chaz5500

I have also posted this to the "Kit Building" forum.
I would like to ask owners of the PT40 if they had to make any changes to the CG. The instructions indicate that the CG is 4 7/64" (104mm) from the leading edge, however this results in a tail heavy plane that does not fly well. Please let me know if your CG had to be changed from the provided instuctions/plans.

Specifications:
Wing: Trainer "A-Wing"
Engine: OS .46AX
Monocote coving

I took out my PT40 trainer for it's maiden flight today and the results were not very good. My instructor is excellent and was able to save the plane but there were a few tense moments. The chief instructor of my club checked out the plane and made a few minor modifications - throttle and reduced aileron throws. He also thought the plane was tail heavy. He balanced it off the main spar and decided to add some weight to the nose. The instructor asked me several times if this was the correct CG location from the plans.
Before going to the field I checked every inch of the plane - clevises, linkage, hinges, langing gear, etc. This also included checking the balance of the plane both laterally and cener of gravity (CG). After the model was complete I added pin strip tape to the underside of the wing at 4 7/64" so I could check the CG. In order to get the plane to balance at this location I had to move the battery to mid-tail section (about half way between the tail and wing saddle). I used a home-made plywood base with dowels and pencil erasers to check the CG.

Anyway, we started the engine and got it out to the runway where the instructor ran in up and down the runway a few times to check for tracking and it looked OK. After taking off and gaining altitute the plane became very unstable. Even my instructor with 20+ years of experience had a hard time controlling the plane, but was ultimately able to save the plane by coming down low enough and it tumbled into the grassy field.

Any help would be appreciated in determining the correct CG for the PT40. I don't believe the plans are correct. I know the PT40 is a very popular kit and many people learn to fly on them. However, I also believe the instructions are incorrect with regard to CG location. Any help would be appreciated.

Bax

My Feedback: (11)

Hmmm...the PT-40 Mk. II has been around since 1996, and the original design since the 80's. We've not heard from anyone that there's a problem with the location of the model's balance point. Usually, flight problems are caused by inaccurately-built models, and too much control throw. Also, trying to fly the model at full throttle can be problematic. As a trainer, full-throttle would only be used for takeoff and climbs, but the throttle would normally be reduced after takeoff, just as you'd do in a full-size aircraft.

As far as your belief that the model is "tail heavy", you don't state what the problem actually is. If the belief is based upon a strong tendency to climb under full power, then nothing's really wrong...the model will normally tend to do that. You just come off the power a bit.

We'd really need more information about how your model handled, and upon what basis the judgment of "tail heavy" was made.

Thanks,

rogsteele

I also built the GP pt 40 and found it to be very tail heavy when balanced as per plans, I have a tower 46 upfront turning a 10-6 and on takeoff it wants to go straight up and stall a very hard plan to fly. I put 5 oz of lead over the fuel tank and it was better.
I have now rebalanced on the main spar (4'') replaced the weels with 3" took out some of the down thrust, added a little right thrust and it has become a very nice plane to fly.
but it took a whole lot of tinkering and tuning, it was sure not the perfect trainer balanced as per GP, my grandson is now learning to fly it.

rog[&o][&o][&o]

chaz5500

Rog, thanks for the feedback. A few other folks who have built the PT40 or PT60 have had similar experiences (see the main "Kit Building" forum). As a beginner, I find it a little frustrating to follow the kit instructions/plans and end up with a plane that does not balance correctly on the specified CG. All I can say is that I'm glad I had an experienced test pilot from our club maiden this plane. Without an experienced test pilot it would have been a disaster.

I did a lot of research before buying this kit and it got many positive reviews. I agree the kit has been around for a long time and many people have learned to fly on it, however, not without all the changes to CG, engine thrust, etc. Experienced flyers can do these things and determine how to tune the plane to fly it's best. However, beginners can't.

Is it too much to ask for correct instruction manual and plans? Image how many more kits GP could sell if it truly was a "perfect" trainer. I don't have any complains about the quality of the kit, but there are several aspects of the instructions that need correcting.

Bax

My Feedback: (11)

Hmmm....interesting. One thing nobody's mentioned is what they mean by "unstable". A more exact description of what the handling problem happened to be has not been given.

Nowhere does anyone mention the control throws they used. The instructions mention 1/4" up and down for the elevators. Ailerons slightly more. Too much throw makes the airplane very sensitive, and could be interpreted as "unstable" by some.

The O.S. .46 AX engine is a LOT of power for a PT-40, so full throttle will make the airplane move out and, again, seem "unstable" because it would cause the nose to pull up very steeply. You'd need to add MORE downthrust to compensate.

As designed, the model flies very, very well. Throttle does need to be moderated on a trainer. Most flying would be done at about 1/2 throttle or so. The model should trim out and fly easily and smoothly when the throttle is properly used.

chaz5500

Thank You for your suggestions. I will try them out.

cmjm

I have finished a pt40 kit for my 8 years old son about two weeks ago and i have no problem with CG.
In the manuals says to glue the servo tray in place aft or back to assured the cg; i put mine as back as possible near the landing gear block and the battery is between servo tray and fuel tank.
This is my third pt, one pt20 (1991) and two pt40 (2003), (2009), the 2003 pt40 is still flying with no crash.
Try to move the engine a litlle bit forward or place the battery down the tank, like the manuals says.