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Losing speed on landing

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Re: Losing speed on landing

Postby ethan3391 » Mon Jan 25, 2010 7:54 pm

To those who are interested I seem to have been wrong in a few of my previous posts.

I looked up the article I was referencing before and found this;
8. Your plane isn’t a car. Don’t drive it onto the runway. The accepted method for landing heavier aircraft is landing attitude. The nose is positioned in a positive angle of attack, and this angle of descent is held by using power to maintain the correct altitude—if the plane goes below the glidepath, power up; if it goes above, power back.

I'm assuming this means only a few degrees of positive AOA though not the 20-40 degrees you see some simmers fly on you tube.

To my knowledge, Plane & Pilot published two articles, and for those who may be interested, here are links to both of them.
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Re: Losing speed on landing

Postby redneck » Tue Jan 26, 2010 5:00 am

Well, AOA is dependent on aircraft. Pitching to an AOA of 15 in an F-4 Phantom, may be the same as 7 degrees in a c172p. Since civilian planes don't have AOA indicators, I can't accurately judge AOA all the time; I just sorta feel it from the way the plane behaves, and the amount of input needed on the joystick.
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Re: Losing speed on landing

Postby Thorsten » Tue Jan 26, 2010 9:25 am

My flight instructor used to distinguish 'Lufthansa landings' from 'proper landings' (the ones I was supposed to perform with the ASK-21). The meaning was that when landing the ASK-21 I should flare with spoilers extended half a meter above ground till the plane became incapable of flying (i.e. reached a condition beyond max. AOA) and then touched the ground without the possibility of bouncing back. In contrast, he referred to 'Lufthansa landings' as touching the ground with slow sinkrate but not really close to max. AOA, so the plane would be capable of rising again after touchdown, but would not bounce due to the small vertical speed, and would then be decelerated on the runway instead of in the air.

So, it's very much a question of aircraft - a plane with very long wings and a center wheel like the ASK-21 is difficult to control on the ground, so you decelerate it in the air and touch down at the last moment. On the other hand, if your plane has a weight of a few hundred tons, you want to hit the runway in a soft and controlled way to minimize stress on the gear and not essentially fall down from a few meters. Supersonic planes like the Lightning or the Blackbird which rely on a braking chute anyway but have *really* bad slow flight characteristics are essentially 'driven onto the runway' to minimize the risk of a stall on final approach (when these planes stall, your chances of recovery are really poor...). For carrier landings, hitting the wires is way more important than anything else - you can touch down with the most beautiful AOA and sinkrate, but you'll have to go around anyway if the hook doesn't catch.
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Re: Losing speed on landing

Postby ethan3391 » Tue Jan 26, 2010 5:52 pm

@ Redneck,
You seem to be missing the attitude indicator, artificial horizon if you please. It shows pitch, that is aoa, and roll.

@Thorsten,
That was what I was thinking about when I said not to hold high aoa on descent. The A-4 Skyhawk does what is called a pancake stall, which is not recoverable at low altitudes.

Another thing about those aircraft carrier landing is if you catch the wire in the middle of a flare, you would probably land in the infirmary, and the aircraft would most likely have to be dumped, which is probably a little more of the logic behind why Navy pilots don't generally flare.
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Re: Losing speed on landing

Postby MD-Terp » Tue Jan 26, 2010 7:09 pm

ethan3391 wrote:You seem to be missing the attitude indicator, artificial horizon if you please. It shows pitch, that is aoa, and roll.

WHOA. No no no. Pitch and AOA are not even close to the same thing. Related, maybe, but not the same thing.

Pitch is the angle of the plane to the horizon.

AOA is the angle of the airflow to the wings.

You can have a pitch of +5 degrees; but at a higher speed, this usually gives you a low AOA resulting in a climb. At a lower speed, you can still have the nose +5 up, but your AOA could also be 5, giving you level flight, or >5, giving you a descent. (Of course that assumes that the wings are pitched level with the nose of the plane, which may or may not be the case based on the particular plane.) (You aeronautical engineers correct me if I'm wrong. I know that this is a simplified explanation, but basically correct, no?)
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Re: Losing speed on landing

Postby ethan3391 » Tue Jan 26, 2010 7:14 pm

Okay, it was my understanding that airflow was not much more than a side effect of angle of attack, and as such pitch.
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Re: Losing speed on landing

Postby MD-Terp » Tue Jan 26, 2010 7:36 pm

To understand the difference, try this:

Take off in your favorite plane (it makes absolutely no difference which one -- the basics are true for pretty much all of them, except helis that is) and climb to a nice safe altitude and normal cruising speed.

Now pitch your plane to five degrees nose-up. At cruise speed, that should put you into a slight climb.

Now, maintain +5 pitch, but reduce to near stall speed. The AOA should increase (which is what actually causes the stall, by the way, not the speed itself) and the plane should descend. The nose might still be pointed slightly up, but the AOA now exceeds the positive pitch, resulting in negative climb.

When the angle of the wings to the airflow is too steep to provide lift (think of it this way -- it's "mashing" through the air instead of "cutting" through it), then you have a stall. The published "stall speed" for an aircraft is sort of a misnomer, then, or a guideline -- the lack of speed doesn't cause the stall. The lack of speed causes lower lift, meaning the plane has to pitch up to maintain level flight; and when that pitch up exceeds the max AOA for that wing, you have a stall as described above. A stall can occur at speeds above the published stall speed, for example, if you pull a high-G maneuver (climb or turn) and try to change the angle of the wings to the airflow too quickly. Dave Culp's planes model this particularly well, and most have an AOA indicator on the upper left of the panel.

