C172 "Autoland" Procedure

petrolero

petrolero

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petrolero
Just remembered something a C172 owner once mentioned that an instructor demonstrated to him in his plane. I owned a 172 at one time but I don't think I ever tried this.

As best I recall, he said you could trim the 172 for, say, a 70 knot stabilized short final, with full flaps, and just maintain directional control (I'll stipulate no xwind for discussion), and the thing will flare itself once it reaches ground effect, and land. The instructor, he said, jokingly called this the C172 "autoland".

Now I'm recalling this from a conversation years old so the flap setting or speed may be off or perhaps it requires throttle manipulation, but I'm casually curious if any C172 drivers here have tried this or do it regularly.



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Not unless the plane somehow becomes sentient on short final and decides to pull back on the yoke out of self-preservation.

In any case, you could "land" the plane that way but it only works once. :yikes:
 
That's an emergency maneuver for non pilots. It won't be pretty .
 
My understanding is that the flare happens once it reaches ground effect.
 
At 70 knots, your descent rate for a 3-degree approach path is only about 350 ft/min. Ground effect will reduce that somewhat (don't ask how much, or the aerodynamics behind it, 'cause it's been a while...), so the impact shouldn't be too bad.

Big thing to watch is make sure there's enough positive pitch so the mains touch first. Airplane will probably be useable again!:D
 
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I suppose if you do a shallow enough approach it could work.
 
Long ago, I was taught to land a C-150 without touching the yoke, but that's a far cry from autoland. :confused:
 
bflynn said:
Aerodynamically, how does that work?
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Lift is increased in ground effect.

I don't see how you'll get much pitch-up, so you'll need to be configured nose-up (at least a little) on approach. I don't think full flaps will do that for you. Maybe 70 KIAS with flaps up. If you had the nose up even a little on descent, you'll get some loss of descent speed in ground effect. But trim is going to put the nose down if you lose any speed, so you'll have to touch down at that real high speed.

Seems survivable on a big wide runway. Not pretty.
 
I would think it would work better on a low wing, due to the increased ground effect. The problem I see is that the elevator is going to go in to ground effect too (first, if the nose is pitched up) which should have the effect of pitching the nose down.

I wouldn't want to try it hands off.
 
MauleSkinner said:
At 70 knots, your descent rate for a 3-degree approach path is only about 350 ft/min. Ground effect will reduce that somewhat (don't ask how much, or the aerodynamics behind it, 'cause it's been a while...), so the impact shouldn't be too bad.

Big thing to watch is make sure there's enough positive pitch so the mains touch first. Airplane will probably be useable again!:D
Click to expand...

So just how much will ground effect reduce the descent rate are what are the aerodynamics behind the descent rate reduction? :D
 
I would think you could probably lower the speed to the lower 60 knot range,for less damage.the older 172 s stall at around 50 kts
 
Cessna did promote the 172 with "land-o-matic" gear at one point...
 
MAKG1 said:
Lift is increased in ground effect.
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No, induced drag is reduced. Lift remains the same but since the counter-acting force is reduced the effect "feels" like the plane is creating more lift.
 
I had demonstrated to me a similar thing in a 1956 C 172. 2 notches (20 deg) of flap and trimmed to 65mph....no knots. Adjust throttle on final and keep the nose centered with rudder. No hands. Airplane landed fine on the mains by itself. It was a calm day and we were light.
I wouldn't try this with any of the 172s that list 70knots as the approach speed as I think they're heavy enough to not touch down as softly as the old models.
I have done some stuff in my 175 with a 160 lycoming and a STOL kit where I set up the approach at 65 mph, 2 notches of flap (20deg) and control the approach with the throttle. I've not been able to keep my hands off the wheel for flare though....just can't make myself do it since I'm paying for repairs and maint. I don't do it with more than 2 of us in the plane either.

So I think that while the original posters concept could be done I'm not sure if it would work well with all the 172's but I know it works with the early lightweights. I wouldn't call it auto land though.

