Our sailplanes have to fall forward tofly right. Trim & Balance is a trip thatis tremendously interesting once youget your head beyond the aerodynamicjargon that causes us to dismiss thecommon sense factors. I’ll be on thisroad for awhile; hope you can comealong!

n

ZIKA

GONE SOARIN’


HAVE SAILPLANE, WILL TRAVEL!

By Tom H. Nagel904 Neil Ave.

Columbus, OH [email protected]

This month’s travel saga is written byKurt Dumas. Kurt lives in Burbank,California and works for Warner BrosAnimation as a story artist. He is a privatepilot as well as a hang glider pilot. Thiswas his second trip to Maui to fly sail-planes.

Hawaii 2002

by Kurt DumasBurbank, California

Many things led to the development of a practical RC travel

glider but I believe that my trip toVenezuela was the proverbial laststraw. I was traveling to an island offof the coast of Venezuela for a friend’swedding and learned that the condi-tions might well be favorable for flyingRC sailplanes. Not certain of the windor landing conditions, I decided to taketwo planes, a foam Zagi and a CR Fun-1 with a two-piece wing. Taking thesetwo planes required a 14x11x37.5 inchbox for transporting. I emphasize,BOX. And it just so happened, unbe-knownst to me, there was a boxembargo.

I don’t know why there was an em-bargo on the container rather than thecontents, but the Venezuelan securityofficers decided that my box must beopened and searched, perhaps formore boxes. For me this was a huge

inconvenience. I don’t speak Spanishsave for “Dos huervos por favor.Donde esta les banos?” and “Cervesacon manzanas, gracias.” You can’t holda discussion of RC sailplanes with saidvocabulary. Thankfully, they under-stood hand waving and looks ofconfusion and eventually I wasallowed to repack my airplanes andunderwear to search for my bilingualfriends that had long since vanished.The trip itself is the subject of anothertravel article but for now I will focuson the convenience or lack thereofwhen traveling with sailplanes andhow to make things a whole lot easieron yourself when flying for fun bybuilding a plane that will fit in yourregular suitcase.

There are several ways one mighttravel with model sailplanes, each ofwhich has its advantages and, unfortu-nately, its disadvantages, as well.

Shipping ahead:

Shipping ahead makes things simple atthe airport but adds expense and timeat the post office. It also requires moreplanning. You do, after all, need aplace to ship this large peculiar box to,some place where it will be held foryou safely and out of the rain. Youreally don’t know for sure. Then at theend of the trip of course you need toship it back.

Packing in an over size luggagecontainer:

Some pilots pack their planes in golf orbow cases and claim it costs no morethan flying with regular luggage. It’spossible to get it past the steward but itis oversized and therefore subject toadded airline charges, if the airlinefeels like it. The golf or bow cases arebulky and you need to consider thiswhen hiring a car, train or rickshaw.

Custom making a box for your planesthat doesn’t look like a standard pieceof luggage can lead to problems whenthere’s a crazy box embargo, even ifthe box fits within the size restrictions.Silly.

If you’re thinking of packing yourradio as a carry-on consider the addedsecurity at airports these days. Takingyour radio and odd shaped batterypacks through the x-ray machines andmetal detectors often will subject yourbag to search more often than before.

If you’re entering an internationalcontest and must have that 20 footspan scale sailplane that requiresshipping then deal with it, but if you’rea sport flyer that is taking the airplanealong as an afterthought than my voteis a portable/packable travel planethat is designed to be flown in avariety of conditions.

Suitcase with 2 travel planes and gear. The Nomad MP iscircled and the arrow points to the small box it fits into. Thisphoto shows 2 travel planes, radio, charging gear and repair

supplies. There was still space left in the suitcase.

Page 11November 2002

This column is dedicated tosoaring vacations. If you have a

favorite sailplane saga, considerwriting it down for RCSD. If youare planning a vacation thatincludes your plane and transmit-ter, consider making notes as yougo, and working up an article later.Take photos. Collect maps. Andsend your story to Tom Nagel [email protected] for gentleediting and suggestions.

Tom

Voila, the Nomad Sailplanes MP. TheMP stands for multi-purpose. It is a 2in 1 travelplane.

The MP is a super-modified 55"Dynaflite Skeeter. When disassembledit will fit into a box with dimensions of5"x7" x22". The tail is removable, thefuselage separates into two pieces, andthe wing into three.

