I have a Modeltech .25 size P-51. Running a .32sx. The plane flys beautiful at low to higher speeds. Soon as I open her up I have to hold opposite stick in the turns to keep it from rolling inverted toward the turn. I'm not sure where the cg is supposed to be. I bought this plane a long time ago ready to fly. But it had a wimpy motor in it at that time. I balanced it on the main spar with no fuel. I also installed retacts in it. It goes very, very fast, but it is not that much fun to fly. Any ideas?

Thanks,

Steve

Normally I'd say it sounds slightly spirally unstable but you say it only happens at higher speeds. That leads me to suspect the wing is flexing at higher speeds.

Try to see if it happens during climbing turns at full throttle but with the climb holding back the speed a little. Then try it at lower throttle but after coming out of a dive so the speed is high but the engine thrust low to eliminate any thrust input to the study.

I'm not familiar with the Modeltech P51. Is the wing D tube sheeted or just built up of ribs and spars? If it's D tube then I would be surprised if it's flexing but if it's open framed then this becomes a real possibility.

i belive that part of the problem may be weight placement vs. dihedral... hte top of the fuse may weigh too much causing the plane to narutally keep rolling because of gravity... try doing a fairly steep turn and let it stall.. if it flops on it's back, may be the weight

Thanks for the ideas. I will try to get out today for a few more test flights.

Steve

Steven - I tend to go with wing flex also. I have a mini QuickFly called a FleaFly + 10. Strange name but it is 10percent bigger than the FleaFli which was a Phil Kraft desing. It is a small airplane of conventional structure. It called for around a .19 to .25 in the original form (I think I am remembering this correctly). I am running an older model of )S .40 in it. It is a nice strong running motor but it is not as fast as the new OS 40 that I have.

Anyway the overall planform vs mass relationship is not that far removed from the P-51. Most airplanes that fly nicely will have the general layout the same. Scale them all to the same wingspan and it is interesting.

Anyway my FleaFly (never can remember if it's FleaFli or FleaFly) flies great. The big motor just makes it go faster. I would expect the same from the P-51 IF the wings are not flexing. A bigger motor on a strong enough platform just goes faster, it doesn't roll reverse.

rogwabbit - The mass considered at the CG and it's moment arm is small compared to the aerodynamic forces the airplane is experiencing at high speeds. A good example of low aero forces happens in a stall turn. When the airplane stops at the top of the straight line up, and a lot of rudder given to start the turn it will flop over and do a yaw wiggle until the speed builds up again.

The P-51 in a high speed turn pulling a lot of g's has a bank angle between 60 and 80 degrees. The moment due to weight could be around 25 inch pounds (5 pound airplane with a 5 inch moment arm from the wing location). That amount of moment is small compared to what the ailerons can produce. For an aileron moment arm of 24 inches both ailerons would have to be responsible for 1 pound of force or 1/2 pound per aileron. Pretty easy to do going at a high velocity.

Steven again - a limp and flexible wing will twist the opposite way of the input and causes a reversal in roll from what was commanded. It is a problem in full scale and model scale both. Most of our structures are so strongly built that we never have it happen. To stop it you just need to remove the covering and put in some diagonal wing ribs to give some more torsional rigidity.

Or, of course, sheet the leading edge top and bottom back to about 40% and cap strip the rest back to the trailing edge.

Ben, it's "FleaFli". Like you I had a +10 built from the kit. I "only" put a Veco 19 into mine and it was a stormer at that. I can't imagine what it would be like with the 40.....

Mine only lasted one day. It was flying fine but I pulled up across the face of the sun (no sunglasses either)for an Immelman and the loss of vision blanked out the model for a moment too long. When I saw it again the nose was pointed straight down from coming around the loop to far. I then did one of those.... "UP... NO... DOWN.... NO.... UP ! ! ! " thingies and it was all over...... The 4 or 5 flights I did have on it showed that it was a lovely flyer too. The kit came out way overweight but the thick section just didn't seem to care.

Thanks for the name check. It does really scoot but the handling characteristics are so honest that I don't seem to notice it even with less than optimal reflexes. On the ground I think, good grief that is a big motor. In the air I think, this is a nice airplane.

As far as the go-up-down-should I-wait thing goes, don't you hate it when it happens. I have almost trained myself to pull up and wait. You then just have to fly one loop radius high.

If I remember from all those years back as I flew across the sun and as the model was lost for that moment I actually realized what was going to happen and pushed some down onto the stick thinking that it would go into an inverted climb but I guess I did that too late and when I saw it again it had actually looped over and was in the process of tucking down from a 70 degree dive into the bunt I was inputing. That's when I realized that I should just finish the loop but THAT decision took just a millisecond or two too long and I would have been better to bunt it out after all....

So all I got was a ZIG... ZAG... CRUNCH.....Boy, did I feel stupid....

Yeah, I HATE it when that happens....

Keep yours safe Ben, she's a classic !

.... Meanwhile, back to your regularly scheduled thread... :rainfro:

I own a few examples the Model-Tech .25 Mustang ARC.

Very strong plane box stock. I had a 32SX in one with retracts , radared at nearly 100mph. The wing is not a problem on this model. I have another with a Nelson QM40 engine.

Your CG on the spar is right.

Are you hunting for altitude? Lots of down trim?

Any rudder trim?

The one common ailment this model seems to have with power and speed is known to be associated with the stab incidence. If I recall its rather high, should be around 0 degrees for best performance at speed.

I think that puts to bed the aileron reversal question. It seems to handle speeds nicely.

One thing that ocurred to me is the chance that the turns may not be coordinated adequately. If you bank over and don't pull enough up elevator the airplane will tend to fall in to the turn. Opposite aileron would be needed to keep the airplane level. Try pulling a little more up elevator.

Ok, here's a silly question : does it do it the same when turning in both directions? I've noticed a lot of guys tend to always turn one way when going fast so I had to ask.

One of my P-47s would roll into tight left turns and out of tight right turns. It turned out to be a rudder trim problem. You might also check your lateral balance.

If it's only at high speeds, how about your aileron hinge gaps? Are they sealed? If not, then that would be my #1 guess.

Thanks for all the replies. I still have not made it out for more testing.

mglavin: I did notice some down trim on the elevator, about an 1/16" deflection on the elevator. I will measure incidence asap Rudder trim seems o.k. although plane seems to be constantly weaving slightly on yaw axis. It doesn't seem to bother it much other than it does not look great.

Steve h