Ground Effects and Airspeed

[CBR F4i]

Would a 757 type aircraft be able to fly 400-500 knots at VERY low altitude (20 feet)??

Would Ground Effects prohibit such low levels?

At what altitude would one really begin to feel GE??

THANKS:ok:

[WHBM]

At what altitude would one really begin to feel GE??
Within half a wingspan from the ground.

[CBR F4i]

I've heard the "half the wingspan" definition and that it is based on 'chord length'.

Any ideas on how airspeed affects GE?:p

[The Greaser]

Magnitude of downwash must increase with airspeed hence it seems logical that ground effect will be felt at greater heights.

[atr-boy]

If my memory serves me well....

Induced drag decreases and parasite drag increases with increased speed, so at a fester speed at low altitude (400 kts at 20 ft) I guess there wouldn`t be as much ground effect compared to a lower speed at the same altitude....

But really... there is only one way to find out, right???? ;o)

[jabberwok]

See WIG (http://www.aerospaceweb.org/question/aerodynamics/q0130.shtml) for some interesting details.

They quote that induced drag is halved when an aircraft flies at a height 1/10 wingspan.

I can confirm that a notable increase in airspeed (we got 10%) can be obtained from flying very close to a smooth surface. I won't say any more though in case ex employers are reading this..

[Tinstaafl]

Don't think I agree but I'm not sure.


Does the magnitude of the downwash increase with speed? *Provided* the a/c is in S&L flight then L=W at any speed. L is a force & is a reaction to accelerating a mass (the air) downwards.

F=ma

The air mass flowing over the wing is constant (barring temp/press changes to the body of air) so acceleration must be constant for the force remain constant.

Maybe I'm wrong? On the other hand:

At a slower speed there is less air in a given time - but the wing is operating at a greater AoA. Maybe that means the reverse: Less mass in a given time so more acceleration to the mass is needed to maintain F.

[CBR F4i]

L=W

With Ground Effect, you get more downwash/lift so to remain level, you need less AoA.

BUT, I heard a 757 has an AoIncidence of 4degrees!
You couldn't maintain ZERO AoA?

I guess I need to borrow a 757 for a bit....:suspect:

[Intruder]

Would a 757 type aircraft be able to fly 400-500 knots at VERY low altitude (20 feet)??
Only over a surface already at high altitude...

Tthough I don't know the Vmo of the 757, the 747-400 (which is a very speedy airliner) has a 365 KIAS Vmo. You would have to be at an altitude where TAS would be over 400 Kt at Vmo (e.g., 5,000-7,000' for the 744).

[Notso Fantastic]

I don't see why a large jet can't fly really fast at 20 feet, 10 feet, 50 feet whatever? WHat is stopping it? The Air Force does it all the time in smaller jets and large turboprops chucking things out of the back!

On a stabilised approach in a large jet, you will start going high on the glide path at about a wingspan plus height as you start feeling ground effect. On the 747, you should take a small chunk of power off at about 300 feet to stay in the slot.

I don't see what will stop a 747 speeding at 350 kts at 20 feet apart from the pilots nerves! I know a VC10 at an airshow did a fast, very low pass over an airfield without any problems apart from the Flight Engineer who was immensely not amused (he told me so himself).

[CBR F4i]

Do swept wings feel less Ground Effect relative to straight?

I am just having aa argument with a guy who says that at high speed, GE would force the plane higher unless you reduced speed...

If a 747 has an angle of incidence of 4 degrees, could you fly slightly nose down with an AoA of 1 degree:confused:

hehe!

[Notso Fantastic]

The aeroplane will go where you make it. If you fly at high speed getting closer to the ground, GE will make it reduce its rate of descent, so you will change descent angle slightly. By lowering the nose, you can maintain the descent angle. You are not interested in aeroplane angle of attack- you don't even know it. All you are interested in is where the aeroplane is going. You may have to lower the nose slightly to counter ground effect when you experience it, but the aeroplane can fly at high speed at 20 feet because you make it!

<<Do swept wings feel less Ground Effect relative to straight?>>..... I seem to recall they do have a slightly lower effect than straight wings, but comparison is a bit hard to make! All that matters is that there is a slight GE.

[CBR F4i]

Making it fly at 20 feet is how I felt about it but I don't fly, so what do I know:O

If it's experiencing enhanced lift, counter it with some elevators......level flight.

[BOAC]

Having experienced some ULL flying, GE does not stop you. It is necessary to actually 'push' down through it. From experience, the limitation on speed is a 'comfort' thing, where a blink or a sneeze can spoil your day. Around 260-280kts was the most comfortable swept-wing speed in my experience, and any faster with twitchier controls makes other parts twitch:D

[BigGrecian]

I know the link is a little random but just been sent the link included.

