Serious question for a QFI

[Diablo Rouge]

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.

[Fox3WheresMyBanana]

I'll PM you

[Sven Sixtoo]

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.

[jxk]

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.

[L J R]

zzzzzzzzzz....

[P6 Driver]

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

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

[Fareastdriver]

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.

[Lightning Mate]

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. :)

[ktk]

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

[newt]

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:

[Wholigan]

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

[Lightning Mate]

Ah yes newt but if you keep pulling the stick back, the Earth will eventually get bigger again.

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.

[ktk]

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!!

[BEagle]

...pulling the stick back...

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

00

[Ali Qadoo]

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."

[Lightning Mate]

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.....


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

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

[Fox3WheresMyBanana]

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!

[teeteringhead]

... 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) ;)

[ktk]

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

Why so high?

[spekesoftly]

I don't believe any of these replies are from real QFIs, otherwise they'd be in at least 3 colours
Depends on their age. When I was a stude, QFIs used "talk & chalk" ......:{

[Lightning Mate]

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

Zero you idiot. :E

The wings are level...................

The most important killer is:

Where is the nose????????

[E L Whisty]

Surely that should be 180 degrees LM.

Please forgive me for calling you surely.

[newt]

Er on the front of the aeroplane LM:ugh:

[Diablo Rouge]

To those who gave a serious answer / PM; Thank you very much. :D

To those in the playpen; :rolleyes:

[sycamore]

E L W, LM is correct; the wings are level. but inverted and the nose is above the horizon ,by the same amount it was at the bottom(lowest part) at the start of the `barrel`. If it`s not above you may end up lower and faster as you recover..

D-R; if you can maintain a constant speed around a loop,then the momentum will stay the same throughout,( if you fly an F-15/6/8,Typhoon etc). Most aircraft can`t so there is a loss of speed going up,due to gravity and Drag,mainly lift induced by pulling `G`,and then gained on the way down. If you try to maintain the `G` you will stall; ie if the stall speed is 80 kts in level flight,in a 4 G loop it is 160 kts(sq.root of load factor); practically, starting at 250 kts in a J-P,over the top ,the speed may be between 90-130 kts,but the `G` will be down to possibly less than 1,if you are trying to `shape` a vertical circle,and you will have gained about 3000 ft,depending on how hard you pull..
Now , in level flight ,starting and maintaining the same speed,the radius of turn of 1500ft,gives a bank angle of about 75-77 degrees for about 4G.
Is that good enough for a B2 ?

[newt]

Much more fun in the playpen DR!:ok:

[Sloppy Link]

Teeters,
I thought that if I pulled the stick back, I came to the hover, it's the uppy/downy lever on the left that makes the cows get bigger or smaller or have I been doing it wrong all these years?

[BEagle]

Now , in level flight ,starting and maintaining the same speed,the radius of turn of 1500ft,gives a bank angle of about 75-77 degrees for about 4G.
Is that good enough for a B2 ?

Close enough - I make it 74.8° AoB and +3.8G without resorting to a 4 colour board brief!

Next - explain (without hand waving) why the 'victory roll' was such a dangerous manoeuvre. Nothing to do with the tired old excuse of "You might have unknown combat damage", incidentally.

(Chopper pukes needn't bother to answer - not something they would understand!)

[Diablo Rouge]

I dare say that the Playpen is more fun, but all we need now is for Seldom to wear his Membership Police hat in here and it will have descended to the immature levels of much that surrounds it. It would be nice if the rubbish could be aimed at Jetblast and at least some mature conversation maintained without interfearance.

Beagle: One suspects fuel starvation as the Spit tends to cough a black cloud worthy of a Wessex when it is done to this day.

[BEagle]

Nope - the manoeuvre shouldn't result in -ve G!

Mrs Shilling later fixed the problem of the rich cut with her famous orifice!

[middleground]

Whats a victory roll?

[newt]

Well DR most on this thread are very mature with loads of experience! A little light hearted interference (I think thats how you spell it!) should be encouraged!

Maybe you should try Google if you want a serious reply to the question!:*

[teeteringhead]

(Chopper pukes needn't bother to answer - not something they would understand!)

Oh BEags .... as if it weren't* bad enough for my self esteem to be castigated as a (sniff) direct entry (sniff sniff) grammar school oik (sniff [you get the idea]) who drives a diesel car and owns a gas BBQ ....

...... but to be outed as a "chopper puke" (a naval expression surely?) :{

I am not worthy to share a forum with sky gods like yourself :{:{:{

But I am an A2 QHI :ok:


* Note correct use of subjunctive mood.