Of course, I haven't discussed the effects of flaps, weight changes, and other effects which can change the plane's AOA at level flight. This is just the basics. Plus I'm not as well-versed in this stuff as many of the developers are.
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Re: Losing speed on landing

Postby ethan3391 » Tue Jan 26, 2010 7:47 pm

Yes, I understand about the stalls I read about those a couple of days ago. I just misunderstood aoa.

From the above statements, then, would it be possible to use a, more or less, simple equation using the vertical speed, forward speed, and the pitch to get, at least, a decent idea of the aoa?
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Re: Losing speed on landing

Postby MD-Terp » Tue Jan 26, 2010 7:52 pm

ethan3391 wrote:Yes, I understand about the stalls I read about those a couple of days ago. I just misunderstood aoa.

From the above statements, then, would it be possible to use a, more or less, simple equation using the vertical speed, forward speed, and the pitch to get, at least, a decent idea of the aoa?

Yep. Absolutely.
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Re: Losing speed on landing

Postby eeK » Tue Jan 26, 2010 8:03 pm

Adding to what has been said already:

A wing stalls at a particular angle of attack, the airspeed of which depends on loading. So for example, a C150 will stall at a higher speed if the weight is increased. The angle of attack at stall should remain the same (for a given configuration).

Mil pilots are taught to be at 1.1 times stall (in landing configuration) at threshold. The problem with FlightGear <=1.9.1 (perhaps different in the new FG) is that you can't get a "g break" to determine the stall speed. Have a look at the real world data for the aircraft you're flying to try and get an idea of stall speed. Also in the real world, you'll discover "speed stability". Once trimmed at a certain speed, small throttle movements will not change airspeed but rather rate of decent (or climb). Rule of thumb fixed pitch props, every 100 RPM change will give you (roughly) that change in FPM. So having trimmed hands off level at 70 knots 2300 RPM for example, reducing RPM to 2000(-300) will give you 70 knots at -300 FPM. Not sure if FG exibits those traits though.

One last thing, if the wing is a laminar flow type with high loading, fast and thin (eg SF260 and probably most fighters?), you normally don't flare when landing, instead flying "onto" the runway (I guess like a soft carrier type landing). This is because such wings have a propensity to "let go quickly" at higher angles of attack.

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Re: Losing speed on landing

Postby Merik » Tue Jan 26, 2010 10:44 pm

Can you also explain how flaps reduce the stall speed and help with landing?
I'm using FlightGear Git version on a Win 7 machine with an onboard ATI graphic card and 3 GB of RAM.

These are the link I've found useful: Types of AirspeedE6B emulatorAirspaces explained with examples
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Re: Losing speed on landing

Postby flameout » Tue Jan 26, 2010 11:01 pm

I'm sure there's a more informative flap article on Wikipedia, but here's a very short summary:

Flaps are surfaces on the back of the wings designed to increase the curvature and increase lift. While the stall AoA remains the same, you have more lift, so for a given airspeed the plane has a higher vertical speed. This causes the AoA for any airspeed to lessen, so you can have a lower airspeed and still be above stall AoA. Therefore, you can fly slower.

Another effect (and the reason the wings aren't just shaped like this in the first place) is that they increase drag, so you can descend and slow down more quickly.

Many aircraft (mostly larger ones like jet airliners) also have slats, which are also controlled by the flaps lever. They are on the front of the wings. I am not sure whether they add any lift or not, but if they do, it's a very small amount. Their main effect is to increase the stall AoA. This allows you to fly even slower without stalling. Combined, flaps and slats allow you to fly slower for landing, allowing larger aircraft to land safely.

I hope this helps.
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Re: Losing speed on landing

Postby ethan3391 » Wed Jan 27, 2010 12:41 am

When MD-Terp pointed out my mistake about aoa I went on a search to find a simple a way of figuring out the angle of attack, I have not as of yet found the answer to my question. One of the things I found instead however, was an articles about the contribution Newton's 3rd law has on lift. This may be a reason why flaps work.

I used to think that flaps increase the surface area of a wing, but when I thought about it today it seemed less likely. The only thing it appears to do is increase the slope of the trailing edge, so, if this is true, this article would go a long way toward explaining how a flap increases the lifting force.

Here's the link; http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/angatt.html
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Re: Losing speed on landing

Postby nickyivyca » Wed Jan 27, 2010 5:36 am

One thing that I've been wondering is if I should have the flaps one or two positions above full when on approach. On the ground I put them at full, but on approach I usually leave them at one above full and it's hard to bring the wheels down without lifting the whole plane up. I haven't really tried two above normal, which I probably should sometime. On long trips it's hard to use the 787 (which I really like, I can use small spoiler movements or the whole speedbrake system, which is useful) because the autopilot is REALLY spazzy.
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Re: Losing speed on landing

Postby Thorsten » Wed Jan 27, 2010 9:45 am

One thing that I've been wondering is if I should have the flaps one or two positions above full when on approach.


That would depend what airplane you're in and how steep you have to fly the approach (in the mountains, a standard approach glideslope may just not be possible for instance).
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