Why do planes spin?

Just a small point to counter something in the first post.

A helicopter can autorotate straight down with no problems…or backwards, sideways or forwards or any combination of the above. I used to enter an autorotation from a hover at 2000′ AGL over the intended landing spot, then move backwards until having the correct approach picture and then start to get forward speed for the flare and landing.
You do want that forward speed as you try to reduce the rate of descent before setting down.

Ken

Yes, that was interesting… but it did not clarify well where the yawing couple comes from.

It was explained with the drag differential. Well, the inside wing might have higher coefficient of drag because of higher angle of attack – but then again, it surely has much slower or indeed negative airspeed….

The inside wing is more stalled than the outside wing… so the outside wing actually produces a bit more lift… partly due to the faster rotation than the inside wing… that extra lift creates a roll moment that is translated into yaw… the spin becomes stablized when the aerodynamic forces and the mass moment forces come into equilibrium… that is the developed spin.

On some aircraft the yaw force must be maintained to remain in a stable spin. Bottom line is that the yaw due to roll and roll due to yaw are coupled with mass dynamics (e.g. the engines, location of fuel, other mass points).

There are other topics that can illustrate some of these concepts… check out my website… the links page has some video of normal and accelerated spins… it is a work in progress.

Whereas the “normal” tailed planes ensure that if the main wing stalls, the tail is in clear airflow beneath the wing wash… and turns the airplane to normal, unstalled AoA.

Transport aircraft have to meet certain criteria under JAR25 & JAR23 in the stall to get certification, one of these is a pitch down moment at the stall, this is caused by the rearward movement of the CP (Centre of Pressure) not by clear airflow over the tail, swept wing aircraft have design features build in to induce root stall before tip stall, the problem lies with the fact that if the stall is left to develop further then the CP will still move forward after it has moved back causing a pitch up moment, the problem with pitch up moments is that longer fuselage aircraft are hard to recover because the bottom of the fuselage is presented to the relative airflow further increasing the pitch up moment, then vast elevator authority is needed to correct this problem, this is also known as deep stall.

Hope this helps

Dean

The TU154 is not a training aircraft, WD means your average Piper or Cessna (I assume),

Ah, I see. What WD stated above was that only aerobatic aircraft were capable of stable spin…

when you start talking about T tailed & swept wing aircraft you are in a different ball game, T tails are prone to “Deep” stall, which is virtually irrecoverable, the tail is blanketed by the wing wash which destroys the laminar flow making control authority near impossible,

Indeed, they are notorious for this. Like the BAC 1-11 prototype, or the recent Caribbean MD-80 crash where the pilots stalled the plane.

Whereas the “normal” tailed planes ensure that if the main wing stalls, the tail is in clear airflow beneath the wing wash… and turns the airplane to normal, unstalled AoA.

If you don’t have it already and you want a greater understanding of the spin, you might want to purchase this:

Anatomy Of A Spin

Well worth it in my opinion.

Cheers

Paul

chorn I think you are getting confusesd, are you sure you completely understand stability? that being, positive static & dynamic stability, negative static & dynamic stability & neutral static & dynamic stability? and all three in the 3 axis of aircraft movement, Lateral, Longitudinal & Directional?

Then how does a plane come down from FL390 in 3 and half minutes or so? This is roughly 200 km/h… If spin weren´ t stable, you would expect some sort of recovery over 3 and a half minutes…

I’m not sure how you could expect a recovery from a spin if your example aircraft was unstable as you stated, I think an aircraft in a spin is generally holding neutral dynamic stability, (which is an unstable state?) and that basically means the spin is not getting any worse or better (hence the neutral) and it stays in this state until an input from the pilot, be it a rudder deflection in the opposite direction to the yaw which corrects it and straightens the aircraft

What about Tu-154? Attempted to climb over a thunderstorm near or above ceiling, then something like an attempt to turn or a gust provided the upset… It did not right itself in a couple of turns (and inside the thundercloud they were attempting to avoid to start with) – it continued all the way to the ground. Or so the explanation of the Pulkovo Airlines crash goes.

