Cubdriver

Fly like a fighter: Down we go...

(AOPA Online, 12/30, L. Brown) "...Aside from drag devices, fighters also have the option of performing a low-power but high-G descending spiral to keep the airspeed under control while making a rapid descent. But, alas, not all of these options are available in our general aviation airplanes. From cruise at 8,000 feet to 12,000 feet msl, would you consider ripping your throttle to idle to expedite your descent? Not a good idea since shock cooling a piston engine is not a healthy choice. Most GA airplanes don’t have speed brakes, and most already have the gear extended. We do have an added trick that doesn’t work as well in fighters--the forward slip. Overall, though, what is your best option? Follow the guidance of “proper prior planning prevents pretty poor performance” and you will pre-plan your descent point in your aircraft for your particular flight that day. If you do get behind, then go ahead and make a 360-degree turn (or two) to allow you to descend comfortably and not rush your arrival..."

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This article reminds me of the knife-edge spiral some skydive dropzone pilots use in Cessna 182s. Close cowl flaps, set power to 15, bank to ~85, kick full opposite rudder, and adjust attitude using aileron (not elevator) to stay in yellow arc, adjust g's using elevator to hold about 3.5 on the meter. This is the fastest way to get down short of using a drag chute plus speedbrakes. It is faster than forward slipping or a full-flap descent. Shock cooling does not occur using good power management technique as any drop zone owner knows, although a digital engine meter helps fine tune it. Many drop zone Cessnas have ten thousand hours on the same engine doing this with no problem. I had a thread on it some time ago, see below. You do not want to do it without a good bit of practice because it involves pretty fine adjustments of attitude, speed, and power. Also, never attempt without proper training, parachute, and g-meter lest you overstress the airframe and find yourself without one.

Knife Edge

UAL T38 Phlyer

The high-g profile described in the article works because fighters are generally low-aspect ratio wings. At high g (AOA), there are huge drag penalties due to tip-losses/vortices.

The same principle works in GA aircraft, as you have noted, just not quite as well. Most fighters have an aspect of about 6; GA is usually around 10-12, if memory serves. GA losses due to vortices are a much smaller percentage of the drag on the airplane.

The article said fighters can't use a slip: I have, and continue to do so (this is in the F-4 and the T-38). Fly-by-wires may not cooperate as much, but I think it can still be done (ie, step on the rudder in an F-16, and it deflects---but the ailerons would auto-deflect to prevent the roll that happens in swept-wings with yaw).

I did an emergency decent in the sim (for a checkride) simulating cabin-pressure loss in the Lear 35 a little differently than the book. We normally just went to idle, dropped the gear, popped spoilers, and held it straight-ahead at redline speed. From 40,000 ft, it took about 7 minutes to get down to 10,000.

I did all that, but went into a 60-degree-bank turn and used bank to keep the pitch at its normal mark (I think it was 10 degrees). IE, I was holding 2-gs.

The Sim guy said "Hey, what are you doing?" I had my FO give the "ATC, we are orbiting at present position in our descent" call.

When we levelled off, the Sim Instructor said "Wow...you got down in half the time of normal.......you're going to have to explain to me how you did that...."

USMCFLYR

I've seen cross-controlling use to slow closure too. Usually the result of poor closure control to start out with - but effective.

USMCFLYR

Cubdriver

This maneuver of course is all about converting potential energy (altitude) into kinetic energy (motion) by wasting it at the stress limits of the airframe. AoA gives lift which is wasted to the side as well as induced drag which wastes energy in the form of wingtip vortices as mentioned. The added value of a slip or knife edge is you get the rudder adding to it through unused yaw and associated drag, while it angles the fuselage to get form drag as well. All this challenges the structures, but I used to see minus 5,000 fpm in the C182 without speed brakes or evidence of any rivet stress. Fly-by-wire may be why they do not use it as much in military jets. I am guessing, but you guys would know.

Ewfflyer

When training guys in tbms or meridians I would use this setup, max safe(simulated versus actual situation) speed, 45 bank, and point that nose down! Very effective when the only concern is just getting down. Now if there's some distance to still cover I'd say keep the wings level, but everything is situational.

