Spiral Stability and Vertical C.G.

[Don Cooper]

Can anybody expound on the effect on spiral stability of changing nothing but vertical C.G. location?

[Dave Robelen]

Hi Don,
Boy, that's a tough one. As you probably know the fin and dihedral are the major players. I would expect the spiral stability to increase with lowering the CG because that will have the effect of increasing the effective dihedral. How much that would be offset by the inertial effects in what is a dynamic situation muddys the water for a clear answer without some tests with a simple balsa glider. Actually, that would shed more light on the issue than a bunch of discussion.
Regards, Dave

[Don Cooper]

Thanks for the reply Dave.
The question came about because I had (prior to a death spiral) a rudder/elevator slow flyer with a low C.G. (light airframe, battery on bottom of fuselage) and high vertical tail CLA. Its turns were erratic, sometimes starting quickly, sometimes not, and spiral stability was terrible. After it crashed, it occurred to me that with the low C.G.-high CLA, the rudder force would cause a rolling moment opposite to that of the dihedral. Consequently, when starting a turn, the rolling moments would battle each other and when applying opposite rudder to come out of a turn the tail rolling moment would tend to try to steepen the bank. I was wondering if anybody had had similar experiences.

I am in the process of designing a slowflyer with low tail CLA and variable dihedral to do some experimenting.

[Dave Robelen]

Hello Don,
Now that we are getting down to cases, I have another data point. I built a low cabin 280 class park flyer that has ample dihedral and a rathr tall vertical with all of the area above the cg. It flew fine but would not roll with the rudder even though the power and speed were adequate. I chopped the tail off and redid it with a rudder that has about 60% of the area below the cg. That did the trick. The flight is smooth and the rudder rolls are nearly axial. Good luck on your project.
Regards, Dave

[Don Cooper]

Dave,

Thanks for the input. It appears to add credence to the thought that the location of the vertical tail CLA relative to the C.G. can have a substantial effect on flight performance of slow/park flyers.

My son ( a plane and car R/C addict for years) keeps telling me to get a GWS or Wattage ARF and quit fooling with the problems. I keep telling him that solving the problems is at least half the fun of R/C airplanes. I haven't convinced him yet.

Don

[Richard Miller]

Don,

This is something that has occupied me for many years. I always intended to write a serious-type paper it about it, them, but so far haven't gotten around to it.

There are two issues, really: The size of the vertical surface and its centroid. Way back in the 30s it was customary to stand half a stabilizer on its end to seerve as a rudder, or fin. Ah, what havoc that wrought!

Sense came by degrees. My own experience came with the reduction of the fin size of a Thermic 50 to about 30% of the size Frank [Zaic] made it on the plan. Before that the model did 3-1/2 turns, count 'em, off the tow every time. Afterwards it was as docile as ever you please.

I got that size by cutting everything off, gluing on some scrap balsa, trimming that until there was a hint of dutch roll, then going back one cut.

Rule: The vertical should have *just* enough authority to keep the airplane shy of dutch rolling at it lowest airspeed.

The other matter is more complex. It is the effect of prop wash on the fin, and is there ever a lot to say on this! It was a favority topic of Bill Winter, and you can find some wonderful insights in his old columns in Model Aviation. Maybe 10 years ago, maybe more.

The centroid of the vertical surface usual ends up above the longitudinal axis. It's hard to put it anywhere else, things being as they are. This induces - did somebody already say this? - adverse roll.

It was long experience with hlgs that taught me so much, like the time I glued the fin to the bottom of the fuselage, just to see what would happen. Roll and yaw combined. Eureka!

Late, tired.

-Richard