I recently bought myself a trainer, just because it was so cheap and I had enough spare gear to equip it. The trainer is an AT-40 (see pic stolen off the net). I'm past the trainer stage and now fly low wings and 1/2A models, but I bought it as a bit of a hack, a plane I knew I could fly just for the sake of flying it. And I had a few ideas in mind to 'pimp' it up a bit.
I had on of these as my first trainer and it flew very well with no bad habits. The original was powered with a GMS.47.
This new one, I have converted it to a taildragger, built the wings with zero dihedral and installed twin aileron servos. For power I have a Super Tigre Sport .40 that is almost new. Runs great on a 10 x 6 or 10 x 5.
My problem is that this beast does not want to fly.
The instructions say to balance it at about 33-35% MAC, which is a bit far back for my liking, so I moved the engine forward and put in a bit of nose weight to bring it to about 27-28% MAC.
When I go to take off, it uses the whole field to get airborne - if it does get airborne. It will lift the tail no problem, but just won't lift off the ground. And if I am lucky enough to get it airborne, its very squirrely to fly. It seems to be very tail heavy, despite all the weight I have in the nose. I don't know where to go from here - more or less weight in the nose?

Sounds underpowered or overweight.

I wondered if the .40 would be enough. It's a .40 size trainer, so I thought a .40 would haul it. I might take the .47 off another plane and try it out.
I weighed it and its about 1.9 kgs. If the .47 doesn't fly it, I have a .61 with a pipe that may just lift it off the ground - vertically

I agree, it sounds like it is underpowered. On most of these trainers, the CG is about 2 to 3" from the leading edge of the wing. If your throws are set to 100 TO 125% nd it still doesn't get airborne, I would try a .51 to .61 sportng a 10/5 sport prop. If you have a tach, make sure you are getting about 12,000 rpm on the high end. you may want to go with a more agressive control arm on your elevator servo to gain more lift, assuming the servos can gain maximum travel. Be careful though. If it is this dificult to get off of the gound,
it will be squirly in the air, and I would think that a dead stilk landing would not be he easiest. Plane looks great though, but bet it would look better in the air!
Good luck!

1.9 kg shouldn't be too much weight for any .40 to fly. Besides, you say there's something wrong with the handling too, which would indicate some problem with the setup.
Have you worked through any flight trimming? Specifically, thrustline and CG tuning is in order before you change anything else.

on that airplane, If I din't have an instruction manual, I'd set the CG at the wing spar.....all the incidences correct?
Good Luck

Think I would try a 11X4 or 5 prop . Maybe a incidence change is needed ,try a 1/8" under the leading edge .The .40 should fly it OK. Good luck, Max H. AMA 14958

The first thing to do is to set it up like the manufacturer recommends. Try the correct CG.

Right on dbacque. Find out what the CG is and put it there before you do anything else.

Hi!
Plane looks fine too me. But the picture shows a rather strange wing form, especially the tips. Are you sure the wing isn't upside down!!??
A .40 high winged trainer should just leap of the ground with .40 engine like your Super Tigre .40.
11x5 or 11x6 APC or RAM is the prop of choice for all .40 trainers if you fly at sea level. 10x5 or 10x6 is not that good. If you use an ASP .40 (Kyosho .40) and run 10-16% nitro a 12x5 or 12x6 APC is the prop to use.

Cof G should be on the wing spar on all high winged trainers. Just put your fingers under the spar and the plane should balance slightly nose heavy without fuel.
Elevator travel should be around 1,0-1,5cm up (and as much down) measured on the trailing edge of the elevator.

When you fly a plane at full throttle it should be able to just hold its heading without you doing anything. It should fly straight as an arrow as long as you have trimmed it correctly!

where did you mount the gear when converted to a taildrgger?too far forward will make for difficult liftoffs also as will all other sugestions

Hi longdan
Could your prop be mounted backward? A backward mounted prop will move the air in the correct direction but will only move about 60% as much air.

I think Villa may have gotten it.... I was about to post the same thing but he got to it first... take a look man..

I agree that the ST-40 is plenty of engine for this plane. I have one on a SPAD trainer that weights almost 6 pounds, and it has plenty of power.

I would go with the 11x6 prop, make sure the main gear is near the leading edge of the wing, make sure the CG is on the spar, and there is no reason it shouldn't fly.

Good luck

If it lifts the tail during take off, and the model runs at more than 50 km/h, it will lift up, even with a forward CG; unless the wing is at zero or negative incidence angle.

I recommend to verify the wing incidence and also the angle relative to the stabilizator (decalage).

You may want to read and follow the adjustments and trim sequence explained at this link (Three part document):

http://www.modelaircraft.org/mag/ftg...29/29main.html

TURN THE WING OVER!
>>>-> Bassfish

I'm liking the reversed prop theory too.
Or the prop might be a pusher.

OK longdan, you've got my interest. Did you finger it out?

looks to me like you have a prop on it for electric motors......or its backwards.....dont look right.

Actually, it sounds like it's suffering from something that happens a lot with tricycle geared model planes.

If the front gear is shorter than the mains, the plane will have a stance that places the wing nose down. If it's too much, when the plane starts to accelerate, the wing builds up lift...... toward the ground...... and builds it more the faster the plane moves. Depending on the CG and a couple other things, the horizontal tail just might not overcome the screwed up lift vector.

Look at the airplane from the side. You want the stance to position the wing's AOA near the zero lift AOA of that airfoil. Designers want that so the wing won't adversely affect acceleration on the takeoff roll until the pilot wants more lift. That also throws less downwash at the h.tail so there's less pitch up generated until the pilot wants it, and he pulls some elevator to get it.

Hey guys don't look at the pic, it is not the bashed plane; he got the pic off the web.

What kind of airfoil? The .40 should fly a 6 lbs plane like that. I think the wing incidence is out. Also on the CG set it up at 25%; if that is what the plane wants, then it is what it wants.

longdan: can you please clarify something for us? Is the pic u supplied of your actual plane? or off the net? If thats not your actual airplane, can we please get some pics?
Like others have said, the plane in the pic dont look right. According to that pic, the wing looks obviously upside down. The prop in that pic looks like an APC prop, and if you look closely, its mounted right.
If your plane isnt the one in the pic, and everything is set up right, I would say its the CG. With the CG too far forward, performance will drop rapidly. The stall speed will be higher (the angle of attack for a given airspeed will be closer to the critical angle of attack) The plane will be more stable (unless the CG is waaay to far forward) and the plane will just not want to fly.
Also, if the trim is out of wack, it can cause symptoms like this too. On my kadet, I accidentally put the wrong controll arm on my rudder servo, and it allowed the gears to "skip" It made the plane just not want to fly. (kinda in a continuous slip) It needed full power, alot of controll input and a bit of luck to fly straight and level and land safely.

Actually I think Longdan has either figured it out or gave up and sold the plane becasue this thread is four months old.

BTW, he said right up front, "pic stolen off the net".