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Planning a club “hack”

Old 03-05-2019, 12:22 PM
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Default Planning a club “hack”

Gday all,

I posted in the beginners section asking if anyone knew of a giant scale trainer to use as a club trainer/hack. I got a couple of great replies, but I got to thinking (always dangerous)...There’s nothing out there that fits my exact desires, so why not create one myself?

My goal is to create an airframe that’s easy to build and repair, fly like a trainer and do a little bit more as the pilots grow.

At the moment, my rough idea is to build something with a 12 foot wingspan, about 8 ft loa, high wing, tail dragger for about a 70 to 100 cc gas engine.

The wing;
Ailerons and flaps, 1 servo per surface
Symmetrical, no idea what airfoil yet
0 degrees dihedral, 1.5 ish degrees washout
Old 03-06-2019, 07:03 AM
Hydro Junkie
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I'd make a couple of changes to that wing design:
Ailerons and flaps, 1 servo per surface With a 12 foot span, are you sure one servo per surface will be enough?
Symmetrical, no idea what airfoil yet I'd go with semi-symmetrical as that would be more stable in a training roll
0 degrees dihedral, 1.5 ish degrees washout I'd go with a couple of degrees of dihedral to prevent the wings from looking like they are drooping and to make it easier for an inexperienced pilot to fly
Old 03-06-2019, 08:32 AM
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Smoky, welcome to the world of designing. I would like to make a couple observations of your plans so far. I think a symmetrical airfoil is a great idea. Many guys had learned to fly on the original Ugly Stik and the Bridi series of trainers. I would not however incorporate any washout. While inverted it becomes wash in. The tip stall tendency you are trying to eliminate can be done with other techniques such a keeping the weight in check, correct CG placement and the use of aileron differential. Another tip would be to reduce the tip cord just a little for the last 10" of the wing similar to what Bridi did on the Kaos. With today's available servos one per wing surface is not an issue, what could be an issue is flex of the surface. An aileron or flap that will twist easily can flutter. No dihedral is a personal choice but will reduce the airplanes self righting capabilities. That being said, a cabin mounted wing will exhibit some dihedral effect. Your airfoil could be a hot topic all by itself. I have found that unless you are building a pylon racer or a sailplane, the actual curvature of the airfoil is fairly unimportant within the same family ( flat bottom, semi symmetrical and symmetrical ). Where you place the airfoil high point and airfoil thickness does play a part though. For what your goals are, I would go with 14% to 16% airfoil thickness with a high point at 30%. If you are doing a foam core wing you can move the airfoil high point forward 5%, another little trick to avoid tip stalls as well as place a little additional drag out at the tips that will aid in directional stability. Good luck with your project. I hope you will do a build thread.
Old 03-07-2019, 10:15 AM
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Firstly, I have to apologise for my post, the design came to me over a few too many drinks, and the post came after a few more, sorry about that��
Now, I’ll try and make a bit more sense, and incorporate some of your suggestions.
Re flex in the control surfaces, that’s actually one thing I did consider, I thought of incorporating an arrow shaft into the all of the ailerons, flaps and elevators.

A bit of dihedral sounds like a great idea, not sure why I was thinking of 0 degrees, apart from laziness. I’ll forget the washout, I can achieve the same result with a bit of aileron reflex for slower flight (aren’t modern radios and flight conditions wonderful��)

The wing and empennage will be built up, using fairly conventional methods (“I beam” spar, cap strips on the ribs). For ease of transport I think that a 3 piece wing is the way to go, I’d like to make it a quick and easy setup at the field so any suggestions regarding a quick joining/take down method would be very much appreciated.

More to come


Last edited by Smokeyr67; 03-07-2019 at 10:19 AM.
Old 03-08-2019, 05:50 AM
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Senior Telemaster.
Old 03-13-2019, 09:14 AM
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Smoky, the washout is a better idea than relying on aileron reflex. While it's true that reflexing the surfaces reduces the angle of incidence compared to the inner portion it also alters the airfoil shape to one which is not as stall resistant. So you gain some, you lose some. On the whole you gain more than you lose which is why it works... to an extent. And it's certainly better than nothing. But if you build the proper washout twist into the wing and keep the aileron neutral where it should be it's all gain and no loss. Plus you don't get folks asking why the ailerons are "like that"....

The other thing is that if you use flaps consistently for takeoff and landing the flaps being deployed also induces a LOT of washout twist in the wing. Even a few degrees of flap will do as much or more than a couple of degrees of washout. And if you wanted to make dead straight panels instead of building in the twist I'd suggest you droop the flaps a couple of degrees instead of messing up the outer airfoil with reflexed flaps.

The choice of airfoil is open for a lot of variations. And if some of the hack duty for this thing will be casual aerobatics and a fair amount of time inverted I'd say that the symmetrical airfoil is a slam dunk good way to go. And make the wing straight and use flaps to add in washout effect for takeoffs, initial climbing and then landing again. In between it can be a blast. But if' it's more for casual upright flying around and letting low timers and raw newbies get a feel you may want to consider more like a NACA 2415. The shape and thickness are known for having good flying characteristics. And the 2% camber isn't so much it will wreck the aerobatics side to a great degree but it will certainly favor a slightly lower stall speed and better manners for slow flying. And the thickness will tend to turn minor stalls into more of a slight mushing which can be easily recovered with a whiff of down to get the speed back.

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