Each outer panel of the wing has anaileron with its own servo. Whenassembled the pilot has the option offlying the plane with the wing’s centersection in place as a thermal sailplaneor without the center section forsloping.

The Travel plane in its box easily fitsinto a medium sized suitcase, withroom for radio gear and charger andstill extra room for enough clothing fora week on the islands.

It was with this new sailplane that Itraveled to Maui to fly once again withmembers of the Maui Island SoaringOrganization.

FLYING INPOLY CONFIGURATION

FOR HLG GOLF

I contacted Duane Asami through theMISO web site and made plans to meethim at the Glider Golf contest theyhave once a year. It was held on thefoothills of the Haleakala volcano atPoli Poli. (For directions to the Mauiflying sites go to http://groups.msn.com/SoarMauiRC andclick on FLYING SITES.)

The flying conditions had been poor atbest during the previous week. Theday before the contest the site wascompletely socked in. Apparently mostof the regular pilots were apprehensiveabout the possibility of flying ininstrument conditions so there was alow turnout as the sky cleared on theday of the contest. The pilots that didshow up were good. Really good. They

Poly config. and slope config.

had many morehours of flying timethan I had. Oneclaimed to fly oversix hours a week atthis spot. Yet, stillwith my little travelplane, I managed toplace third with ascore of a 19, just one“stroke” behind thefirst and secondplace pilots whomboth scored 18.

The travel planeperformed very well.The ailerons allowedme adequate controlin the turns as longas I kept the speedup and the planeclearly indicatedwhat little lift therewas. I was flyingfaster than I expectedbut the plane is 2 oz.

heavier than the Skeeter recommends.In addition, the cg may have been off.But over all it flew great.

FLYING THE SLOPECONFIGURATION AT MALUHIA

With a successful day of flying at PoliPoli complete, the next test was to flythe travel plane in its slope configura-tion at Maluhia, an exceptional thermaland slope lift flying site about twentyfive minutes from the Kahului airport.

It was about 4:30 p.m. and the windwas averaging 15 miles per hour. Iheld the travel plane up to feel thewind and it was obvious I was goingto need lead to keep control as I flewout past the turbulent air and topenetrate into the wind. I added threeounces to the belly and gently tossed itinto the wind. With a few unsettlingdips she was above the tree line andclimbing. Six ounces would have beenbetter but even with it being a bit toolight it was solid in flight. I was usedto the slow roll rate of the thermalwing, which requires a generous upaileron differential throw to counteractthe adverse yaw, so the roll rate of theshortened wing with full aileron alongthe span was quick to say the least. Thelanding was relatively uneventful inthe field of freshly mowed grassbehind the ridge.

Over all I was pleased with the perfor-mance. The short wing doesn’t providemuch of a silhouette at great distancebut it flies great and is very responsiveto control inputs. Overall, the plane Idesigned and built flew great. Theoverwhelming response to the NomadMP was, “Man, you’ve got too muchtime on your hands.”

Here’s an abridged explanation of thestructural modifications beginningwith the fuselage:

Fuselage:

There are a number of planes withremovable tail designs and wings builtin multiple sections are common.These features help make a modeleasier to transport but, to make it trulyportable, the fuselage must also packsmall. The fuse on the MP breaksdown.


The fuselage is built similar to theoriginal except that it separates just aftof the wing’s trailing edge. Brass andaluminum tubes glued into the lowerlongerons form connecting rods whichprovide lateral support and keep thefuselage from twisting. Strips ofplywood bolted into the upper long-erons keep the fuselage from separat-ing in flight.

Tail:

The original Skeeter is designed with asolid balsa tail. To keep the weightdown I cut the stock into strips andbuilt the tail over its planform.

Adding ailerons meant adding weight.I eliminated the rudder to help offsetthis weight. To counteract adverseyaw, I added the aileron differentialadjusted to provide greater throw inthe up position.

There is an option to add a rudderservo under the trailing edge of thewing.

Wing:

The wing is built into three sections.Ailerons, each with its own servo, areadded to the outer port and starboardwing sections and aileron servo wireswith connectors run through the centersection.

The basic idea of this modification isthat the outer wing thirds, or wing tipsections of the full thermal wing, arebuilt to remove from the center sectionand then connect to each other forming

Building theremovable tail

over themodified

plans.

a wing suitable for slope soaring. Thewing is aligned using a basic male/female tube system at the spars. The 3/8-inch thick ribs at the ends providestrength for the connection.

