Hey guys,

I have a strange problem with my JKA P-51. Last time I wrote about it, people started chiming in with their experiences with their P-47's, but please keep in mind this is a P-51. Thanks

She balances at the recommended point extremely nose-heavy (more so than Jamie Farr).

BUT, she flies tail-heavy.

Weird. I checked the horizontal stab and wing incidences by eyeball, and both sit as they should. The wing is in the saddle properly, and the stab sits on the flattened area on the fuse tail. If anything, the wing and stab leading edges are slightly higher than the trailing edges (engineered that way I suppose)...so I don't think she should want to keep pointing her nose up.

I had to dial in a ridiculous amount of down elevator to get her to fly right...but now with all that wind resistance in the tail, she does not fly very fast.

Any suggestions?

Thanks!

-Dan

How does the thrust line of the engine match the wing/stab? And, what does she weigh?

JESTER

By the way, here she is. Yup, I put her up on a pedestal

Jester,

I don't know what she weighs, and I assume the thrust line, since I used the provided template, is correct. By eyeball, it does not appear to have any upthrust that would cause such a strange way of flight. I don't know how to accurately check it, though I am a bit naive...

-Dan

Just looking at that pic, it looks like it might have some up thrust. (Look at the line of the muffler in relation to the wing saddle.)

Does it still fly tail-heavy when the power is off (gliding)?

Thoroughly check the vertical and horizontal thrust lines and correct until it is either at 0/0 or a very little bit of down/right.

For what its worth, my JKA P-51D flew rock solid from the start with the CG 1/4" forward of the recommended location. So, your's should too.

Also, check the angle of your ailerons at neutral. The bottom surface of your ailerons should follow the line of the bottom of the wing's airfoil.

HTH,
D

Hey Darin,

I have no real way of checking the thrust line, but you did ask the right question. During gliding, she still flies tail heavy, as I have to apply ample down elevator to get her to fly level when the power is off.

I put the plane on my Great Planes airplane balancer, upside down, with the balance point 1/4 " forward of the recommended location (as you told me to do in a previous thread), but it still flops over nose first.

I am going to move my battery pack from the rear of the fuse to the cockpit just over the CG, and see if that makes a difference. If it does, then it will become abundantly obvious that I don't know how to balance planes.

Thanks.

-Dan

Dan,

It sounds like you have it balanced good enough, if it were tail heavy then it would be uncontrolable. I have to agree with Demon, check your ailerons. It sounds like the may be acting like flaps. If your P-51 climbs with the engine off then it would not be the thrust line. Just a thought, but worth a try.

Bob

What do you mean when you say it "flys tail heavy" ?

To me, "flying tail heavy" means it snaps violently with just a little elevator deflection, is extremely abrubt or twitchy with elevator input, and requires almost no down elevator when flying inverted.

Pitch changes with throttle or airspeed aren't necessarily due to being out of balance.

Bob and Darin,

Ooops, forgot to mention it. I placed a ruler on the bottom of the wing (had the ruler going chord-wise), and set my ailerons at neutral (parallel to the bottom of the wing nearest the aileron since the wing is semi-symetrical).

Of course, I could have screwed that up too

Bob,

My experience has been that a plane that is tail-heavy is still controllable (though a pain to fly), as long as you compensate with plenty of down-elevator.

Thanks for both of your suggestions. I will let you know what happens after I move the battery pack forward.

-Dan

Montague:

"is extremely abrubt or twitchy with elevator input, and requires almost no down elevator when flying inverted"

describes my P-51 perfectly. She does not snap violently, though, unless I give full up elevator.

-Dan

Dan,
Try this: Set the prop up straight up & down. Measure the distance from both prop tips to the stab LE at the root (blue lines):


On your P-51, those two measurements should be very close to the same if the engine has about 1 degree of downthrust. If the bottom measurement is significantly more than the top measurement (>3/16"), the engine deffinantly has up thrust.

Be sure to turn the prop 180 degrees and re-measure to cancel out any variation in the prop tips.

Check the left/right thrust in much the same way with the prop horizontal and use the TE of the rudder for the reference point.

This is a very rough method of checking thrustlines, but should at least get it in the ballpark.

HTH,
D

I had a JKA P-51B with a norvel .15 and it was awful to fly...until I realized that I had left thrust in my motor and the ailerons were set a little down (like flaps). I would hand launch and it would violently snap left almost immediatly. I added 5 degrees right thrust and a little more down thrust, also added some reflex (raised the ailerons 3 degrees in an up flap position) and it flew great!!! I now have a JKA Mig 3 with .25 and did the same thing to it. It flies hands off/on or upside down with no bad habits at all. I also used micro radio gear to make the wing loading real light so slow speed is also predictable with no snap happy tendencies.

I have two whole pages dedicated to warbird related material including AVI videos of launching and landing these little planes at www.nearc.rcclubs.com