sellener

I dont understand the PHAK way of describing a graveyard spiral. I can explain it the way the PHAK explains it to a student, but I dont understand it's explanation.

So you are in a banked attitude and losing altitude due to the loss of lift in a turn. You feel as if you are straight and level for reasons I understand. Naturally you pull back to slow the descent.


The PHAK says that pulling back on yoke tightens the spiral (I agree) and increases the rate of descent (I dont understand).

When Im doing a steep turn and need to correct for an initial tendency for a descent/loss of altitude I can increase back pressure and increase angle of attack and lift thus slowing/stopping descent.

The danger of stalling or spinning or over stressing the load factor on the aircraft I can understand.

I guess if you banked enough to be inverted, pulling back would increase your descent rate (ha ha), but I dont think thats what the PHAK is talking about.

So my question is why in a banked descent are you going to increase your rate of descent by pulling the yoke back?

NoyGonnaDoIt

Lift in a turn has both a vertical and horizontal component. When you pull up in a steep turn you are increasing the horizontal component and the increase of the horizontal component comes at the expense of the vertical so at a certain point vertical lift decreases (descent) as horizontal lift increases (tightening the turn).

In the normal steep turn maneuver, you can compensate for small losses in altitude with early response on the yoke, but even then, the goal of learning and teaching the maneuver includes an understanding that larger excursion require a decrease in the bank angle to recover.

If you're the seeing is believing sort, try this – do a commercial steep turn (might as well get the "advantage" of the extra bank angle). Once in the turn, establish a 100-200 fpm descent and then try to level off using only the yoke (as a CFI candidate, you might try this with your instructor). Do it enough altitude so you can give it enough time to see what happens before you roll out and recover.

shdw

Not with a constant bank angle it doesn't.

detpilot

A graveyard spiral doesn't have a constant bank angle... that's the point.

Cubdriver

The graveyard spiral is a classical "unstable system". The long answer to this topic is found in the study of aircraft stability, flight dynamics, handling qualities, and more generally system dynamics. You won't see these topics in flight school unfortunately- they are engineering subjects for the most part.

But in a basic way, pitch, roll and yaw axes are defined in relation to the airplane body while flight path and gravity are defined in relation to the earth. So when you roll past a certain point the vertical component of lift goes to the horizontal. The airplane was trimmed for level flight, which means it was stable there. So, it starts descending in an attempt to get back the vertical lift lost, which can only be had by an increase in airspeed per the lift equation. It trades potential energy in the form of altitude for speed (kinetic energy) to get more lift. It descends a little at first.

A mild bank, perhaps a wind gust caused the original loss of lift. The airplane could actually could find a new equilibrium by speeding up in level descending flight, we do this all the time to descend. But in this case the outside wing begins to develop slightly more lift than the inside wing, something it is supposed to do. Lift on the outside wing goes up with speed, increasing the bank. As the airplane has a tendency to roll towards its slower wing it loses even more lift to vertical and the airplane must now speed up even more.

Now going back to the original issue, which was a loss of lift. We are losing even more now, so the airplane has to seek a faster speed to get that equilibrium it needs. It is an unstable system that continues until all the available energy is used up and the altitude runs out. The end of that of course is a crash, and I suspect investigators using radar tracks and flight data recorders have linked a few to spiral dives.

As long as the roll has not exceed 90 degrees the airplane could possibly be recovered from the turn by merely pulling up on the yoke. It will exceed its structural limits at about n = 4, however. The right response is to therefore to remove bank first to get the real energy waster out of the equation, then recover by any of a number of actions which may or may not include pulling back on the yoke. You may need to apply back elevator, possibly no elevator if it was trimmed for level flight, or even forward pressure depending on how it responds. You are flying at speeds you did not expect most likely, perhaps quite surprised as well. You need to go a little slow on the recovery until speed is under control.

I do steep spirals at 70-110 degree angles all day in my part-time skydiving job. Speed is about 150 mph. The graveyard spiral is tame if you manage things properly. If airspeed goes crazy you always roll out first then adjust pitch. As explained that could be either way, pitch up or pitch down. I actually do them kicking full rudder in a slip, which gives a knife edge maneuver. I put some pictures up a few months ago of this from my own airplane in the Your Photos section (see Knife Edge).

Here's one of my favorite non-engineering links on the subject.

Denker on Steep Spirals

shdw

I guess I should have quoted his entire sentence, "When you pull up."

I know a graveyard spiral doesn't have a constant bank. My point is, back pressure, applied between 1 and 89 degrees of bank, will never increase horizontal at the expense of vertical or vice versa. It will increase both.

Vertical component = sin [bank angle] * total lift
Horizontal component = cos [bank angle] * total lift

So 3 things: speed, AOA, and bank angle change the components of lift. One doesn't change at the expense of another. (Can one of you aero engineers confirm those formulas? I don't have a book in front of me to check them.)



My suspicion with the PHAK sentence quoted, but without further review I am unsure, is that the resultant decrease is from a decrease in speed due to increased induced drag. Meaning, the initial experience will be a small decrease in descent, from the increase in lift.

However, when speed falls low enough total lift will decrease below the point where the back pressure was introduced. At that point, the turn will still be tighter but a descent would result.

IMO this coupled with the continued increase in bank angle would account for the continued decrease in vertical lift component. Absolutely nothing to do with the increase in horizontal at the expense of vertical.

sellener

I think i understand what I didnt understand :0)

1)A graveyard spiral bank is increasing throughout the situation, as opposed to a consant bank.

2)The bank angle in a graveyard spiral can/will pass 90 degrees at which point back pressure will obviously point the nose at the ground increasing the rate of descent.

Reading the PHAK it explains it poorly. It really doesnt talk about the dynmaic situation IE increasing bank....rather it tooks about a disoriented pilot returning the aircraft to its orignal bank

Then the PHAK goes on to say pulling back on the yoke increases the rate of descent....without mentioning anything about a 45 degree bank as opposed to 110 degree bank and how back pressure effects the two, not to mention the load factor involved trying to stabilize a descent rate in a 70 degree turn with back pressure alone.

shdw

And this is why I didn't try to explain it. Such an aerodynamic nightmare.

I do have two little nit picks in there. One, it is assumed that the pilot is likely uncoordinated, in a slip. Directional stability is greater than lateral in most non delta winged aircraft. So I would add that into the mix as to what causes the increase in bank.


Two, you said, "So when you roll past a certain point the vertical component of lift goes to the horizontal."

I suspect this would be better represented with wording like, "with an increase in bank angle we decrease the vertical component and increase the horizontal component." I think that is what you were getting at there at least, right?


Otherwise, nice explanation. Also, you cheating punk! Using rudder to get some vertical lift you, what a pansy piloting technique, Vne limits are for sissies.

KC10 FATboy

Thank God I was military trained.

jonnyjetprop

Lol........