Packing tape may be used to hold thewing tips on. However, I chose to boltthem to the center section by screwinga nylon bolt horizontally through theend ribs. This requires an access hatchthat can be covered with tape prior toflight.

For more photos and information onhow to order the 32 page detailedconstruction manual with completeplan modifications, photos and step bystep instructions for building theNomad MP (AKA 2 in 1 travel plane),go to:

nomadsailplanes.comn


GORDY’S TRAVELS

Gordy StahlLouisville, [email protected]

RC Soaring…It’s Not About Flying!

So often I get asked,”Why sailplanesversus helicopters or powered

planes, or even electrics?”

To start at the beginning, I started withglow powered planes like so many ofus. I had a ball with it but it always leftme just a little bit short of beingsatisfied with my hobby. I did build anew model, fly it and began lookingfor something faster, bigger or fancier.

When I got into RC soaring, it was thesame thing all over again; before I hadthe latest and greatest model finishedand flown I’d be looking and lustingafter the next new sailplane. Frombalsa to bagged to molded, from E214,3021, 7037, 7035, to MH32, it was amistaken quest.

It took a lot of miles of soaring in orderto understand the answer to thequestion, and that it was misguided tothink that some new ‘gimmick’ wouldgive me an edge toward ‘enjoying’ myhobby.

The answer? Simple... Or maybe Ishould say, it is now, after all theseyears in the hobby. The differencebetween flying power planes andsailplanes is that one is about buildingand flying, and one is about buildingand HUNTING... with the emphasison HUNTING.

It really came to me the other daywhile flying with Bruce in Nashville atthe Percy Priest Dam site. The windwas slightly sideways to the dam;Bruce had one of Dave’s Aircraft’samazing 60” EPP Schweizer 1-26’s, afoamy suited for rough landing sites.But I only had my Fred Sage 126”Compulsion Thermal Duration contestship so, needless to say, I wanted tomake sure conditions would warrantlaunching.

I watched Bruce’s launch closely, but itwas his comment that gave me thatsort of Zen realization. He tossed theplane and headed out along the damface, but what he said was,”I think thelift will be right over that corner.”THAT WAS IT!

Thinking back over every launch Imade or watched with sailplanes,slope or thermal, it was always about

ZIKA

finding air... It was about the HUNT.

When we step on a field, we scour thearea for clues signaling lift. When wecome off the winch our thoughts are,“Where is the air, which way to turn?”Never about ‘the flying’.

RC soaring is about the hunt for lift,because we don’t have a motor. Surewe can do rolls and loops and virtuallyall the stuff the powered planes can do,but because there is no motor or fueltank, finding more energy to fly is ahuge part and in fact the primary goalof each flight.

We all can think back (and remembervividly!) to that first huge thermal thatsky’d us out... And it’s that memorythat drives our yearning each morningof a flying day, and our day dreams onthose rainy days.

The Hunt, that’s what separates RCsoaring from all the other variations ofRC (well maybe not so different thanRC sailboats)!

Good luck on your next ‘hunting’ trip!I’m off on another of mine...

n


Figure 1 - Primary forces affecting pitch.

Greg CiurpitaSomerset, New Jersey

http://ciurpita.tripod.com/rc

“As the forces approach zero, the center ofpressure moves to infinity. For this reason,the center of pressure is not always aconvenient concept in aerodynamics.”

- Fundamentals of Aerodynamics,John Anderson Jr

The total pitching moment about thecenter of gravity (CG) is

CMg = CM + CLwx wg - CLt x tg S t / (S w c)

where

CMg is the total moment of theaircraft about the CG,

CM is the moment coefficient of thewing (nose-up is positive),

CLw is the lift coefficient of the wing,x wg is the distance between the

aerodynamic center of the wingand the center of gravity,

CLt is the lift coefficient of thestabiliser,

x tg is the distance between theaerodynamic center of thestabiliser and the center ofgravity.

St is the area of the tail.Sw is the area of the wing.c is the average wing chord

length.

Figure-1 illustrates the primary

forces affecting pitch. The wing isshown on the left, with an upwardarrow indicating lift, represented byCLw in the equation. The lift is shownoriginating at the aerodynamic center(AC) of the airfoil. An arc is shown atthe AC indicating the moment createdby the wing in the direction caused bya negative CM. The stabiliser is shownon the right, with a downward arrowindicating the force it creates, due to anegative CLt. The circle with a cross init indicates the center of gravity (CG).Both moment arms between the CGand the lift generated by the wing,Xlw, and lift created by the stabiliserXlt, are shown.

