MTK

Depends on amount of control authority you want in roll. For aerobatcis we use around 15% of the wing for good control that damps quickly. In your application where you will fly very slow, maybe a bit more would be better but it may also get twichy.

Just a general observation, with 120" of span an avg chord of around 18", that's a sizable amount of wing area for a model plane. The loading would be quite reasonable at a couple #s per sq foot. The 2 56's would produce enough power for sustained hover at probably 1/2 throttle. You probably don't need an 18% section but it would be thick and sturdy. I don't think you will feel that much difference in speed compared to a 12% section; the area dominates drag regardless

The model you are describing doesn't need to weigh 30#s tho. I would think it could be easily built at 20-24 #s using typical balsa-ply-foam techniques. Your perceived min speed would decrease accordingly. And don't forget to soft mount those gas engines. I've got a thread here on RCU that describes how to do that. I have my DLE55s and BME 58 Xtreme soft mounted (full rubber isolation) on mounts that weigh 5 ozs. Will make the airframe and radio last much much longer and reduce the noise footprint considerably.

2walla

My Feedback: (10)

At the size and weight you are proposing it probably won't make much difference what airfoil you use for just flying around. Be careful making the wing too thick as it can have a much larger effect on the flight characteristics than the overall airfoil shape. Since most of us control the models pitch with the elevator vice using the power settings you will need to consider the incidence- a flat wing generates lift at 0 AOA and a symmetrical wing will not.. Think of what you are going to use it for and fly it.. For a sport model you to set it up so once trimmed the throttle setting will not change the pitch.. this leads to putting downthrust in the motors, negative wing incidence relative to the tail-feathers etc, in a flat bottomed wing plane to counter the increased lift at higher speeds and throttle settings..

physicsprof

Good stuff guys. The weight, as of right now is only estimated and a very rough number. I will be using kiln dried aspen for the wing ribs. It is readily available at most hardware stores, so I intend to use most of my supplies where the average person can pick up materials. Also, theenginesare about 5.4lbs each(with stock muffler and no tank), hence, my #'s for the added weight. . I will be fabricating my own aluminum brackets for mounting. I think the 18%- thickness would help out with the weight of theengines.

tony0707

Hi
You can just build a very large 60 size STIK
The plane does everyting slow and easy and relaxing to fly,Big flys better !
I have my most flights to date ,after 22 years at RC ,on a 40 Sig Sweet Stik and a 60 size Sweet Stik
Just enoy that design for easy breeze day at the field and those are some of the best and most fun
Regards Tony

Propworn

My Feedback: (3)

Check out the SAE Aerodesign competition http://students.sae.org/competitions/aerodesign/ they design a high lift low speed aircraft to lift as much weight under competition limits. How about a 14 ft span powered by a .65 Jett engine lifting near 45 lbs. The fuselage designs could be better but the main thing is all of them pretty well use the Selig 1223 airfoil.

http://archive.stevens.edu/ses/me/fi...13/graphs.html

http://www.demec.ufmg.br/Cea/Discipl...da/artigo4.pdf

This is the University of Windsor's 14 ft span with a Jett .65 for power. http://www.youtube.com/watch?v=g8xC-pE6RoA

Slightly different is the year we used an OS .61 limited to a 1000 square inch wing plan. http://www.youtube.com/watch?v=vBgWUceKmR0

Dennis

guamflyer

I like what you intend to do but I am also in agreement about the trade-offs..once heard or read...theres no free lunch in concerns to aerodynamics...I scratch build alot and keep to a certain criteria as you are doing.. look up the motorglider airfoils as they're used alot for crusing and slow speed..I also agree that weight should be watched carefully..specific airfoils for a particular purpose may need different build techniques..all the previous mentioned airfoil designers got some great foils...interestingly I have been using rc flight simulators to test fly the aircraft with the set-up I'm gonna use..it's one thing to get numbers and yet another to feel what its gonna probally do in flight...A guy once told me " can't really say what its gonna do till you throw it out"..you've built alot and flown alot , rely on that experience of how the planes flew...I came to the concluson after 35 years that you either fly slow or fly fast but you're still flyin

guamflyer

Geek1945

130 lbs with a 100cc engine seem almost incredible, it demonstrates models don't follow airfoil Reynolds numbers since their cords are much smaller giving more lift per wing span. I would imagine as you said very few in airfoil design have delved into accurately plotted performance with such narrow cords, since there aren't big $$ doing so. Fortunately this lack of data has been supplemented by a few PhD's who also are model flyers, plus a wealth of practical experience by modelers.

I have noticed a few commercial and military UAV's have adopted PSRU's to gain more performance out of engines vs. direct drive the Tern has. No doubt the Tern has no problems with cooling either compared to most pusher designs. Among it's other pluses are those large diameter wheels, disc brakes, and box fuselage with Hersey bar wing.

