Spins & Stuff

[jrenwick]

I'll bet the idea that spinning doesn't overstress an airplane comes simply from the fact that the wings are both stalled -- and as such, it's not possible to impose positive G-loads above the design load limit. This is true as far as it goes -- but it doesn't address other considerations of wear and tear like those George is talking about.

[GAHorn]

Uhmmmn.... thanks John. I suspect you didn't actually mean that both wings are stalled in a spin. (One is stalled, the other isn't, which is what imparts the rotation, as you know, of course.)

Spinning an airplane actually imposes a twisting-moment upon the door-posts, and it also places a differential twisting load on the wing in a fore-aft plane. On single strut aircraft (like A/B models) it causes the stalled wing to unload, then abruptly re-load in torsion, as the strut re-accepts the load of the aircraft's weight. In other words, it tries to twist the wing trailing edge off the rear doorpost attach. This is much more violent than the ordinary creation of lift during takeoff. It's more like re-bending-a-paper-clip again.... except you are in-flight when you do this to it.

[jrenwick]

Uhmmmn.... thanks John. I suspect you didn't actually mean that both wings are stalled in a spin. (One is stalled, the other isn't, which is what imparts the rotation, as you know, of course.)

Better get ready for incoming, George! (I'm teaching a ground school right now, and I'm up to speed on the FAA test questions for Private Pilot....)

[jrenwick]

I should clarify. I believe I was taught, 35 years ago or so, that only one wing is stalled in a spin. FAA doctrine now is that both wings are stalled, and there's a PPASEL exam question covering that.

The FAA text "Airplane Flying Handbook" (http://www.faa.gov/library/manuals/airc ... _handbook/) page 4-12 says: "As the airplane rotates around a vertical axis, the rising wing is less stalled than the descending wing creating a rolling, yawing, and pitching motion."

John

[GAHorn]

Perhaps we are dealing in semantics.

Page 309, para 2, of Aerodynamics for Naval Aviators: "The rolling velocity (due to yaw) tends to increase the angle of attack for the downgoing right wing (in a RH spin) and decrease the angle of attack for the upgoing left wing. At airplane angles of attack below the stall, this relationshop produces roll due to yaw..." In para 3, " If this airplane is provided a rolling displacement when at some angle of attack above the stall, the upgoing wing experiences a decrease in angle of attack with a corresponding increase in CL (lift). In other words, the upgoing wing becomes less stalled."

Para 6: "The fundamental requiirement of the spin is that the airplane be placed at an excessive angle of attack to produce the autorotation rolling and yawing tendencies. Generally speaking, the conventional airplane must be stalled before a spin can take place."

I agree with your comments, about the "old style" and the "modern" principle of spin instruction. The FAA can change the semantics all they want, the spinning airplane has one wing producing lift and therefore rotation regardless of their choice of words.

[voorheesh]

This is not "incomming" for George and I agree it is not the best thing to be subjecting our old planes to unnecessary stress, BUT spin instruction is important. As a CFI, I would not solo a student without first demonstrating a spin entry (1/2 turn works fine and puts minimal stress on airframe). The important lesson is to demonstrate that the rudder stops the rotation. If you don't demonstrate this, the student will try and use ailerons to raise the low wing which just makes things worse. I believe that the dive during recovery puts more stress on the airframe and I have learned to control that to a speed below Va. I believe it is good practice for even experienced pilots to do stalls, slow flight and a spin entry once or twice a year for proficiency. My spin entry is as follows: start with wings level, power off stall and apply full rudder in direction desired. The wing on the side of the applied rudder will drop, as will the nose. Maintain neutral aileron (very important). Apply immediate opposite rudder to stop the rotation and very slightly release back elevator pressure. The airplane will dive because the nose is low but gentle back pressure can stop this before too much airspeed. I have never felt any adverse stress on the airframe. Performing this practice builds confidence and could save a life if you ever experience an inadvertant stall. At my glider club, we practice this maneuver in an old Blanik L13 because inadvertant stalls are common when thermalling. It is amazing how even experienced pilots will crank in aileron when the wing drops. Practice cures that bad habit. Only do this in a plane/glider certified for spins and in the correct configuration.

[canav8]

This is not "incomming" for George and I agree it is not the best thing to be subjecting our old planes to unnecessary stress, BUT spin instruction is important. As a CFI, I would not solo a student without first demonstrating a spin entry (1/2 turn works fine and puts minimal stress on airframe). The important lesson is to demonstrate that the rudder stops the rotation. If you don't demonstrate this, the student will try and use ailerons to raise the low wing which just makes things worse. I believe that the dive during recovery puts more stress on the airframe and I have learned to control that to a speed below Va. I believe it is good practice for even experienced pilots to do stalls, slow flight and a spin entry once or twice a year for proficiency. My spin entry is as follows: start with wings level, power off stall and apply full rudder in direction desired. The wing on the side of the applied rudder will drop, as will the nose. Maintain neutral aileron (very important). Apply immediate opposite rudder to stop the rotation and very slightly release back elevator pressure. The airplane will dive because the nose is low but gentle back pressure can stop this before too much airspeed. I have never felt any adverse stress on the airframe. Performing this practice builds confidence and could save a life if you ever experience an inadvertant stall. At my glider club, we practice this maneuver in an old Blanik L13 because inadvertant stalls are common when thermalling. It is amazing how even experienced pilots will crank in aileron when the wing drops. Practice cures that bad habit. Only do this in a plane/glider certified for spins and in the correct configuration.

Voorheesh, I applaud you for your student requirements being above the minimums required. It is sound judgement. I to have followed this line of training for years. The only difference was I could not make it manditory but generally that was never questioned as a student who trusts their instructor generally will recognize the value of the training recieved. When I do flight reviews we always do stall series, with emphasis on the cross control stall. THe statistics do not lie. I have complained to the FAA about their symantics regarding spin description. They are already revising their last publication and it will not be in the next revision. I am shooting for the following revision. Just like anything with the FAA, it is subject to interpretation. I keep old copies of the Handbook around for that very reason. V/R Doug