The TU154 is not a training aircraft, WD means your average Piper or Cessna (I assume), when you start talking about T tailed & swept wing aircraft you are in a different ball game, T tails are prone to “Deep” stall, which is virtually irrecoverable, the tail is blanketed by the wing wash which destroys the laminar flow making control authority near impossible, and swept winged aircraft are prone to tip stall, where the tips stall first, this moves the wing CP forward rapidly which gives a pitch up moment making the stall far worse

As for a stabilized spin. Other than aerobatic aircraft I don’t think it’s possible to have a stabilized spin in an airplane unless the CG is out of limits. Airplanes are too stable to permit a stable spin.

Then how does a plane come down from FL390 in 3 and half minutes or so? This is roughly 200 km/h… If spin weren´ t stable, you would expect some sort of recovery over 3 and a half minutes…

Just by letting off the controls in any training aircraft and the airplane will eventually right itself in no more than a turn or two. Most airplanes are so stable it can be a chore to get them into a spin for training purposes.

What about Tu-154? Attempted to climb over a thunderstorm near or above ceiling, then something like an attempt to turn or a gust provided the upset… It did not right itself in a couple of turns (and inside the thundercloud they were attempting to avoid to start with) – it continued all the way to the ground. Or so the explanation of the Pulkovo Airlines crash goes.

That’s very nice of the kids, but shouldn’t you be able to get him the explanation Dean? 😉

Moggy

HAHA, they could probably explain it better than I can 😉

Yes, that was interesting… but it did not clarify well where the yawing couple comes from.

The yawing moment can come from a number of sources, the by-product of roll is yaw (adverse aileron yaw) and the by-product of yaw is roll, it could be from a catastrophic failure of an engine (does’t have to be this severe), also BR is right, the down going wing has a greater AoA in the spin but it will have passed critical alpha, causing more drag, as long as the AoA exceeds critical alpha you will get autorotation and a wing drop even more (different to spin, autorotation can cause a spin), hence why aileron is not used to stop autorotation or a spin, the down going aileron will increase the camber thus increasing it past critical alpha even more making the situation alot worse.
With any turning moment you need a force to keep the aircraft in the turn, this is where centrepetal force comes into play.

In which frame of reference is the less stalled wing going up? I suspect that the whole airframe, including the less-stalled wing, is going down, very fast, in a spin…

Of course, but a less stalled wing, will be the right side of critical alpha compared to the more stalled wing, thus giving greater lift than the other, this will cause one wing to be “higher” than the other, just the same as any aircraft in a rolling moment, to turn left the right wing must be “up going” & the left wing “down going”

The question in the first post was why spin is a stable state – so referring to fully developed spin. After all, a logical way of looking what happens in incipient autorotation is to look at what happens at fully developed spin, and figure out what happens in between.

Whiskey D answered this for you

chorn just as a matter of interest, where are you getting your information from?, and purely for information purposes do you know the difference between autorotation and spinning?

Dean

Very simply most aircraft are designed to fly in one direction. They fly when moving in that direction because their aerodynamic surfaces generate maximum lift and minimal drag when moving in that direction. Moving sideways or rearwards generates more drag and no lift so even in a flat spin when your forward momentum is gone you simply fall. The easiest way to get into a spin is assymetric thrust or assymetric drag. Loss of one engine or loss of lift from one wing. Once you are in a stable spin the air is no longer flowing properly over your control surfaces so they no longer provide proper control. If the problem was assymetric thrust then shutting down the other engine and trying to start the other might solve your problem. Once spinning you have momentum and no normal control surface to counter that force to stop spinning. With vectored thrust engines recovery would be easier. Some aircraft are designed to recover naturally from a stall or spin. For others some types of spin are unrecoverable (ie F-14 flat spins).

In a spin, one wing is moving leading edge forward, the other is moving trailing edge forward. Why is this a stable state?