[noprobs]

There is another variation on the theme of which lever does what. With a substantial amount of thrust vectoring available, as in the VAAC Harrier, it is possible to introduce computer control that enables odd, but logical, responses. For instance, the right hand lever can control vertical movement, while the left controls fore-and-aft acceleration; both would have a central position for zero demand. Thus, to take off, pull the right hand back. If sufficient power-to-weight margin is available, thrust will be vectored to vertically down, and the engine will go to full power. When the required altitude is reached, put the right hand lever to the central position. To accelerate to wingborne flight, move the left hand lever forwards. When the required speed is reached, move the left lever to the centre. To slow down, pull it back to get the necessary combination of engine power, thrust vector and airbrake.

The above manoeuvres keep the aircraft in a constant attitude, so you would probably use a speed filter to get to a regime where the right hand controls attitude, enabling you to point weapons if necessary, and to make conventional turns using Gz at high speed.

Chopper pukes might argue that they have a logical control system, with a go-up/go-down lever that goes up or down, but they never need to concern themselves about transition to high speed flight.

;)

[BEagle]

Grammar school oik? Well, at least it wasn't a 'comprehensive' school....

Direct entrant? Not many other options these days.

Diesel car? Old chap, historically diesel was simply not an officer's fuel. But if it's powered by a high performance turbocharged engine, you can always say that the car is turbine powered and simply uses the reciprocating element as a gas generator.... Audi A5 3.0 Tdi quattro, for example. But people will still know....:hmm:

Helicopters? Someone has to do it, I guess.

A2? Definitely enhances the status and more than compensates for all the above. So mote it be...00

But gas? Gas?? Sorry, but you fell at the final hurdle!

Subjunctive? :eek: - that brings back memories of 'Ut and the subjunctive' and other terrors of prep school Latin lessons... I never did understand why Gerundives were considered 'attractive' either. If you didn't have to study such horrors at your school, you were very lucky.

Anyway, back to the dangers of the 'victory roll'....:(

Usually observed to be a high speed beat-up, balls-out on the deck, followed by a climbing aileron roll. As the aircraft runs in at high speed, it will very likely be trimmed for such speed. Pulling up, it will begin to decelerate. Full aileron and rudder to balance will cause a lot of drag, so it'll decelerate even more. It has now slowed down and the untrimmed state will tend to pitch the aircraft nose down as the roll progresses; the pilot may well pull against this, reducing the apex height accordingly. As the roll reaches the final wings level state, the aircraft will probably be descending and significantly out of trim. In addition, the attitude needed for level flight at the new speed will require a higher pitch angle than did the initial beat up. So the pilot may well need full aft control column to stop the descent and to oppose the out of trim state...:\...and recovery to level flight may well take longer than anticipated, or may not be achieved until terra firma prevents any further descent.


How many of us have seen Bloggs cock-up a barrel roll and bury the nose on exit....:uhoh:?

[teeteringhead]

BEagle, Sir, you are too kind (it is of course a TDi!)..... and I shall be replacing the BBQ next year so who knows what may not happen ....

...... but what about the fly landing on the ceiling? Is it from the top of a loop, or halfway around a (slow? hesitation?) roll? :confused:

[cazatou]

BEagle

"historically diesel was simply not an Officers Fuel."

Quite true - but that was in the days of the PCL when petrol was 50p a Gallon!!

[jindabyne]

DR

It would be nice if the rubbish could be aimed at Jetblast

Rubbish?! As an ex-A1 sky-god, and ex-display pilot, I can assure you that this is all serious stuff your'e reading here:(

[newt]

And you still need a QWI to keep an eye on you LM:ok:

Sky God! We all bow to you Jindy:cool:

[Lightning Mate]

....and there are quite a few A1s/A2s around here.....:)

...except Newt of course....:E

Plus one or two ex-display pilots....;)

[Lightning Mate]

Newt, good morning mate. :)

You are more clever than I thought - you replied before I posted!!!!!

[jindabyne]

newt

lower mate, lower;)

[Lightning Mate]

Newt doesn't do low mate!!

Spent a lot of his time up at 100 feet. :E

[airpolice]

Spent a lot of his time up at 100 feet.:D

Now that IS funny.

[newt]

Clearly I'm still hypoxic from the experience!!

Must be all the clear air in Scotland:ok:

[cats_five]

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


The sum of all lift in straight and level flight (positive and negative, from all parts of the plane) = gravity, otherwise you would be going up or down.

[Lightning Mate]

Owza fishin' mate?

[Lightning Mate]

The sum of all lift in straight and level flight (positive and negative, from all parts of the plane) = gravity, otherwise you would be going up or down.

Sorry buddy - lift is a FORCE and cannot equal gravity because that is an ACCELERATION.

Back to Sir Isacc Newton.

[Fox3WheresMyBanana]

Lightning Mate is right.

Lift is defined as a force.

However, formally, 'g' is referred to as the "acceleration due to gravity" to avoid this kind of argument.

The force on a mass caused by the acceleration due to gravity is, formally, weight.