PearlPilot

Very interesting stuff. Gets into some advanced aerodynamics and you'd better know what you are doing. There are a couple of jump operations closer to where I operate out of and since we are located in a TSRA area I doubt they use techniques such as this to get down.
On a similar note, please forgive me if the said note is dissimilar to what we are discussing here. I was giving a photo flight and the gentleman (who was an Air Force pilot) wanted to take a "vertical shot" of the target since the gear of the Skyhawk would be in the way. We were at 1,000 foot AGL surrounded by mountainous terrain and he wanted me to "try doing a crab" for which I scratched my head. I have received adequate spin training and I was not about to "crab" or be uncoordinated at 1,000 ft. AGL so I refused such a maneuver and settled for a 45 degree bank. I actually climbed to 2,000 AGL and made a descending 45 degree bank to 1,000 ft. AGL. If I would have tried to crab or be uncoordinated in the name of getting the landing gear out of the way, I think I would have found a very fast way to get down indeed.

JamesNoBrakes

I assume he meant "sideslip" rather than crab.

Sideslips/forward-slips (same thing, just differences in the relative wind direction and path along the ground) are used by all sorts of GA aircraft all the time to land! Many people teach this as a technique to get better visibility of the ground, but I never went crazy with it, as being uncoordinated is usually not the best thing for passengers and you need to pay attention to maneuvering speeds when making control deflections obviously. Still, I would have my students do it at times. Don't let your speed drop excessively low, but realize that sideslips and forward-slips are done all the time close to the ground. Remember that a spin requires a yawing moment, if you aren't yawing when you stall, it will not really start to spin (unless the stall then causes a strong yaw, but this can usually be managed). Use good judgement, don't let your speed get dangerously low, but it can and is done safely all the time. You might not be able to maintain altitude for long while doing this, so it's best to limit it for a while, climb back up, repeat if necessary. Obviously each aircraft is different and not all will react the same, but you can do this all day in standard cessna single engine, and while I may not want to stall in some other aircraft while being "uncoordinated", the airspeed and any POH/AFM limitations are really key here. I've had people come to me and say they were never taught to do full forward-slips to landing in that airplane with full flaps, and while a tail-oscillation buffet is possible (it's been very rare for me), it does not result in loss of control and as long as you got fuel in both tanks, you can do this all day too, not restricted in the manual or anything.

On the other hand, flying low over mountainous terrain is something that has to be managed correctly. There are things you can get away with, and then the right way to do things. If you didn't feel comfortable getting low/not having an exit, then more power to you.

PearlPilot

Testing the operating envelope of an airplane requires a higher degree of skill and knowledge. The topic at hand in this post certainly says that. I have and teach the use of slips constantly and quite frankly I enjoy doing them especially when landing with no flaps on a Skyhawk. The combination of high terrain, and low altitude, made me feel like a recipe for a disaster to try being uncoordinated. I just did not feel comfortable. I listened to that infamous inner feeling that said "don't do it."

N9373M

It's knowing the airplane and knowing your skills. Slips with the 40 degree flap setting in the 172 is not good - that's why the newer models only go to 30. I'd let pax know, "we're a little high, so I'm going to turn the airplane sideways to get down, it's better to be too high than too low".

I was taught to use slips whenever needed in every plane I've flown and have done so (on final only, not at altitude or in the 3 other legs of the pattern)

Somewhere in the recesses of my brain, I remember hearing of a 737 (? big jet, regardless) driver slipping to get down.

Great thread - I've enjoyed the Lear and jump driver posts especially

2nd best pilot

From my limited aerobatics experience, I was taught that you cannot spin the airplane from a slip, but only from a skid. If stalled while in a slip, the plane will do a "falling leaf" maneuver. Quite a weird feeling to hold it in one, but the falling leaf is pretty unstable and can oscillate to a skid pretty easy and start a spin if you're not careful.

Being experimental, I've tried slips in a 182 with 40 degrees of flaps. It pitches the nose down pretty violently. Before I tried it, I heard in a lecture, at an ESP CFI seminar, that the violent nose pitch is the reason planes are placarded for no flap slips (he was an FAA examiner so I'll consider it a credible source).

And that knife edge maneuver is awesome!