I just built a SPAD that is a conglomeration of a few different airplane designs I found on the web. I set up the CG for the plane that the wing, tail feathers, and fuse length were copied from, which figures out to be 25% from LE, right on the spar. My problem is that even though the plane becomes airborne very quickly and smoothly, at about 30-40 feet up it goes nose over and is impossible to recover. It acts like the elevator is not even working. On all 3 flights (you have to love SPAD) the elevator worked just fine when I checked it on the ground, after the crash. Do I need to change my CG? Increase elevator control surface? Throw it away? Any suggestions are greatly appreciated.

A pic of the airfoil.

Newbie76
From the picture in your second post, it looks like your wing may have excessive positive incidence. The only way that I have to measure it is to hold a ruler up to the CRT screen and look at the angle of the wing compared to the angle of the fuselage and horizontal stabilizer. I am not exactly sure what you mean when you say “it goes nose over and is impossible to recover”. But with excessive positive incidence the model may climb out nicely at low speed and as speed builds take more and more positive angle of attack until it stalls and comes right back down. If the model is not in a stalled condition when it impacts the ground then it may be something like the control surfaces twisting/warping under normal flight loads. I do not think that it is a C/G problem. If it balances at the spar then you are approximately at the 25% point. Plus, If it was a C/G problem you would know it immediately at the time of launch. I would check the wing incidence and set for zero or no more than a positive one or two degrees. If that does not solve the problem, then launch and immediately throttle back and nurse it up to a safe test altitude. Slowly advance the throttle and observe the results. If it starts to pitch down immediately pull back on the throttle to recover then look for excessive down thrust of the engine or the horizontal stabilizer bending under load.

Let us know what you find.

Do you have experienced pilots around to give it a test flight as well? It might be worth it.

It's hard from your description, but it sounds to me like a classic "leap off the ground and stall" type take off. The plane is leaving the ground too soon with out enough airspeed. One most RC models, the power-to-weight is just enough to climb nearly vertical to about the height you mention before gravity out does the motor and down comes the plane. Since you are stalled, up elevator is making it worse, not better. The old "up elevator makes you go up. Lots of up elevator makes you go down" thing.

Try taking a really long and fast take off run, and go easy on the elevator. when the plane leaves the ground, be ready to give a little down elevator to keep the climb as shallow as you can manage to avoid hitting anything. Do your first couple of turns with as shallow of a bank as you can manage, nice smooth and open, no bank-and-yank stuff.

There's a good chance that your elevator is too sensitive as well, so maybe turn down the rates there a little bit. I also agree with Truncator that you should double check the wing incidence, as it will make everything else worse.

Thanks for the tips. I measured the incidence and it seems as if I have around 7 degrees. How does positive incidence stall the plane? Does an excessive amount of impact lift create enough drag to stall? Thanks for your help and I'll post the results as soon as I can.

Newbie76
Some of the other guys can probably give you a more involved technical explanation but basically the excessive positive wing incidence will cause increased lift and drag. As the speed goes up, the nose will pitch up until you exceed the critical angle of attack of the wing. At that point the wing is stalled and down she will come.

It certainly seems like a stall situation. If it is, the triangular airfoil section will aggravate it. The sharp leading edge will cause a stall at a pretty small angle of attack. Most any wing section will fly with enough power, but that section must be flown fast and will stall easily.

7 degress is a lot. You can think of that has having a ton of up elevator trimmed in to the plane, so when you lift off, you're already holding way too much elevator, so you stall quickly and crash. (It's not a totally accurate description, but it's close enough). Adding more up elevator makes it worse. You might be able to acheive a sort-of counter to that with lots of down elevator trim in the air, but it's not the way to go.

Raise your TE a little and get the wing down to 0-2 degrees positive compred to the stab and try flying it again.

While your wing does have a flat surface airfoil, You do have the flat on the top of the airfoil, which makes a huge difference. That's good. And your LE is sharper than you might find on some planes, but isn't all that sharp as these things go. You should be able to fly that wing just fine. LouW is correct, though, that those elements of your wing can cause a more pronounced stall, depending on the wingloading and other factors.

Oh, one other thing, since your ailerons are coming off the under side of the wing, check to make sure they are slightly reflexed upwards, and don't droop at all. That just adds to the incidence and creates other problems as well.

Once the tail comes off the ground, try holding a little down elevator to keep the plane on the ground longer. The positive incidence is causing the plane to jump up in the air before it has enough airspeed to fly