Hi guys, I am playing around with an old trainer I have, bashing it into a Stick more or less. It is a PT -40 that I converted into a tail dragger, and I have now cut the wing in half and taken all the dihedral out of it. I have a spare OS .61 SF that will fit into the front of the plane. I am doing this because I am advancing beyond my current Trainer's capabilities, I want something a bit more challenging and fun to fly, but I want to keep my old trainer around because I do enjoy the ease of flying it.

Here is my main question, will that .61 just be too much for the airframe? I built this plane a while ago, it is well put together, but not a ton of flight time, I know it is not designed for a .61 but the motor fits into the frame, now I am wondering, should I do it? Will I have too much speed and overstress the airframe ect.

I would like to get your thoughts on putting that much power in a .40 trainer.


Thanks

Manks

It will be a lot faster than if it had a .40LA in the nose but not a speed ship by any means. You will also have great take off run and vertical. Be careful how much weight you add. You may end up putting so much ballast in the tail to get it to balance that you end up giving all the power back just to haul around the lead.
The air frame will only go so fast and then thats it.[)]

OK, Manks,

Here's what can or will happen when you put a .61 into a .40 trainer.

Strength:
1. Rubber banded on wing. Not very strong. If you yank hard, you can lift the leading edge of the wing. It will scare the hack out of you. In particular, install the rubber bands from FRONT TO BACK. This stretches them tighter in the front so they hold the leading edge down better.
2. You wing has balsa spars, more than likely. Making a turn or pitch maneuver at a higher speed puts more g's on the plane. There is a possibility that you can break the wing.
3. It has a thin wing. Sticks tend to have a fairly thick, symmetrical airfoil, making then inherently stronger than a thin wing.
4. Center joint. Most trainers do not have a big wing joiner. You need to use fiberglass cloth 4" wide or so and 30-minute epoxy on the top and bottom of the joint for added strength.
5. Stab & fin. Can be easy to break off on a trainer with high g maneuvers. I recommend guy wires. You can use weed eater line. It's cheap, easy and plenty strong enough.
Servos and pushrods. You standard seervos will be OK. Check the pushrods to make sure they can't flex and allow the controls the straighten out.
6. As has been mentioned, reinforce the firewall using 30-minute epoxy. the 30-minute kind soaks in more and gives a stronger bond. Use tri-stock or 1/4" square in the corners.

Aerodynamics:
1. Flat bottom airfoil. As you increase the speed, your plane will tend to climb. When you reduce power to land, you'll need to hold a good bit of up elevator or retrim. I recommend raising the trailing edge of the wing with 2 popsicle sticks to reduce the incidence and alleviate the clim.
2. Your CG may be out. Rather than add weight, move the elevator and rudder servos to the read fuselage. Cut a hole, add a little piece of ply at each end and screw them in place. Doesn't have to be fancy, just has to hold.
3. Wing tips. If your trainer has plastic or wood wing tips that are beveled upwards, take them off. They act like more dihedral, which you don't need.
4. Flutter. Keep an ear open for a buzz. Most of the time it is the aileron tips. Try cutting the tips off at an angle starting 3" in from the tip to a point at the tip.

Engine:
1. Use a low pitch prop. No more than a 6, but a 4 pitch is better. It will give you the climb, but not the speed which will stress the plane. A low pitch prop will also help you slow down for landing.
2. Use power management like the guys with the really big planes. Half power in level flight and full power for straight up climbs.

Other:
1. You may need a longer landing gear or larger wheels for prop clearance.
2. Since you are changing to a tail dragger, don't glue in the new gear block with CA. Use epoxy and tri-stock in the corners.

I have a .46AX on my PT-40 and I can go straight up indefinitely with it. Not sure what more you would want from a .61.

I made mine with a bolt-on wing. I believe the spars on the PT-40 are spruce, they definitely feel denser and heavier than balsa. The shear webbing is horizontal grain, however... I wish I knew about vertical grain webs when I was building the thing.

James

Do eeeeeeeeeeeeeeeeeet. Do eeeeeeeeeeeeeeeeeeeeeeeeeeeet!!!

Well I am trying to turn a trainer in to a much more aerobatic plane. I want to use the 61 because I do not have a spare 46 to put into it.

I have a 46 FX sitting in another PT-40 it runs well, but there not unlimited vertical with it.