Apparantly it can be done according to the FBI it happened on Sept 11.
Interesting conspiracy theories of this @
Bangedup.com (http://www.bangedup.com/bu_posts/pentagon121qwedc.swf)

[CBR F4i]

The Pentagon was what I was "secretly" talking about with all this GE business:p

I've heard a few say it was impossible for the plane to hit it due to ground effects...:)

[Notso Fantastic]

If so, no plane could fly into the ground!

[Foxx]

"CBR F4i Quote..."The Pentagon was what I was "secretly" talking about with all this GE business.

I've heard a few say it was impossible for the plane to hit it due to ground effects..."
-------------

'Notso Fantastic' Reply...

"If so, no plane could fly into the ground!"

----------------



Foxx Reply...

" 'NotSoFantastic' gives a perfectly logical answer to the way the question is worded.

The question asks is it 'impossible' to crash a plane because of GE?

I don't recall anyone saying that it would be impossible to 'crash' a plane into the pentagon building because GE would stop or hinder a plane from 'crashing'.

What I have said from the beginning is that a 757 cannot be brought low enough in a stable landing attitude at the cruising speed of 400 knots or so to hit the pentagon as low to the ground that it was hit. If the pilot had slowed to a landing speed of 150 mph he could get the plane stabily to that height. Without lowering the speed as well as application of airbraking systems, lift dampeners, and trim flaps to slow enough to get to that height above the ground, the trajectory is 'impossible'.

The Boeing 757 is not known as an 'acrobatic aircraft'.

I would doubt the most experienced military / stunt fliers in the world would attempt entering into GE at high speed. In order to 'push' through the dynamic ram flow of air between the wing and the ground surface, split miliseconds of control factors would have to be accounted for in order to compensate for changing AoA. Ground interferences such as buildings and topography would also seriously affect the pilots ability to make control adjustments at almost impossible speeds in order to keep the craft stable for as long an approach as was achieved.

Compounding all the above issues, you now ask me to make a 'leap of faith' that, inexperienced Cessna flier Hani Hanjour, (who had never before controlled a 757), miraculously controlled that plane in ever changing GE conditions more professionally than any experienced Boeing pilot would dare attempt.

It has often been pointed out that Hani Hanjour could fly the 757 with a minimum of experience. I can believe that - the 757 is often quoted by pilots to be a very easy aircraft to fly. Flying at cruise speed at altitude in a 757, I think could be accomplished by almost anyone if someone showed you the basics.

Bringing the thing into 'terrain-hugging' GE flight at full speed, I would think would require a much more extensive training. You will have to get some serious evidence to prove the case to me that Hani Hanjour could have possibly performed this manuever.

Actually, there is a schematic 'scale' drawing superimposing the 757 profile against the building which shows that IF the body hit at the height of the impact zone, the port engine pod would have been dragging through the...

Amazing PentaLawn.
http://killtown.911review.org/pentalawn.html

[MonkeyintheCockpit]

There's a few things at work here:

1) Lowered drag in ground effect, so higher airspeed theoretically possible.

2) Increased lift; it's doubtful that the increase in lift @ 400kias would be greater than full control authority.

and

3) Wingtip vortices are greatest in clean config at slow speed, so would therefore be minimized in this example.

Based on the above, it would seem that the aerodynamics of the situation would allow a greater speed within ground effect. But at 400 kias, there would have to be a very very fine control of the aircraft to avoid some sort of mishap.

Q: What is maximum airspeed for a 757 at sea level?

[Foxx]

'MonkeyintheCockpit' Quote:

"1) Lowered drag in ground effect, so higher airspeed theoretically possible."

2) Increased lift; it's doubtful that the increase in lift @ 400kias would be greater than full control authority.

and

3) Wingtip vortices are greatest in clean config at slow speed, so would therefore be minimized in this example.

Based on the above, it would seem that the aerodynamics of the situation would allow a greater speed within ground effect. But at 400 kias, there would have to be a very very fine control of the aircraft to avoid some sort of mishap.

Q: What is maximum airspeed for a 757 at sea level?
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Foxx Reply - The lowered 'drag' only results in greater 'efficiency' (not necessarily 'speed') at these heights. At higher speeds the GE factors rise in relation to wingspan and therefore increase the distance above ground surface, (apart from slowing the craft to 'push' through the GE dynamic ram cushion).

My experience in investigating this field is more related to WIGe craft...

http://foxxaero.homestead.com/indwig.html ...

Although WIGe craft are specifically designed to operate within GE (as opposed to a typical aircraft, which are not)... the physics related to GE flight will affect ANY craft with foils / wings operating in close proximity to a 'ground' surface. The higher the speed, the further away from the surface the craft will be forced...(apart from entering close proximity to the ground at an angle of attack which defeats the physics of GE flight - 'nose-dive').

If a Boeing 757 entered GE in normal landing attitude it will HAVE TO apply lift dampeners, air-braking systems, and changes to A0A to overcome the GE effect, OR, it will settle to a certain height, and then, will just continue to sail on into the sunset at that particular height until such time as something gets into the way or someone decides to 'apply the brakes'.

Cheers

Foxx