The total moment of the aircraft, CMg,must be zero for an aircraft to remainin level flight, otherwise the aircraftwill either nose up or nose down. Theelevator setting affects the lift pro-duced by the tail, defined by Clt, and isadjusted to neutralize the total mo-ment, making it zero.

This equation is fundamental tounderstanding pitch stability. Itidentifies the key forces and momentsof the aircraft. Inherent in this explana-tion is the fact that the lift is stationaryat the location described by xwg. Thisequation accounts for the fact that awing produces lift and a moment, andbecause the moment is separate from

the lift, the location of the lift (center ofpressure) is stationary and does notmove as the lift changes.

Pitch stability is determined by thelocation of the CG. I continue to beimpressed by experienced flyers thatcan so easily see that my model is noseheavy by the way it flies. I’ve seenthem use various methods: the divetest, flying upside down and seeinghow much elevator is required,observing how the plane turns, andobserving that a plane is thrown out ofa thermal. (I sincerely wish someonewould write an article on these.)

In the 3rd edition of Aerodynamics forModel Airplanes, Martin Simonsshows two polar diagrams for theClark-Y airfoil. The first, from 1924,shows a center of pressure measure-ment. The second, from 1931, showsseparate lift and moment measure-ments. In this article I show theequations that relate the center ofpressure curve from the 1924 measure-ment, to the CM curve in the 1931measurement. This is not an article onpitch stability. I’ve shown the equationabove to emphasis that a wing canproduce both a pure moment, CM, andlift which also produces a momentaround the CG.

A review of moments seems an

Center-of-Pressure, Again


Figure 2 - Center-of-pressure curve for a few Cm values.

appropriate place to start. A moment,or torque, is a twisting force such asthat created by a wrench twisting abolt. The magnitude or size of themoment is the force times the length ofthe moment-arm. The moment can beincreased by either increasing the forceor moment arm.

M = F * d

where M is the moment, F is the force,and d is the moment arm.

A translational force, F, would normallypush an object in the direction of theforce. But if the force is applied offcenter of the center of gravity of anobject, the object rotates, due to theresulting moment. A translationalforce can create a moment, but acommon rotational motor is an ex-ample of a device that only produces amoment and no translational force.

Most cambered, or non-symmetricalairfoils produce both a translationalforce, lift, as well as a rotational forcedescribed by CM, as shown in figure-1.The exception being those with areflexed trailing edge used for flyingwings. The aerodynamic center (AC) ofthe airfoil, typically 1/4 chord location,defines the center of lift.

The moving center-of-pressure conceptassumes that an airfoil produces asingle force, lift, and that the momentis the result of the lift force. But sincethe moment of many airfoils is fairlyconstant even though the lift changes,this concept implies that the moment-arm, d, changes, as the lift changes. Tomaintain a constant moment, themoment arm must increase as the liftdecreases, moving the center ofpressure rearward.

What becomes difficult to believe is

that as the effective AOA and the liftbecomes small, a point is reachedwhere the lift is actually behind thetrailing edge, and ultimately goes toinfinity as the lift becomes zero. Then,as the lift decreases infinitesimally andbecomes negative, the center-of-pressure moves an infinite distance infront of the leading edge.

Center-of-Presure vs.Moment Measurements

The center-of-pressure curve on the1924 airfoil plots from Simons’ indicatea most forward chord position atmaximum lift, and more rearwardchord position as the lift decreases.

Figure-2 illustrates typical curves fora moving center-of-pressure as in the1924 plot from Simons’. A typicalcurve is close to the 1/4 chord locationwhen the lift is near its maximum onthe right hand side of the plot. Moving

to the left, the liftdecreases. The center-of-pressure decreasesslowly, at first, butthen falls very quickly.The center-of-pressure,on all three curves, hasmoved to ~60% of thechord as the liftdecreases to 0.25.

Consider using theprevious equation, tocalculate the center-of-pressure (CP) on anairfoil knowing theaerodynamic centerand a constant momentcoefficient, CM. The CPlocation must be somedistance from theaerodynamics center(AC) so that themoment arm, d, issufficiently large toproduce the requiredmoment for theavailable lift, L.