I have made a decision that I am going to make the mods and put the motor in the plane, I will be gentle with it and see what happens. Will let you know how it goes

the trainer i learned on was an arf had a supertiger 61 on it and i had to put lead in the nose to balance. it had a bolt on wing. if i were to try making a sport aerobat i would take out dihedral have a bolt on wing and split the difference between 40 and 60 and get a 50. then the difference in weight is going to be about the weight of you battery so you can mount it where you need the balance. clip the wings acouple of bays and increase control throws a bit. its amazing what can be done with a trainer if you are kit building it. you can get it to do just about anything.

Good luck and have fun, absolutely do it and have a blast. The SF's are great engines, one of the better O.S. has made. Just use good throttle management and have a blast. Don't come in on a downwind dive pass and pull up too hard. [)] Add a few more #64's, watch the prop clearance , balance her proper, and just have fun. No need for any long drawn out mods. I am always for taking what you have, and making it last even when you outgrow a plane, changing engines, etc, and fly her until she gives up, then buy a new plane for the next level of experience.
Peace Out. Reg

The whole point of using a .61 is because I have a spare one, and do not really want to buy a new motor. If the plane is way too over powered I will strip the motor out and try to do something else. If I overstress the plane and it goes it, I just hope I can find it. I am spending a ton of time right now making the wing very strong to handle the extra stress I think it will be ok - I will take is very easy to start - should be a lot of fun playing the role of test pilot

i think most of the problem with putting a 60 on a 40 sized airframe when it comes to a trainer isnt structural its balance. the supertiger 61 is 8 ounces heavier than the 45 for instance. i think you will find it will work just fine you just have to tinker with the balance a little. could also move servos into the tail etc for more weight to balance with. i frankly have only had one plane i used lead to balance with. i do everything i can to get balance with stuff that needs to be on the plane. trainers i think are overbuilt to begin with.

ps the OS 61 SF i think was only 19 ounces so you might not have balance problems after all with the battery balanced aft of the cg.

it won't be too much ...it won't need the regular wings to fly...
just the two that's mounted to on the engine will do
it'll hang on the prop, you can hover on it all day long
You don't need to go fast to hover.

Just back off of the throttle. Fly it around at 1/2 throttle... i dunno
or use lower pitch props.

well yeah after a while didn't your instructor told you to back off of the throttle on the second half of the loop
to make it look purdier on a .40 to begine with anyway.

Just don't snap it at full throttle or when in a decend.

I have really spent some time and effort making the new wing joint as strong as possible, it looks really good right now. I put three plywood braces in the joint, filled the gap with good, strong balsa and 30 min epoxy, then filled any small gaps with expoy and micro balloons, sanded it down and put a four inch wide strip of thick fiberglass cloth covered with 4 hour epoxy. The wing will hold for sure. That said, I will not be snapping the plane around at full throttle!

IT is fun doing the mods!

The problem of zooming is going to be exagerated, so, you might want to jack up the trailing edge of the wing about an 1/8". That will help kill the zooming tendency.

You hit it right on the head when you said you would be gentle with it. Apply full throttle only when going straight up, or in a high load situation, then back off for normal flight. Running a larger prop with slightly less pitch than normal might help. If you don't have the clearance for a larger prop, try running a 3-blade prop of the appropriate size.

Any airframe can be destroyed from over stressing. Give that wing joint a break and try not to stress it too much. Otherwise, you should have a lot of fun.


Ed Cregger

Well it took some time, but I finally got that big .61 under the hood of the plane. I am moving the radio and servos tonight, all that will be left to do is recoverving the wing. Don' t think I will get to fly tomorrow, but next weekend for sure!

Post pix.....

this is what modeling is all about.... Trying new things if you want to... and having fun doing the same ol' same ol' if THAT's what you choose!

We'd all like to see!!!!

I learned on a flat bottom airfoil trainer and I hated it. I couldn't wait until I flew something with a semi-symmetrical airfoil just so it would fly more realistically. If I were you, I'd give that PT-40 to a newby and get something made for a .60 that's meant for intermediates. My 2nd plane was a Goldberg Falcon 56 MkII and I learned more with it than any other plane I've owned. If you have a .60, try the Skylark ARF or something similar, you'll have a lot more fun than trying to bash that PT-40 into a more maneuverable plane.

I apperciate your comments, I have the plane to play with, I have the motor, the work is pretty much done. I have a .91 FS motor that I will be putting on a 4* .60 that I plan to buy in the next few weeks, right now this project is about having some fun with an old trainer.

