... how does one recover from a flat spin.

I have been able to get almost every plane on my Dave Brown simulator library into a flat spin but I can't get any of them out. Is this peculiar to the sim or is it me?

From : Don Stackhouse

The answer for that simulator could be peculiar to that simulator. However, there are some fairly straightforward answers with regard to both models and full-scale.

Some airplanes, typically due to C/G, mass distribution and tail design problems, might be able to enter a flat spin spontaneously, and then not be able to recover. Certain full-scale light twins, particularly ones with a lot of mass (fuel tanks, etc., out near or at the wingtips), are notorious for this. "Deep stall" characteristics of some airplanes, certain T-tailed civilian jets being one example, where the tail gets caught in the wing's wash, locking the plane into the stall, can also have similar results. However, the true flat spin can be deliberately entered and recovered from by many aircraft, and is fairly common in quite a few airshow routines, both with models and full-scale.

The best discussion of intentional flat spins I've seen is in Neil William's book "Aerobatics", Airlife Publications, copyright 1975, ISBN 0 9504543 0 3 This is by far the best book I've ever seen on the subject, and has a whole chapter on advanced spins including flat spins. It also has an entire chapter on the various kinds of Lomcevaks.

In a spin, the airplane is flying in a corkscrew flight path. As such, there is some rotation about the roll axis, but also about the yaw axis and the pitch axis as well. The total amount of rotating that happens is relatively constant, but you can trade off some of the rotation about one axis to get more rotation about one of the others.

To get into a flat spin, typically you first enter a conventional spin using an intentional power-off stall, with a big boot full of rudder right at the stall break. The sudden yaw from the rudder gets the rotation started, and also stalls the aft-yawed wing while keeping the forward-yawed wing unstalled. One wing is still making lift, the other is not, and the two start chasing each other around like two dogs chasing each others' tails. Meanwhile, neither of them is paying any attention to their fundamental job of holding up the airplane, so gravity takes over. The result is the familiar vertical corkscrewing downward flight path, with the helical airflow past the wings keeping the angle of attack above the stall angle on one side, and below the stall angle on the other.

The spin is one of the few "auto-stable" maneuvers an airplane can do; i.e.: a maneuver that requires no changes in control inputs from the pilot to maintain after it's been initiated. Some of the old barnstormers and mail pilots would use it for this reason if they got caught on top of clouds without any gyro instruments. Trying to manually fly through clouds without proper instruments is very literally suicidal; the pilot will inevitably end up in a "graveyard spiral" (not the same thing as a spin at all, the plane is not stalled), and most likely pull the wings off from too much "G" or from excessive airspeed. Instead, if the unlucky pilot was confident that there was enough clear air between the bottom of the clouds and the ground to recover, they could simply pull the airplane into a spin, hold the stick all the way back and hold full rudder, and wait. The plane would spin through the clouds all by itself, and when the ground was visible again, simply center the elevator and apply opposite rudder to stop the rotation, then smoothly pull out of the resulting dive.

OK, so now we're in a stable conventional upright (i.e.: positive "G") spin. What about turning that into a flat spin? Here's where we get into trading off the different kinds of kinetic energy. We want to take all of the energy about the pitch axis and the roll axis and force it to concentrate about the yaw axis.

Oops, one little detail I left out. It works best if you are spinning in the correct direction. With a conventional right-handed prop, that means an upright spin to the left. Now, open the throttle, and apply aileron AWAY from the spin (in this case that would be right aileron). The gyroscopic precession from the mass of the spinning prop will try to raise the nose, and the opposite aileron will resist the rolling energy component, and the energy taken from the pitch and roll axes will show up as faster yawing.

At this point the plane is really spinning fast, but with a rate of descent that is actually less than in a conventional spin. The nose is still maybe ten degrees or so below the horizon, and the world is starting to become a rotating blur in the eyes of the pilot.

There is one more thing to do to get the spin even flatter and faster. While still holding high power, full in-spin rudder and out-spin aileron, APPLY FULL DOWN ELEVATOR! Yup, you heard me, down elevator. At this point the plane is spinning so fast that even with full in-spin rudder, the vertical tail is actually impeding the rotation. By applying full down elevator, we cause it to blank the vertical tail, and the rotation rate can go even faster. Since the wings are already pretty much level (no roll energy left), and that extra yaw energy from the faster rotation has to come from somewhere, it gets pulled from the pitch axis. This causes the nose to rise even higher, so that in some cases the plane's nose may actually be a little above the horizon.

To recover, we reverse the process. First, get back into a conventional spin by pulling full "up" elevator. This unblanks the vertical tail and slows the yaw, which then causes the nose to drop a little. Now, cut the power and center the ailerons (if the plane has a big enough prop that it can blank the tail if it is windmilling at idle, you might want to leave just a tiny bit of power on to keep the air flowing back there). This drops the nose further and also puts more of the motion into roll instead of yaw. At this point you are back in a normal spin. If you apply opposite rudder, it stops the rotation about the yaw axis, and down elevator (as the rotation stops) gets the wing's angle of attack back below stall. Your wing is now flying again, and smooth back pressure (not enough to re-stall the wing or to pull enough "G" to break something!) will pull you out of the dive.

It's all in the timing. The effects of some of the controls during one phase of the spin are not the same as they are in other phases. The key thing to note in all of this is that adding down elevator while still holding in-spin rudder has the opposite effect of what that same down elevator does if you first stop the rotation with out-spin rudder.

Of course whether the folks who wrote the simulator understood all this and correctly incorporated it into the program is anyone's guess. Best bet is to just try it and see.

Don Stackhouse
DJ Aerotech