CP = AC + d

The moment arm canbe determined bydividing the momentby the lift

CP = AC + M / L


We should all be familiar with the liftequation

L = 1/2 r V2 S CL

where r is air density, V is airspeed, Sis the area of the wing, and CL is thelift coefficient. A similar equation isused to calculate the total moment,given a moment coefficient, CM, asindicated on the more modern 1931plot from Simons’

M = 1/2 r V2 S CM

These two equations provide valuesfor the moment and lift produced bythe wing, and can be used to replace Mand L

CP = AC + (1/2 r V2 S CM ) / (1/2 r V2 SCL )

Fortunately, since the lift and momentequations are so similar, many termscancel out, and the result is simply

CP = AC + CM / CL

This equation relates the momentcoefficient on modern airfoil measure-ments to center-of-pressure on pre-1930 airfoil measurements. Figure-2

illustrates this by reproducing thecenter-of-pressure measurement forthree constant moment coefficients,assuming an aerodynamic center at the1/4 chord position.

While both the lift and momentdepend on airspeed, it is very commonfor the moment coefficient to berelatively constant while the liftcoefficient varies with AOA. It appearsto be standard practice for airfoilmeasurements to be made and plottedwith respect to the 1/4 chord location.A moment coefficient that is notconstant but varies linearly (in astraight line) with AOA indicates anairfoil where the aerodynamic center isslightly different from the 1/4 chordlocation. In these cases, the measure-ments account for the aerodynamiccenter not being at the standard 1/4chord location, that the moment armbetween the actual aerodynamic centerand standard 1/4 locations is not zero,and therefore that the lift contributesto the moment that determines CM.There are, however, (less desirable)airfoils where the moment coefficientvaries non-linearly with AOA similarto the way lift varies non-linearly nearstall.

Working out these equations andseeing the resulting plots helps me seethat ... “the center of pressure is notalways a convenient concept in aerody-namics.” With this understanding, I canunderstand why some people wouldsay the center of pressure is behind thewing, but I don’t think this helps meunderstand pitch stability.

My sincere thanks to Dave Registerand Lee Murray for their review onthis article. Their comments verysignificantly shaped and improved thisarticle. Thanks to Dave for help withthe graphics.

References:

Anderson Jr, John D., Fundamentals ofAerodynamics, 2nd ed., (New York:McGraw-Hill) 1985.

Gale, Ing Ferdinando, AerodynamicDesign of Radioguided Sailplanes,(Olalla, Washington: B2Streamlines)1996.

Simons, Martin, Aerodynamics for ModelAirplanes, 3rd ed. (Great Britain:Nexus Special Interests), 1994.

n


The information in this column has been derivedfrom manufacturers press releases or othermaterial submitted by a manufacturer about theirproduct. The appearance of any product in thiscolumn does not constitute an endorsement ofthe product by the R/C Soaring Digest.

NEW PRODUCTS


NEW PRODUCTS


T.W.I.T.T.(The Wing Is The Thing)T.W.I.T.T. is a non-profit organization whosemembership seeks to promote the researchand development of flying wings and othertailless aircraft by providing a forum for theexchange of ideas and experiences on aninternational basis. T.W.I.T.T. is affiliatedwith The Hunsaker Foundation which isdedicated to furthering education andresearch in a variety of disciplines. Fullinformation package including one back issueof newsletter is $2.50 US ($3.00 foreign).Subscription rates are $20.00 (US) or $30.00(Foreign) per year for 12 issues.

T.W.I.T.T., P.O. Box 20430El Cajon, CA 92021

The Eastern Soaring League (ESL) is a confederation of Soaring Clubs, spread across the Mid-Atlantic and New England areas, committed to high-quality R/C Soaring competition.AMA Sanctioned soaring competitions provide the basis for ESL contests. Further guidelines arecontinuously developed and applied in a drive to achieve the highest quality competitionspossible.Typical ESL competition weekends feature 7, or more, rounds per day with separate contests onSaturday and Sunday. Year-end champions are crowned in a two-class pilot skill structureproviding competition opportunities for a large spectrum of pilots. Additionally, the ESL offers aRookie Of The Year program for introduction of new flyers to the joys of R/C Soaring competition.Continuing with the 20+ year tradition of extremely enjoyable flying, the 1999 season will include14 weekend competitions in HLG, 2-M, F3J, F3B, and Unlimited soaring events. Come on out andtry the ESL, make some new friends and enjoy camaraderie that can only be found amongst R/CSoaring enthusiasts!

