Serious question for a QFI
 

Ladies / Gents,

Ones son has asked a question concerning the four forces on an aircraft when under 'G' and I would be grateful if someone were to offer a simplistic answer.

He understands the four forces and that to maintain flight lift has got to be equal or greater then weight or gravity. He has stated that when pulling '+G' the aircraft and occupants weigh many times their static weight. Logic therefore states that increased lift is required when pulling +G.

In trying to answer, I mentioned that G is always married to momentum at which time he shot me down by posing a question about approaching the top of a loop and losing momentum whilst maintaining +G. Clearly the aerofoil section and surface area are a constant on all but swing wing aircraft, therefore how is the need for increased lift met on a conventional aircraft whilst sustaining G.

To keep things resonably simple; in a scenario of a run-and-break for example. Genuine question requiring sincere replies please.

I'll PM you

Lift varies with, amongst many other things, angle of attack - looseley, the amount by which the leading edge of the wing is pitched up relative to the air. So, to increase lift, pull the nose up. The first thing that happens (before the aircraft goes anywhere since we haven't yet generated any additional force) is that the leading edge pitches up relative to the air passing over it. Result - increased lift. The aircraft now accelerates at right-angles to the wing - increased G. if you don't add power at this point, speed will reduce (increased lift => increased drag) and since lift also depends on speed, the excess lift will diminish until the forces come into balance. Pull some more and the angle of attack again increases, and the manoeuvre continues. The limiting factor is the stalling angle of the wing, or the point you run out of speed, which in turn depends on how powerful the engine is, or the extent you are trading height (= energy) for turning performance.

This works whether the aircraft is initially level - entry to a loop, for example, or whether it is banked - entry to a break.

Health warning. I'm not a QFI so that may be overly simple / just plain rwong.

When I did my AFI course many years ago everything was explained by the following formula:

Lift (L)= coefficient of lift (C/L) aerofoil, air density/2 (p) , speed squared (V**2), area (S)

So for instance if assuming C/L and S are constant then if p reduces V must increase to maintain the same L.
or say increasing S by use of flaps then V can be reduced.
or say banking S reduces (cosine of angle) then V has to be increased.
or increasing AofA then C/L reduces until a stall is imminent then V will need to be increased.
Pulling G increases need for L therefore one of the other variables must increase or else you will be going down.

Hope this helps.

zzzzzzzzzz....

Diablo Rouge - "In trying to answer, I mentioned that G is always married to momentum"

I'm recording 1G, but sitting fairly still...
:E

Whilst in level flight you weigh 1 G, as does the whole aircraft. Pulling up and opening the throttle to maintain the speed will cause the whole mass to go up and around. Inertia will now cause the whole aircraft and you within it to weigh more than 1 G so the wings have to be made to increase lift to overcome this and continue the loop. As one goes over the top gravity reduces the weight of the aircraft and occupants so for them to remain at + G the wings have to develop (downwards) lift that is equivalent of more than 1G when straight and level. The recovery is self explained.

Simples.

DR, good afternoon.

You may care to point out to your son that there are five forces acting upon an aeroplane during powered flight, no matter what the acceleration value.

In straight and level flight lift must exceed weight because the tailplane is generating a downforce for longitudinal stability.

You may also like to educate him in that g (lower case) is not a weight or force. Weight in itself is a force, but g is an acceleration.

I am sure that Sir Isaac Newton understood the latter statement, hence his second law of motion.

Feel free to PM me. :)

Weight = mass x Gravity

Gravity and mass don't change much (unless both burners are cooking over the top of a loop) = weight stays the same inverted or not.

Aerodynamically there is not much difference between a loop and a 9g tight turn..............zzzzzzzzzzzzzzzzzzzzzzzz

Oh no not another QFI thread!!!!

All you need to know is "When you push the stick forward, the Earth gets bigger and when you pull the stick back, the Earth gets smaller!"

Thats how bored I am on a bank holiday in Scotland:ugh:

Ah yes newt but if you keep pulling the stick back, the Earth will eventually get bigger again. Sorry mate, that's A1 stuff! :E

Schhh mate - Newt ain't a QFI.

Pull back a long way rapidly Newt and the earth not only gets bigger - it goes round in circles.

Smoke off - GO.

No matter what you do in an aeroplane the earth always gets bigger sooner or later!!!

The A1 was always a good line feature for aeros!!

stick? Stick?? Hardly QFI-speak, mon vieux!

00

I don't believe any of these replies are from real QFIs, otherwise they'd be in at least 3 colours (all I can remember, not being a QFI, is that lift is green).

As one of my UAS instructors used to say, "If it wasn't for your mother and a QFI you wouldn't be here."

I don't believe any of these replies are from real QFIs, otherwise they'd be in at least 3 colours

...don't you believe it.....

Personally, I always had to use the designated display line - at 500 feet.

If you pull back on the stick, you can go..

upwards:ok:
downwards (if on the point of stall):(
downwards (if inverted):uhoh:
downwards and backwards (if hovering):eek:
..and more, or less, sideways:confused:

..it's a tricky business!

... and can we also have:

"How do flies land inverted on a ceiling?"

AND

"What is the angle of bank at the highest point of a barrel-roll"


teeters (QHI) ;)

Personally, I always had to use the designated display line - at 500 feet.

Why so high?

Depends on their age. When I was a stude, QFIs used "talk & chalk" ......:{