My goal is to get confident enough on these planes to get my .40 Corsair up in the air, and my 1/4 scale Chipmunk flying.

Should be a fun season!

I've learned something from every plane I've had, and I'm sure you will get something out of this experiment.

The one piece of advice I disagree with here is raising the trailing edge. Just use elevator trimraising the trailing edge is no better, and could be worse, since it is equivalent to taking out some downthrust.

Just think about the wing, stab and engine and forget the fuselage and you will see that positive wing incidence is the same as downthrust, once you trim the elevator. And downthrust would be good on this plane, especially with the extra power.

Have fun, and let us know.

Jim

Agreed. I built a Sig Kadet LT-40 from a kit. Yes, this is a largish 40 trainer with 900 square inches of wing but I heard they tended to build a little tail heavy. Instead of a lightweight plain bearing 40 engine as shown in the instruction manual I opted to go with a ball bearing 46. With that setup, my LT-40 balanced perfectly with no added balast. I HATE adding dead weight to ANY aircraft.

Certainly not a speed demon, with the TT Pro 46 and a low pitched 11-5 prop, it jumps off the ground and can climb not quite vertically. I do NOT recommend overpowering a trainer when it is being used for basic flight instruction.

When I was on the buddy box with my LT-40, the instructor tended to fly mine around at full throttle. When I started getting more control of it I backed off the throttle about halfway and had a very enjoyable flight experience. At full throttle it climbs excessively, requiring quite a bit of down trim.

Now with over 100 successful flights, we've moved the CG back a tad and have bumped up the throws a bit as well. The TT 46 Pro still has plenty of "oomph" for basic aerobatics.

Now that I'm anxious for more advanced aerobatics, I'm progressing to a Sig 4*60 ARF powered by a Saito 100.

ive got a sig lt-40 with an unidentified .46 in it and its got unlimited vertical and its about double the size of your pt. If you get that 60, to balance out right its gonna be pretty nuts.

If I were you theres a few things I would do to that plane.

1. Reinforce the spar, my lt-40 has a full length fiperglass spar and the wing is glassed together as wel
2. put a bigger rudder on it. ailerons are your choice. I can hover/torque roll my sig but the rudder is a bit too small and and you really have to stay on it. needs more aileron too.
3. change the wing to bolt on after you have properly rienforced it.

If you are gonna make it a monster, go al the way.

Once upon a time, many years ago I had a PT40. I built it for my son to learn to fly. One thing I seem to remember is that the wing is rather thin and the wing joining arrangement is therefore not very wide. I eventually took out the original Magnum GP40 which was fine for training and put in a SuperTigre 45. In level flight it was quite a bit quicker and also in level flight the wing joiner failed. Your 61 is going to be stronger than my ST45 so I would be a little worried that you may manage to overstress the wing. If you are sure it is nice and strong in the middle then go ahead, but if not, then either strengthen the centre section or as suggested earlier, add some glass fibre reinforcement on the outside of the wing centre section. A bloke where I learned to fly had a Thunder Tiger Eagle 40 size trainer with a piped Rossi 60 in it. It was built with the Rossi in mind and was a truely awesome beast, especially when he flew over the strip inverted at about 1 foot. Kadet Seniors I have had have had engines from 40 four strokes up. My current one has a 72 Saito which is total overkill. It still flies the same and is not really any faster but its acceleration is impressive and I was able to remove all the weight from the front. I also have a Saito 100 in a Four Star 60 which is a bit over the top but rather nice to fly.

I am a pretty good builder, and have spent some time putting the wing joint together, I have three plywood joiners, then filled with some quality balsa which was put in with Epoxy, all gaps were then filled with an epoxy and microballoon mixture. I finished with a 5 inch wide fiberglass strip, covered with 4 hour epoxy resin. I cannot think of how I could make it much stronger.

From the sound of it, you could use your wing as an axe to cut down trees. I think you can safely go ahead with your power-up plans. All the best. And have fun. Mike

If you can, some pics would be great! Good luck, and remember to have fun!!!

Well she flies!

The plane is very powerful, a bit nose heavy, but once it was trimmed out, she flew great. Very different from the stock trainer!

I am going to extend the elevator and rudder to make it a bit more 3D capable.

Loads of fun to fly on a really windy day today!

manks