I'm in the process of converting a Sig Seniorita to electric and I think I would prefer to also change it to a taildragger. Does anyone think I will need to relocate the main gear in order to maintain proper attitude and balance?

From : Don Stackhouse

There are a lot of rules of thumb on this, but there's one that seems to work best for me. It's the same one used for full-scale taildraggers, and does a more thorough job of taking all pertinent factors into account than some of the other rules of thumb.

First find the C/G of the aircraft, both horizontally AND VERTICALLY. For a high-wing model, it will tend to be a little higher, and for a low wing model it will tend to be a little lower (no great surprise there!). Plot this on a side view of the aircraft. Now, with the aircraft in the drawing level, draw a line downward from the C/G angled 10 to 15 degrees forward from vertical. Use 15 degrees if you want more nose-over resistance, 10 degrees if you want less ground-loop and bounce-on-touchdown tendencies. Draw another line aft from the bottom edge of the prop disk, angled down at the angle you want to be able to nose over to before hitting the prop. Where the two lines cross is where your wheels should contact the ground.

If the model has a very long tail moment arm you can get away with a more forward landing gear location. The higher damping in pitch and yaw from the longer tail moment will help keep the bouncing and ground looping under control. A little bit of toe-in can also help the ground-loop tendencies.

The J-3 Cub has a wheel location somewhat more forward than this, and the associated resistance to nose-over but an affinity for bouncing on touchdown. This is one of the things that make the full scale J-3 such a great trainer; it won't let you break anything, but give it anything less than perfect technique and it will embarrass the daylights out of you in front of any and all spectators. Very easy to land, but very difficult to land WELL. Trainers SHOULD NOT be totally viceless. They should clearly (but benignly) show a student's mistakes, but not injure anyone in the process.

One of the best behaved taildraggers I ever flew was a Peck Polymers "Prairie Bird". It had a wheel location much further forward than this method recommends, which normally would cause horrible ground looping and bouncing problems. However, it also had an unusually long tail moment and oversize tail surfaces (like the rubber free-flight design it was scaled up from), which kept those problems well under control.

The full-scale Supermarine Spitfire has a similar setup, particularly the unusually long tail moment. Despite what you might expect from its tall, narrow landing gear, it is known among full-scale warbird pilots as having some of the most benign behavior on the ground of any airplane in its class. The DeHavilland Chipmunk also uses this arrangement, but with a wider main gear stance for better lateral stability on the ground. I haven't had a chance to fly a full-scale Spitfire, but I have flown a stock British Chipmunk, and it ranks as my all-time favorite of all the full-scale types I've flown. Other than the "Johnson-bar" brake system, it's a shear delight to fly, both on the ground and in the air. We've also prototyped a 49" 1/2A EPP Chimunk, which despite having a non-steerable tailskid, has outstanding ground and flight handling as well.

BTW, for a good discussion of this and other full-scale warbird handling properties, I recommend "Illustrated Classic Warbirds Buyer's Guide" by the late Jeff Ethell, published by Motorbooks Intl., ISBN #0-87938-544-8. A great book by a great pilot and writer, he is sorely missed by lovers of classic aircraft.

Don Stackhouse
DJ Aerotech