4 forces acting on unaccelerated, straight&level flight?

[AvEnthusiast]

As FAA handbook of aeronautical knowledge states that in a straight and level unaccelerated flight the Thrust is equal to Drag and Lift is equal to Weight, but the lift and weight is greater than thrust and drag. I have been asked why lift & weight is greater than Thrust and drag?

[VH-XXX]

I'm happy to be corrected.....

Think of it like this-

If your plane was a helicopter and the prop was at the top, it would NOT have enough lift/thrust to take off. Therefore lift / weight MUST be greater than drag and thrust.

As for WHY they are greater... gravity.

[YoDawg]

If the earth was in a vacuum, the prop wouldn't work!! The engine wouldn't neither..... sorry couldn't help myself.

Well anyway the aircraft still has mass and inertia to overcome.

[Arm out the window]

Sounds a bit simplistic perhaps? Lift is going to equal weight no matter how fast you're going S & L, but if an aircraft with a great big donk sets a thrust greater than the equivalent of its weight, it'd just keep accelerating - drag builds up with V squared until it equals thrust, and both of them are bigger than L or W. Wouldn't work for your average bug smasher but.

[mudpig]

An airfoil requires a certain amount of velocity of airflow to create the pressure differential between the top and bottom surface required to equalise the lift and weight components. The actual difference in pressure is quite small. Aircraft are designed aerodynamically therefore less drag equals less thrust required.
Anyways, it is much easier to move horizonally than vertically.
So, you may notice that your rated output of thrust is quite a bit less than your weight (if you are able to convert the two into a single unit of measurement).
E.G. A mate and I only yesterday were considering thrust outputs on a 747-300 with RR RB211-524D4 engines which had a rated thrust of 51980lbs minimum times 4 engines.
Now if anyone figures something different I'm happy to listen and be corrected if I'm wrong.
This equates to a little over 94 tonnes of thrust total. Awesome.
The 747 MTOW is almost 400 tonnes. Awesome.
Hope that helps.:ok:

[KRviator]

The simple answer is your average light airplane weighs around 3000 lbs, therefore needs 3000lbs of lift.

However, it DOES NOT have 3000lbs of drag, therefore does not need 3000lbs of thrust.

Enter the L/D ratio, which for a half decent airplane is aroundabout 10/1, with the usual variables of manufacturer, flap deployment and all that other rubbish.

So, if your AFM says your L/D ratio is 10/1, and you weigh 3000lbs, your thrust and drag components are 300 lbs each, in S&L flight, and that is why the four forces are not equal to each other.

[Icarus2001]

Lift/Drag ratio of the wing...

You build an aeroplane that weighs X designed to carry Y people etc. Physics shows that Lift has to equal Weight. The ratio of Lift to Drag is defined by the characteristics of the wing...say 10:1...also gives approx glide ratio. Thrust only ever needs to be "a bit more than" drag (over simplification) so those two forces will only ever "need" to be a small proportion of the Lift and Weight.

By the way, there are FIVE forces...CASA.:hmm:

[18-Wheeler]

FWIW in S&L flight lift is slightly more than weight.
The reason is that it needs to counteract the downforce created by the tail, which is used to generate dynamic stability.

[Lasiorhinus]

18-Wheeler - true, but the downforce from the tail acts downwards, increasing the weight of the aircraft in flight, (but making no change whatsoever to the mass of the aircraft).

[muddergoose]

Back to AvEnthusiast's Question:

Have you ever tried to pick a motor vehicle up as opposed to pushing it?

:8

[HarleyD]

Lasiorhinus is correct.

even on small aircraft the stall speed increases with a fwd cog, that is why FAR 23 requires the stall speed to established (amongst other things) at the forward limit at MTOW. other handling issues are required to be investigated at the forward regardless cog.

Net lift will equal weight, ie subtract the downward 'stabilising' force from the upward lift produced by the mainplane(s) and fuselage and the net will be the same as weight, the airacrft just thinks that it is heavier though mass remains unchanged. Also consider that the Vh is reduced at fwd cog due to the increased drag due to the added trim drag of the tailplane and that the increased total lift (up and down) produces added induced drag, ergo slower speed for same thrust. aft cog will reduce Vs0 and increase Vh in all aircraft, the larger the aircraft the more quantifiable this becomes.

HD

[18-Wheeler]

18-Wheeler - true, but the downforce from the tail acts downwards, increasing the weight of the aircraft in flight, (but making no change whatsoever to the mass of the aircraft).

Um, that exactly what I wrote mate.

[AvEnthusiast]

Good goose,

you made it so simple. Thanks for all the replies.

[Lodown]

lift and weight is greater than thrust and drag

A little hard to believe when you're looking at a BAE 146.

[FGD135]

I think everybody is missing the point of the question.

There is no rule that says L&W must be greater than T&D. The two sets of forces are completely unrelated.

The T&D could be greater than the L&W of course, but that would be a rare aircraft to possess that amount of thrust - but note that there is no law of physics that says the T&D cannot be greater than the L&W.

Why are the main wheels bigger than the nosewheel? Same sort of question. They don't HAVE to be, but just usually are.

[jimshutt]

What happens to those forces with the flap application?

[Icarus2001]

The two sets of forces are completely unrelated.
You may want to have a little think about that statement.