Frank
 
I watched this video about landing without primary flight controls the other day, and it demonstrated something *almost* like the auto-land you're describing. They just use the trim for a late flare.

https://youtu.be/yqPFfTRn5So?t=11m7s
 
BiffJ said:
I had demonstrated to me a similar thing in a 1956 C 172. 2 notches (20 deg) of flap and trimmed to 65mph....no knots. Adjust throttle on final and keep the nose centered with rudder. No hands. Airplane landed fine on the mains by itself. It was a calm day and we were light.
I wouldn't try this with any of the 172s that list 70knots as the approach speed as I think they're heavy enough to not touch down as softly as the old models.
I have done some stuff in my 175 with a 160 lycoming and a STOL kit where I set up the approach at 65 mph, 2 notches of flap (20deg) and control the approach with the throttle. I've not been able to keep my hands off the wheel for flare though....just can't make myself do it since I'm paying for repairs and maint. I don't do it with more than 2 of us in the plane either.

So I think that while the original posters concept could be done I'm not sure if it would work well with all the 172's but I know it works with the early lightweights. I wouldn't call it auto land though.

Frank
Click to expand...

Barry Schiff demonstrated this in a 172, as an emergency proceedure in the rare event of loss of flight controls.
Very good DVD set!
http://www.barryschiff.com/videos.htm
 
About a week ago I let the autopilot in my cirrus fly the LPV approach with full flaps and just a little above short field speed at about 75-78KIAS. There was no wind and a very stabilized approach. i was curious how far the autopilot would go, so I kept the autopilot engaged until about 10' above the runway. Right before disengaging the autopilot I briefly felt what felt like a small flair or level off. I know the autopilot wasn't doing that, but maybe there is something to the ground effect that would cause some sort of level off?
 
I've read about a tailwheel aircraft that took off unpiloted and then landed unharmed when it ran out of fuel. A handpropped Aeronca. Many years ago.
 
bflynn said:
Aerodynamically, how does that work?
Click to expand...
The center of lift is generally forward of the center of gravity. Thus, when lift is increased, it will pitch up around the lateral axis behind it.
 
A 172 is a very docile, easy to fly aircraft. Using the proper trim, throttle inputs it will indeed " land itself". ( see videos) it's the main reason it was- is so popular.
 
I would presume the landing would not be so smooth but I'm sure it could be done.
 
Kritchlow said:
The center of lift is generally forward of the center of gravity. Thus, when lift is increased, it will pitch up around the lateral axis behind it.
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That doesn't sound right. If the plane is loaded properly, the center of gravity should be forward of the center of lift.
 
simtech said:
I actually saw a youtube video of a student and a CFI doing this. Here it is

https://www.youtube.com/watch?v=yqPFfTRn5So

the set up and landing is around the 10 minute mark. but he explains a lot leading up to it.
Click to expand...

TexasAviation said:
I watched this video about landing without primary flight controls the other day, and it demonstrated something *almost* like the auto-land you're describing. They just use the trim for a late flare.

https://youtu.be/yqPFfTRn5So?t=11m7s
Click to expand...

They never said which direction they were trimming during the ground effect. The instructor said it will want to balloon once in ground effect and that the pilot must counteract that with trim - so I would assume he meant nose-down trim? Basically just use trim in whatever direction it takes to damp the oscillations - I guess.
 
Sac Arrow said:
That doesn't sound right. If the plane is loaded properly, the center of gravity should be forward of the center of lift.
Click to expand...

You are indeed correct. I must have posted that late, and after a couple margaritas!!
 
bflynn said:
Aerodynamically, how does that work?
Click to expand...

There is more lift when within one wingspan of the ground - same concept as the soft field takeoff - get the wheels off the ground and then accelerate in ground effect. Low wing aircraft are notorious for floating in ground effect far more than a Cessna.

In this case they have it flapped (20 deg) and trimmed for a shallow descent and that extra lift that gets dumped in by ground effect is enough to not only arrest the descent but to cause an oscillation - which one can correct with trim or, presumably, power.