I think herein lies your problem. The stalled wing does not have a reversed airflow as you seem to imply. The airflow is still flowing from front to rear but the angle of attack has exceeded the stalling angle.

Basically because the aircraft is stalled, not flying

The airfoil/aircraft is still “flying”. A stalled wing isn’t producing enough lift to support the weight of the aircraft. This is why an airplane will stall at a lower airspeed when at a lighter weight. The wing will start to stall well before the aircraft finally stalls and that happens when enough of the airfoil has stalled (ie stopped producing lift) that it can’t support the aircraft.

Another way to show that the airfoil/aircraft is still flying (producing lift) during a spin is to roll the ailerons in the direction of the spin. You’ll see that the rotational speed increases. There might not be enough lift to keep the airplane in level flight but the wings are still capable of producing lift and roll the airplane (which is needed for a spin).

The first requirement for a spin is a stall. The second requirement is a yawing motion. The yawing motion isn’t necessarily needed to maintain the spin but it’s needed to initiate the rotation. With the yawing motion one wing is advancing ahead of the other. Higher speed = more lift. Now that one wing is producing more lift than the other the airplane will start a roll. Once the airplane is in a full, established spin the rolling will appear as a corkscrewing decent.

A common reason initial students (or any pilot for that matter) enter a spin during a stall is they aren’t coordinated. In modern training aircraft it takes a pretty good yaw, close to a full ball out, in order to get the airplane to start a spin entry when it stalls. The common stall/spin crash on the base to final turn happens when a pilot overshoots final and tries to correct by yawing the airplane back towards the runway while increasing the bank angle. The increase in bank angle raises the stall speed which leads to an unexpected stall and the uncoordinated yaw control is the other ingredient for a spin.

As for a stabilized spin. Other than aerobatic aircraft I don’t think it’s possible to have a stabilized spin in an airplane unless the CG is out of limits. Airplanes are too stable to permit a stable spin. Just by letting off the controls in any training aircraft and the airplane will eventually right itself in no more than a turn or two. Most airplanes are so stable it can be a chore to get them into a spin for training purposes.

This is just the lead into the incipient autorotation stage not while the spin is fully developed. Try not to get ahead of the game.

The question in the first post was why spin is a stable state – so referring to fully developed spin. After all, a logical way of looking what happens in incipient autorotation is to look at what happens at fully developed spin, and figure out what happens in between.

This is just the lead into the incipient autorotation stage not while the spin is fully developed. Try not to get ahead of the game. The rolling moment is explained in the same terms as my book as on Wikipedia, which I think gives a pretty good account of what happens.

The yaw moment comes in because of the differential drag betwen the down-going (more stalled) and up-going (less-stalled) wing.

..and I don’t start PoF until next Monday, D 😉

In which frame of reference is the less stalled wing going up? I suspect that the whole airframe, including the less-stalled wing, is going down, very fast, in a spin…

The yaw moment comes in because of the differential drag betwen the down-going (more stalled) and up-going (less-stalled) wing.

..and I don’t start PoF until next Monday, D 😉

…. when the kids go out and try and get you an explanation

Dean

That’s very nice of the kids, but shouldn’t you be able to get him the explanation Dean? 😉

Moggy

chord I’ll get my ATPL notes out in a while when the kids go out and try and get you an explanation

Dean

Basically because the aircraft is stalled, not flying

It’s worth reading this.

http://en.wikipedia.org/wiki/Spin_%28flight%29

Moggy

Yes, that was interesting… but it did not clarify well where the yawing couple comes from.

It was explained with the drag differential. Well, the inside wing might have higher coefficient of drag because of higher angle of attack – but then again, it surely has much slower or indeed negative airspeed….

And why cannot a plane in a spin spiral out and return to a forward glide at a reasonable translational speed and low yaw rate?

Basically because the aircraft is stalled, not flying

It’s worth reading this.

http://en.wikipedia.org/wiki/Spin_%28flight%29

Moggy