ESL Web Site: http://www.e-s-l.orgESL President (99-00): Tom Kiesling (814) 255-7418 or [email protected]

The League of Silent Flight (LSF) is an internationalfraternity of RC Soaring pilots who have earned theright to become members by achieving specificgoals in soaring flight. There are no dues. Once youqualify for membership you are in for life.The LSF program consists of five “AchievementLevels”. These levels contain specific soaring tasksto be completed prior to advancement to the nextlevel.Send for your aspirant form, today:

League of Silent Flightc/o AMA

P.O. Box 3028Muncie, IN 47302-1028 U.S.A.

http://www.silentflight.org

SailplaneHomebuilders

Association (SHA)

A Division of the SoaringSociety of America

The purpose of theSailplane HomebuildersAssociation is to stimulate interest in full-sizesailplane design and construction byhomebuilders. To establish classes,standards, categories, where applicable. Todesiminate information relating to constructiontechniques, materials, theory and relatedtopics. To give recognition for noteworthydesigns and accomplishments.SHA publishes the bi-monthly SailplaneBuilder newsletter. Membership cost: $15U.S. Student (3rd Class Mail), $21 U.S. RegularMembership (3rd Class Mail), $30 U.S. RegularMembership (1st Class Mail), $29 for All OtherCountries (Surface Mail).Sailplane Homebuilders Association

Dan Armstrong, Sec./Treas.21100 Angel Street

Tehachapi, CA 93561 U.S.A.

Classified Advertising PolicyClassified ads are free of charge to subscribersprovided the ad is personal in nature and does notrefer to a business enterprise. Classified ads thatrefer to a business enterprise are charged $5.00/month and are limited to a maximum of 40 words.RCSD has neither the facilities or the staff to inves-tigate advertising claims. However, please notifyRCSD if any misrepresentation occurs. Personalads are run for one month and are then deletedautomatically. If you have items that might be hardto sell, you may run the ad for 2-3 months.

For Sale - Business

PARACHUTES: $12.50 (includes S&H U.S.A.)Send check or money order to Dale King, 1111Highridge Drive, Wylie, TX 75098; (972) 475-8093.

Summary of Low-Speed Airfoil Data - Volume 3 is reallytwo volumes in one book. Michael Selig and his studentscouldn’t complete the book on series 3 before series 4was well along, so decided to combine the two series ina single volume of 444 pages. This issue contains muchthat is new and interesting. The wind tunnel has beenimproved significantly and pitching moment measure-ment was added to its capability. 37 airfoils were tested.Many had multiple tests with flaps or turbulation ofvarious configurations. All now have the tested pitchingmoment data included. Vol 3 is available for $35. Ship-ping in the USA add $6 for the postage and packagingcosts. The international postal surcharge is $8 for surfacemail to anywhere, air mail to Europe $20, Asia/Africa$25, and the Pacific Rim $27. Volumes 1 (1995) and 2(1996) are also available, as are computer disks contain-ing the tabulated data from each test series. For moreinformation contact: SoarTech, Herk Stokely, 1504N. Horseshoe Circle, Virginia Beach, VA 23451 U.S.A.,phone (757) 428-8064, e-mail <[email protected]>.

Reference Material

BBS/Internet

Internet soaring mailing listserve linking hundreds ofsoaring pilots worldwide. Send msg. containing theword "subscribe" to [email protected]. The"digestified" version that combines all msgs. each dayinto one msg. is recommended for dial-up users on theInternet, AOL, CIS, etc. Subscribe using [email protected]. Post msgs. [email protected]. For more info., contact MichaelLachowski at [email protected].

Books by Martin Simons: "World's VintageSailplanes, 1908-45", "Slingsby Sailplanes","German Air Attaché", "Sailplanes bySchweizer". Send inquiries to: Raul Blacksten,P.O. Box 307, Maywood, CA 90270,<[email protected]>. To view summary ofbook info.: http://home.earthlink.net/~raulb

There is a growing interest in scale soaring inthe U.S. We are dedicated to all aspects ofscale soaring. Scale soaring festivals andcompetitions all year. Source for informationon plans, kits, accessories and other peopleinterested in scale. For more information:

web site: www.soaringissa.org

InternationalScale SoaringAssociat ion


Documents

R/C Soaring Digest - Nov 2002