You're descending in a stable shallow descent and trimmed as such. The amount of tailplane downward force is going to be constant as long as power and trim (and thus indicated airspeed) don't change. Upon reaching ground effect the wings achieve much more lift - it gets sort of dumped in - but the air flowing over the tailplane hasn't changed thus the tailplane now produces a net pitch-up moment thanks to the increase lift experienced by the wings. This is all assuming no power or trim changes. In the videos they do adjust trim to prevent the oscillation from resulting in a drop-in.

It's similar in principle to trimming for straight and level flight up high and then pulling back on the yoke briefly (without adjust trim) to induce an oscillation and then going hands-off again. We've all experienced that. The pull-up in this case would be somewhat like the sudden additional lift from ground effect. I know it's not precisely the same thing but it is in principle very similar.

But no aerodynamic or physical explanation on this board goes without a fight so you guys discuss... :D
 
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petrolero said:
There is more lift when within one wingspan of the ground - same concept as the soft field takeoff - get the wheels off the ground and then accelerate in ground effect. Low wing aircraft are notorious for floating in ground effect far more than a Cessna.

In this case they have it flapped (20 deg) and trimmed for a shallow descent and that extra lift that gets dumped in by ground effect is enough to not only arrest the descent but to cause an oscillation - which one can correct with trim or, presumably, power.

You're descending in a stable shallow descent and trimmed as such. The amount of tailplane downward force is going to be constant as long as power and trim (and thus indicated airspeed) don't change. Upon reaching ground effect the wings achieve much more lift - it gets sort of dumped in - but the air flowing over the tailplane hasn't changed thus the tailplane now produces a net pitch-up moment thanks to the increase lift experienced by the wings. This is all assuming no power or trim changes. In the videos they do adjust trim to prevent the oscillation from resulting in a drop-in.

It's similar in principle to trimming for straight and level flight up high and then pulling back on the yoke briefly (without adjust trim) to induce an oscillation and then going hands-off again. We've all experienced that. The pull-up in this case would be somewhat like the sudden additional lift from ground effect. I know it's not precisely the same thing but it is in principle very similar.

But no aerodynamic or physical explanation on this board goes without a fight so you guys discuss... :D
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But.. When you pull back on the yoke you are increasing lift on the tail in a downward direction, so I don't see the correlation.

It seems to me that the extra lift would tend to nose the airplane down.

That said, I'm not an aeronautical engineer...
 
Ugh, ground effect is not a product of increased lift. The wing will generate the same amount of lift at a particular airspeed regardless of its proximity to the ground.
 
Kritchlow said:
But.. When you pull back on the yoke you are increasing lift on the tail in a downward direction, so I don't see the correlation.

It seems to me that the extra lift would tend to nose the airplane down.

That said, I'm not an aeronautical engineer...
Click to expand...

I think what he's trying to say is that the rear stabilizer is not affected by (or as much by) ground effect than the wing, so the wing's decent would be arrested by the ground effect cushion where the tail would not, resulting in a nose high attitude.

That explanation seems to make sense.
 
labbadabba said:
Ugh, ground effect is not a product of increased lift. The wing will generate the same amount of lift at a particular airspeed regardless of its proximity to the ground.
Click to expand...

Ground effect is caused by a cushion of higher pressure air that is trapped between the wing and the ground. It indeed increases the lift.
 
Sac Arrow said:
Ground effect is caused by a cushion of higher pressure air that is trapped between the wing and the ground. It indeed increases the lift.
Click to expand...

Respectfully, no. There is no cushion of air. Lift remains unchanged. What DOES change is the lift:drag ratio caused by the near elimination of induced drag when subject to ground interference. This results in the 'feeling' that the plane suddenly has more lift.
 
labbadabba said:
Ugh, ground effect is not a product of increased lift. The wing will generate the same amount of lift at a particular airspeed regardless of its proximity to the ground.
Click to expand...

You need more training. Contact professor Irwin Cory. Ground effect does indeed create lift.
 
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