Regards
Ed

sidgates

My Feedback: (2)

I am not an engineer just an experienced model flyer and some design experience. One time I designed a model to fly from the US Canadian border to Alaska. I designed the model one night and built the fuselage in one week. The pilot built the wing in one week. It was 10 ft square area. 12" cord and 10ft span. It was an eye ball clark "Y" airfoil.
The design goal was short takeoff and landing and approx. 2hrs endurance. I laid a gallon fuel jug on my drafting table and designed a high wing fuselage around the tank with the tank on the C/G. We had a 1 qt. hopper tank right behind the engine. Fuselage construction was 1/2" thick Styrofoam from the trailing edge of the wing to the firewall with 1/16" thick balsa sheet inside and out. Fuse from wing trailing edge back was 1/16" thick balsa with some stringers. We needed to keep the fuel and radio warm so the cabin accommodated two hand warmers. I was powered by a Fox 74 engine and about 15lbs with a gal of glow fuel. Wing was mostly balsa with silk covering.

The test flight was a 10mile round trip cross country. It was very easy to fly. If I were building it today I would probably build the wing with foam ribs. You can see pictures of my models on my web site:
http://www.sidgates.us

Geek1945

Boy, haven't things changed since then now glow engines are cheaper whereas gas ones are higher and electric ones might put both out of business shortly. I'm with you on foam ribs and fir or popular for structure. I have a friend who is in the cabinet business and I get quality wood & plywood from him since the big box stories wood is trash at about the same prices too. Just add some 0.75 or 1.5 oz fiberglass and you can make stress points disappear so I'm designing a UAV to take video not FPV since their essentially shoe boxes with tail feathers and Hersey bar wings. Or as I like to say KISS Planes (simple and cheap) and larger enough not to fly backwards on a windy day. Besides my viewers gallery is cattle, horses, and other critters who never critique your model anyway. To this day I still have no idea what 3D is except maybe hovering like a copter. Another thing is today's adhesives no problem sticking different materials like in the 50's. Things are better yet, more complex since you have so many options doing the same thing.

I dusted off Andy Lennon's RC MODEL AIRPLANE DESIGN 1986 version which I still use to refresh dormant brain cells if not on the web. My wife say's why are you buying so many engines? Of course she doesn't know the EPA is just about ready to eliminate gas 2 strokes and they won't be coming back either. I just saw a 50volt electric weedeater along with battery powered chainsaws so what's left? I might just live long enough to see electric cars become common too.

My degree is math but it's been 40 years since working any calculus problems yet, now most of those have been replaced with computer programs just plug-in your variables and presto there's the solution. So I finally put my slide rule away so my grandchildren will have something for show an tell.

Regards
Ed

tomclark

Well, I sure can't start talking Reynolds numbers or NCAA airfoils, but after over 75 scratch builds of larger models over the last 40 years, I have done plenty of "sketch it out and see how it flies" models, so I will jump in and add a few comments. If you want a high drag airfoil, just make the thickness over 15%, especially on a flat bottom airfoil. Airfoils with the high point up front become very high drag. I prefer mine to be usually around 33% for the best glide ratio.


This year I have finished four scratch builds so far, replacing some of the aging fleet, and having the shop time on my hands. It was time for some slow fliers instead of the high-powered rockets I usually fly. Number one is a 90" Kadet, a real slow flier and an old favorite. Always wanted to blow one up. Other than some Telemasters, hardly ever use flat bottomed airfoils. They are very limited for fun flying as they are not aerobatic at all.

Photo one shows the airfoil. Wasn't planning on airfoil lectures, so don't have exact numbers. Span 90". Cord 18". High point about 33%. Aspect ratio 5. (90 divided by 5 = 18" cord) Normal A/R is 5:1. Short wing is 4:1. Long is 6:1. Your planned wing at 6.7 is a very long skinny wing. Hard to make strong unless very thick - and then it is very high drag.

#2 shows general wing construction. Ailerons cut out of finished wing. About 2.5" wide.

#3 shows 5:1 AR. Quite standard for RC birds

#4 shows a no flap landing. With today's computer radios and a servo (or two) on each aileron, it is easy to make the ailerons drop down about 30 percent for super slow landings and takeoffs. The Kadet is so slow flying that it amazes all at the field, and they all love to fly it compared to most of today's average trainers.

#5 shows another wing going together. Same construction except my usual semi-symetrical airfoil. This wing is a blow up of another old model, a 70s era RCM 60 Trainer. This one has a 102" span, 14.5% airfoil, 40% high-point. A/R was 5.1. Plane takes off and lands at under 10 mph, and inverted flight takes almost no down elevator. It is almost as slow as the Kadet, but a lot more aerobatic.

#6 just shows another 102" span stick. I wanted to power it with a small engine I had on hand, so the airfoil is 12%, very thin. Plane has huge ailerons as today's typical sticks do, so it still takes off and lands very slowly. If it was heavier it wouldn't, but this plane only weighs 11 pounds and has a small 120 engine. It is another really fun flier.

Hope this was at least some help. Happy to answer any questions. In '08 I wrote a small book (The Sunday Flier) on scratch building planes like this, and covered airfoil designs for typical RC designs. PM your address and I'll send you a copy. Check my Gallery for more photos.