The forces on an aircraft are in balance in straight, UNACCELERATED flight, which fits a constant rate of climb or descent as well as straight and level.

I haven't seen anyone correctly use momentum yet.
If you want a clear physical explanation of any of this, please PM me. I do this for a living.

Now, can we get back to interesting stuff like victory roll trimming and LL jokes?

F3WMB
MRAeS, MInstP

[ktk]

not much time to trim here!! Mind you if he got it wrong!!
Touch-roll-touch in a F-104 - YouTube

[airpolice]

Lightning Mate is right.



Fox3, ffs no more of that, it will go to his head and then we'll be right in the deep end.

SI Units. (Expressions to make you sigh)

The way I was tought is that Isaac Newton discovered Gravity when an Apple fell on his head. Four Apples = 1 Pound therefore 1 pound = four Newtons.

Kilowatts is what the Electric fire used to run on and a Litre's like a Metre only better 'cause it's wetter.

[BEagle]

The dear old Frightening was so ancient that its ASI was calibrated in 'furlongs per fortnight'....;)

Is there any truth in the story that the fuel gauge read 'Empty....Fumes....Not enough....Still Not Enough....All you're getting but really not enough'

Or that the altimeter read 'Ground pounders....Mud movers....Learning Command....Truckies....Air Defence Sky Gods and V-bombers only'?

[newt]

Close BEagle but no cigar!!:D

[Fox3WheresMyBanana]

From what I've seen, the Altimeter should be

Groundpounders, Buccaneers, War, Falkland Islands, and then a biiig gap before mud movers. Americans are somewhere above Learning Command.

Lightning top speed 4 million furlongs per fortnight (though that's an awful lot of AARs in a fortnight!)

[FCSoverride]

I think.... momentum is mass x velocity. Velocity is a vector, thus momentum is a vector too, i.e. it has both magnitude and direction.

What this means is that, even at constant "speed" momentum changes around the loop (it swings around with the aircraft's velocity). Newton said roughly that "the rate of change of momentum is directly proportional to the applied force and takes place in the direction of that force".

To loop the loop, we need an extra bit of force perpendicular to the momentum vector to unbalance the forces on the aeroplane, i.e. to accelerate the aircraft mass towards the centre of the loop. This is produced by pulling back on the stick to increase angle of attack, increasing the lift generated by the wings, whilst maintaining airspeed with the throttle.

[jindabyne]

Nobody's mentioned nibble (and flap) :ooh:

[The_Agent]

FCSOveride has a great answer for you right there.

I always like to relate answers to real world examples. Therefore: talk to your son next time you go around a roundabout. Ok, it's lateral 'g', but the principle is the same (get him to lean 90 degrees to the right if it helps).

Before you his the roundabout, there is no lateral 'g', right? Start going round it and he will be thrown to the outside of the car. Keep going round, and the 'g' force will remain. A loop or a break is similar.

In the case of a loop though, we already at 1 (not zero) 'g'. If I pull up into a loop with 4 'g', then that's what I will feel (initially). At the top of the loop I will be doing the same thing with my aircraft but now the plus 1 'g' I would normally feel is acting the other way so I would feel about 2 'g'. (3 'g' in the vertical). You need to picture "plus 1 'g'" acting vertically downwards at all times.

In truth no-one flies constant 'g' loops - it wouldn't look like a loop. A barrel roll is different.

For breaks, if you have the power, 4 'g' will remain the same forever. Depending on aircraft type and power available, it may reduce as, has already been pointed out, increased lift increases drag, reducing speed (unless you have more power).

[newt]

Watch out Jindy as you can be banned for being rude!! Nibbles and flaps is very close to the edge!:\

[teeteringhead]

And if, by chance, you have a helium balloon in the back of the car as you drive your son around the roundabout (traffic circle), he (and you) may be surprised at the way IT leans.........

[Lightning Mate]

not much time to trim here!! Mind you if he got it wrong!!

Did I detect a bit of roll/yaw coupling?

S'all abaht B/A ratio innit.

Morning Newt.

[newt]

Not sure you could get roll/yaw coupling in a F104 LM. More like a dart than an aeroplane!:ok:

[SOSL]

In a loop or a roll or a turn, what's the difference between centrifugal force and centripetal force; or are they the same?

[Lightning Mate]

Not sure you could get roll/yaw coupling in a F104 LM

On the one occasion I flew it, Newt, I was briefed carefully on max roll rate at high incidence.

"....watch the china, alpha..." ;)

[Fox3WheresMyBanana]

SOSL
Centripetal force is that required to keep an object moving in a circle, in an Inertial Reference Frame,i.e. seen from outside the aircraft. It acts towards the centre of the circle, and is usually generated by a component of lift (Bona mates can VIFF too).

