Hi,
I am having trouble with my aerobatic aircraft which is a SuperAir and is superb - better now with the gear repositioned on the fuz. It does everthing I can do and probabaly more. It has (for that read I have) one failing. Trying to do a 3 trn spin and pull out in the right place is proving almost impossible. I count two turns then neutalize and it can pull out correct or overshoot by a good turn.
One of the difficult things in judgint it, I beleive, is the speed in which it rotates. It certainly is fast. What are the factors in determining the rotational speed of a spin. I might add that an inverted spin seems to be more controllable on the exit - ie it pulls out where I expect.
I look forward to hearing any ideas and explanations on this subject as this is the only failing I can find on this model.

Hello Nigel,

I would recommend that you acquire a book called, "Stalls, Spins, and Safety", written by a man named Sammy Mason. The book is out of print but you can easily acquire a copy through Amazon.com. It is the best treatise that I have ever seen on both airplane design considerations and piloting technique related to stalls and spins. Mr. Mason was a renouned exhbition pilot and later Lockheed test pilot, specializing in spin testing. The book is written so that the layman can understand it. I believe I paid about $15 for a very good condition copy.

Dick Fischer

At full scale, one of the biggest factors determining spin rate is longitudinal stick position.

Spins start from a stall. How deeply the airplane is stalled has an effect. How effective each surface is to cause that stall has an effect. How effective each surface is once in the stall and spin has an effect.

You say the inverted spin stops as expected. That would suggest the neutralized surfaces are not blanked by the airplane's attitude in that spin. And that they are in the other. You can't do much about the airflows since they're a result of the airplane's shape.

It could also be a problem with asymmetry, either in balance or shape. You probably don't want to try and reshape the airplane, so look a bit at the balance. Any chance the CG's vertical location is having an effect? It does in a highwing Cub for example. It's quite a bit below the wing and increases roll stability considerabley when the airplane is upright and decreases it when the airplane is inverted. That might be a cause in your situation. At least it's something that can be explored.

From what I have read on the subject, there are some planes [there were]that if you spin to many times it does not want to come out of the spin. You may have a design or set up that has the tendency to do that. I am sure that CG will effect it to. If my memory serves me right,when I took flying lessons many years ago,they warned about doing more that 2 turns in a spin[Cessna 150]. I think that one reason is you can lose a lot of altitude in short order ,but I think the other reason is what I said above.

Some interesting ideas coming. I have tried keeping in the spin for about 6 turns and trying both directions. Neither time did it exhibit any reluctance to come out of the spin. It just takes a good turn to come out. But the problem is that its not even always a full turn. The best I got was a quarter of a turn. I thought I had cracked it - until the next when it took a complete revolution. I have not changed the CG to check that as an effect because it would effect the trimming.
Using different amounts of elevator and aileron have all been tried as also exiting the spin. The normal model exit type where you just release the controls - usualy gives a consistent number of turns before exit - and also giving opposite rudder but that gives a rather erratic turn count before exit.
I am coming to the horrid conclusion it is the aircraft design, in which case I cant do much about it. That is rather a defeatist view though and I am still striving to find an answer that is both simple, explanatory and that gives a result of a clean spin exit in the correct direction.

Another point to consider is that in order to begin or complete a spin the plane must overcome it's rotational inertia.

Center of Gravity (CG) is all the airplane's mass theoretically compressed to fit inside a point and located at that location to substitute for the mass of the entire airplane and this is great for describing behavior when flying without rotation.

However, when the plane begins a spin, mass and CG aren't enough info. You also need to consider rotational inertia (I) which is a theoretical ring of mass (like a giant wedding band) equal to the entire airplane mass with a radius of gyration (k) about the rotational axis which could be substituted for the airplane. This inertia is different for each axis. The equation is: (I = k squared times m) where m represents mass of the plane so (k) is most important. To be clear, (k) would be the radius of that wedding band.

A typical spin is a combination of a barrel roll and a flat spin like sliding on the airplanes belly sideways inside a giant funnel. Airplanes are well designed from a rotational inertia standpoint to begin and end an axial roll. Most of the mass (engine, batteries, fuel) is located right on the axis of rotation (k=0) and wing panels don't weigh very much. Combine this with the highly effective ailerons way out on the tips of those wing panels and you have a very effective system. Some Ultimates can roll so fast they blur! The only difference between an axial and barrel roll is Normal Acceleration which I described in a recent thread so I won't go into it here.

The flat spin is a different case altogether. Much of the weight (engine/fuel) is a significant distance from the rotation axis resulting in a larger radius of gyration (k = 4"or so) which in turn, requires more force to begin and end the spin. Now consider the control surfaces. Only rudder is well equipped to apply force in the direction of rotation. (For those eager to correct me, yes I know its a moment and not a force but it just complicates things.)

How can you improve flat spin performance? Reduce rotational inertia and improve rudder effectiveness is a good start. Try locating the fuel tank directly over the CG. In addition to reducing (k) it will eliminate trim variation with fuel consumption. A shorter fuselage will locate the engine mass closer to the CG while a larger rudder with more deflection angle will move more air and compensate for the shorter tail moment resulting from the shorter fuselage. Also, any general weight reduction will improve all performance characteristics including flat spins but you'll want to start with the heaviest mass furthest from the rotational axis.

Realize that no matter how much you reduce rotational inertia there will always be lag because some mass is present. You have to anticipate and learn to compensate for this. Practice making the spin with the same speed and attitude for consistency. If it takes a half rotation to recover after pilot input then start the input that much earlier. Because rudder surface area is never symmetrical to the longitudinal plane of the CG you will have to compensate for roll while performing the maneuver. Adding a sub-fin and lowering the rudder will help reduce this.

A short version of the stuff written above: Move the fuel tank to the CG if possible. Also, you're probably tail heavy. One symptom of CG too far rearward is you need opposite rudder to help stop the spin. Try moving the CG forward about a half inch at a time. It will probably help.

No books needed
The speed of rotation and the angle relative to direction of spin - control the ease of exit
I can spin from as high as I like and still stop a spin in 1/2 a rotation
HOW?
practice aileron and elevator and rudder settings -change em one at a time till yo u see how it affects speed of rotation and angle of fuselage during rotation.
General rule - the flatter the angle - the slower the rotation
has to do with conservation of angular momentum - -like an ice skater hugging body or extending arms .
The steeper the angle of the fuselage - the more easily the model will assume flying speed when commaded to stop the rotation.
The flatter the angle of the fuselage - the slower the rotation - so set throws till yo u get the happy combo you want
The cg will affect BOTH of these conditions
An extremely light model will stop fastest - because it resumes flying attitude at a lower speed.
My really light stuff can be controlled to 1.2 turn with ease- just relaxing controls
Inverted spins have the advantage of the fin n rudder being a "sub fin"- relative to direction .
easier to stop the spin.
Cessnas etc., I have no experience with doing spins - not my interest ..