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Anti-Spin Ideas

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REVAN

Well-Known Member
Joined
Dec 6, 2016
Messages
668
Location
Tucson, Arizona USA
This thread is derived from a conversation that was going on the thread "Crashes In the News". The discussion had to do with the ways that aircraft design could be improved to reduce loss-of-control accidents, and specifically to improve the spin characteristic of an aircraft by improving/correcting the fundamental yaw instability of the aircraft's wings, a characteristic that is manifested in a wing's differential induced drag distribution which traditionally creates adverse yaw tendencies with aileron input.

Regarding the spin, I had said:
While pilots have become accustomed to this situation and consider it 'normal', I hope that with a bit more careful reflection on the situation, most of you can see that there is a functional problem with the aircraft when the control system is intended to bank left when the pilot pulls the stick left, but it sometimes works opposite, and similar reversals with the pitch control. For decades, the solution has been to have pilots be responsible for recognizing that they need to change the control law that has been reinforced through hundreds of hours of flying and automatically switch to something that is fundamentally different. When there is an emergency and panic sets in, the predominate muscle memory often wins over the known, but rarely or never practiced (because it can be dangerous) control law to recover the aircraft. When it doesn't work and people die, everyone puts the blame on the pilot and inadequate training.


Watch this video to see how a simple control reversal can take something easy we all can do, and turn it into a nearly impossible control problem.
https://youtu.be/MFzDaBzBlL0


Now imagine I made a flying machine that has been carefully crafted to challenge pilots by implementing a control reversal at specific times to correlate with emergency conditions just to see if the pilot can figure it out and do something they are aware must be done, but that is in conflict with their reflex actions and what is familiar. Why would I make such a machine? Maybe I'm an evil SOB, but I'm going to be nice enough to give you the heads up that I put this trap in the design... and by the way, if you fail the test, you will probably die. Would you volunteer to test your skills against my machine of mayhem?


My point of this is that instead of identifying poor pilot training as the main problem behind these unfortunate spin accidents, maybe it's time to recognize the very significant role that excessively challenging aircraft design has been playing all along. There's only so much a task loaded person can be expected to do, and compensating for difficult control problems in an emergency is, too often, more than than the pilot will actually accomplish with success, even though he/she may 'know' what they need to do.

Adverse yaw is one of the contributing factors that will drag a stalling wing into a spin. Adverse yaw is a symptom of yaw instability that requires a vertical tail and rudder action to tame. Fix the instability in the wing and a whole host of problems are alleviated, spin tendency among them. IMO, the guys designing tailless flying wing designs should not be the only ones concerned with making a wing that is yaw stable.

The common misconception is that spin avoidance is achieved through stall avoidance; no stall, no spin. When you get into the aerodynamics of the situation, it turns out that the spin has much more to do with the yaw instability of the airframe than it has to do with a stall. This is why appropriate rudder authority can be used to prevent a stalled airplane from spinning (or force it into one if applied inappropriately). IMO, a plane should be able to stall and maintain full control authority without risk of spinning. The stall is a very useful maneuver when not rendered unsafe by unstable design, and it's a great way to dissipate energy (far more effective than spoilers or speed brakes).

Lift distribution is how roll authority is created. When the lift distributions of the left and right wings are differentiated without regard to the drag profile of those lift distributions, it is likely the wing will exhibit undesirable yawing characteristics. This is what is commonly done. However, the computational tools exist to predict and integrate the lift distributions. Designers could be designing favorable characteristics into their wing designs, but instead the same old wing concepts are repeated over and over again because it is familiar and everyone has accepted that the result is normal.


The analytical tools are there and available for designers to use to create better functioning designs. I'd prefer to not lock anyone's thinking into a single possible solution. There are many ways to add yaw stability to a wing design. I referenced several earlier; the BSLD is one that has a fair amount of understanding behind it already. Wing-grid technology is another very promising architecture that has been around for about 20 years, but has been ignored by just about everyone. I like the concept of using a variable geometry outer wing section (i.e. - wing tip device) to modify the span loadings, as it would leave the entire main wing section available to host flaps for improved low speed flight performance.


Personally, I think the aileron, as it has been applied to contemporary aircraft, is a terrible concept. While it does what is desired for the induced lift profile of the wing to roll the aircraft, it does everything wrong for the induced drag profile of the wing. The solutions for replacing the ubiquitous aileron need to address both lift and drag profiles simultaneously, causing each to differentiate in a favorable manner to control the aircraft.

So, fix the adverse yaw, and you'll likely fix the spin problem as well. I need to say likely, because it is possible to correct adverse yaw without improving spin characteristics. The Ercoupe with interconnected rudders and ailerons is an example of this. Free the elevator travel enough to stall the Ercoupe and it could easily spin. Frise ailerons are another commonly applied method for reducing the 'apparent' adverse yaw of a wing, but that will do little for improving the spin tendency. The adverse yaw should be corrected by favorably differentiating the induced drag profiles of the left and right wings to coordinate yaw with roll.

A Boeing engineer once said to me, "Nothing is harder to fix than something that almost works. Management's position is that it almost works, so don't change anything... just fix it.". My reply was that it is even harder to fix something that works most of the time. That is the modern airplane. Most of the time it works as intended, but occasionally it spins and kills everyone onboard. I think it's time to try changing that situation.

Consider this thread a place to brainstorm and discuss methods to make better and safer flying airplanes through better aerodynamics. The goal is to break the mold, do aircraft control better than it has historically been done, even if it means making planes that look different to do it.
 
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