Centrifugal force is that experienced by the crew, who are accelerating and thus in a non-inertial reference frame. Newton's Laws do not work directly in non-inertial reference frames (Einstein's do). This force acts outwards.

Or, in a thousand words....
http://imgs.xkcd.com/comics/centrifugal_force.png

[Arthur Rowe]

There are only three forces acting on an aircraft in flight. Thrust, weight and total reaction. Lift and drag are just 'lies told to children'. They are artificially defined components of total reaction (a much more valid and useful concept). Of course, take away thrust and there are only two.

[Lightning Mate]

....and I presume you believe that lift, sorry total reaction, is generated because of the venturi effect between a wing upper surface and the free stream air above it. :E

Now then - I know that even an elephant can fly

....seen Dumbo......?

[Fox3WheresMyBanana]

Arthur:

1 Aircrew ARE children. Groundcrew, in my experience, have always referred to the aircrew crewroom as the 'playpen'. Quoth the sign-in desk Sergeant, "What have the children done NOW?".

2 Since the use of physics in this case is to explain, then pontificating about total reaction is worse than useless if it doesn't lead to understanding.

3 Most reference data is provided as separate lift and drag data, so reassembling it into total reaction is inefficient.

4 Acting all superior about it ("lies") does nothing for the reputation of physicists as people worth listening to.

[Wholigan]

Not sure you could get roll/yaw coupling in a F104 LM


newt - it's all about B over A ratio and the 104 has a lot of one and hardly any of the other, so you could get into trouble more easily in a 104 than in most aircraft. That is unless you are a god like you and me mate! :E;)

[Fox3WheresMyBanana]

The clean Jaguar spinning (tumbling more like:eek:) video was a goodie - I think it went unstable in all 3 axes at the same alpha, 17 IIRC.

[jindabyne]

Now then - I know that even an elephant can fly

Don't talk about newt like that;)

That is unless you are a god like you and me

And you and I know different:O;

[Wholigan]

About newt yeah jindy! :E

[newt]

Oh no you don't:ok:


And I really thought you guys were being nice to me these days:{

[HowlingMad Murdock]

teeteringhead....here is a link regardin' your flyin' fly question...cool!
BBC - Bang Goes the Theory - Ask Yan - How does a fly turn upside down to land on a ceiling? (http://www.bbc.co.uk/bang/ask_yan/fly_upside_down_ceiling_land)
:)

[newt]

The real answer is that all flys are really QFIs in the afterlife still looking for an answer to the question....

The real answer is ... Nobody knows!:confused::confused::confused:

[Lightning Mate]

The real answer is ... Nobody knows!

But, Newt, I do of course. :p

[ktk]

The biggest problem in the 104 (apart from no lift) was actually negative g and a high roll rate. Bugging out of a fight was always interesting.

[Wholigan]

Just make sure you never have to bug out mate! ;)

How you doing T?

[ktk]

Good, still flying. :)

[teeteringhead]

Murdock - many thanks, that's a brilliant piece of film, but to me it seemed he (or possibly she) was still "wingborne" when the landing spot was grabbed.......:confused:

And as for Some species won't even be able to land on ceilings (for example, those that don't have any wings). ... then they aren't flies, they are "walks"! Tata boom!

[Wholigan]

Lucky you ktk and good on you. Ageism struck with me sadly.

[jindabyne]

No it didn't Roj - you got friggin old like the rest of us :eek:

[newt]

Jindy.......He's always been old compared to you and me:ok:

[Arthur Rowe]

Lightning Mate

No, that is another lie to children. Don't presume, check. I go with Newton not Bernoulli. See below:

How Airplanes Fly (http://www.allstar.fiu.edu/aero/airflylvl3.htm)

Fox 3

1. Yes, heard that. Very funny.

2. Not pontificating. Perhaps you should be not so set in your assumptions. Greater knowledge leads to greater understanding.

3. More important is relationship between lift and drag (Cl/Cd), which directly relates to Total Reaction.

4. Hardly being superior. Maybe you have not heard the expression 'Lies to children' before. The link below is interesting. I admit that I am not a student of Wittgenstein - more a student of T Pratchett, and I first read about the concept in 'The Science of Discworld'. Valid nonetheless.

Lie-to-children - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Lie-to-children)

[Fox3WheresMyBanana]

Ah!, Hadn't read the Pratchett thing. Many thanks- I had a chat with him about 3 years ago. Very interesting guy.

Still think you are flogging a dead horse with the total reaction thing. Always best to answer in a context people already understand,and not to appear too clever .

Remember a question on Hawk Groundschool exam.

'Does pressure increase or decrease in a convergent nozzle?'

Me: "Sir, it depends. Is the flow subsonic or supersonic?"

QFI "If you're smart enough to ask that, then you're smart enough to know the answer they want"

Wound my neck in after that!