[FGD135]

You may want to have a little think about that statement.

Ok, I've had a little think about it, and I report that, not only was the statement accurate, I will add that ALL 4 FORCES ARE UNRELATED.

Yes, completely separate and unrelated.

To prove that statement, consider an aircraft in unaccelerated, "straight and level" flight. Then, go about removing each of the forces, and see whether their removal has any effect on the other forces.

Of course, removing one force causes a change to the flightpath, which then causes the other forces to change, but note that it was the flightpath change that caused the changes to the other forces - NOT the loss of the original force. This is the proof that the forces are unrelated to each other.

To the original question that this thread is based on: it is possible to build an aircraft, where, in unaccelerated "straight and level" flight, the T&D are greater than the L&D. In fact I wonder if some military aircraft can already do this.

To build your own, obtain an engine with plenty of thrust, say one of the current generation of turbofan engines that can do 100,000 lbs of thrust. Then, attach it to an airframe that weighs less. Think of a tiny airframe, with a tiny fuel tank and that great big engine.

Take it for a flight and have the thrust set at maximum. You will be going fast, and if your airframe (and fuel) weighs less than 100,000 lbs (about 45,000 Kg) then your T&D is greater than your L&W.

Again I say, there is no law that says the L&W must be greater than the T&D.

I think this question has arisen from the fact that the former set is USUALLY greater than the latter.

[tio540]

Also Lift is created by noise, the more noise the more lift. :ok:

[grusome]

FGD135
Yup, and at about M3 you would finally reach equilibrium - you would need supersonic drag to counter that thrust.
I think that what the FAA (or indeed our original poster) forgot to say was that the comment is applicable to GA light aircraft as commonly certified.

[muffman]

To prove that statement, consider an aircraft in unaccelerated, "straight and level" flight. Then, go about removing each of the forces, and see whether their removal has any effect on the other forces.With 30 seonds of thought I would put it to you that the removal of lift would reduce induced drag to zero.

Yes, completely separate and unrelatedSomeone once said you should make everything as simple as possible, but no simpler.

[Lasiorhinus]

With 30 seonds of thought I would put it to you that the removal of lift would reduce induced drag to zero.

True. But have you reduced drag to zero?

[Lodown]

Remove drag and we have lightspeed in an instant. Arthur C. Clark would have loved it.

[Lasiorhinus]

Not quite "in an instant". We still have inertia.

A spacecraft in flight, in outer space, experiences no drag.

They still don't fly at the speed of light.

[TastyBurger]

and the inertia will continue to increase as you approach the speed of light... boom-tish!

gah.

[muffman]

True. But have you reduced drag to zero?

No way - but you've certainly taken a significant chunk out of it!

FGD135's original comment suggested that each force has no effect on the others:
and see whether their removal has any effect on the other forces.

[ChristiaanJ]

AvEnthusiast,
You still looking for a simple answer?

CJ

[Disco Stu]

Icarus2001 reckons there is a 5th force.....CASA.:yuk:

Actually only CASA believe that and besides, the management of the relationship between the Lift, Weight, Thrust and Drag forces are the only Aviation Laws that actually matter to us a pilots.

All the rest of the reams of 'bumpf' in the form of Acts and Regs etc keep bureaucrats in employment and matter little in the real task of flying an aircraft.

Regulating the aviation environment, industry or whatever is unfortunately a necessary evil we are stuck with. It can never be said that CASA is an authority on safe civil aviation. A Regulator yes and that's all. :ugh:

In the pre politically correct and sanitised days "drag" usually refered to a male pilot wearing a 'different' style of clothing:E

[CaptainInsaneO]

I agree, there is nothing to say that the magnitude of Lift and Weight must be greater than the magnitude of Thrust and Drag. There was one of these things flying around the other day, somewhere near North Korea, they called it a Rocket.

Fixed wing aircraft however do have the magnitude of Lift/Weight greater than the magnitude of Thrust/Drag. If you didn't, your glide performance wouldn't be too good if you had a flame out.

[Oktas8]

why lift & weight is greater than Thrust and drag?

Here's the simple answer I think AvEnthusiast wanted.

Making lots of thrust is extremely expensive in fuel. Therefore thrust & drag are kept as low as possible (outside the military).

Current technology in any civil aircraft allows a designer to economically generate about ten times as much lift as drag. So the aircraft will be able to lift ten times as much weight as it generates in drag.

That's why lift/weight is usually much bigger than thrust/drag.

KIS :)

[FGD135]

With 30 seonds of thought I would put it to you that the removal of lift would reduce induced drag to zero.

By "induced drag" you do, of course, mean that component of lift that is aligned with the drag vector. (The lift vector is inclined "rearwards" and can be resolved into components that are, 1, opposite the weight vector and, 2, in alignment with the drag vector).

Is that really drag? Good question. All the textbooks treat it as a drag as that is more convenient when considering total drag.

But, in the strict sense of the word, a force is a vector property, acting from a point, in one direction only, so the "drag like" effect of the lift can only be due to the "rearwards" component of lift - not some separate drag forces.

[ChristiaanJ]

By "induced drag" you do, of course, mean that component of lift that is aligned with the drag vector.As usual, if one doesn't define one's terms clearly before, one can argue until the cows come home.

For the purpose of this subject, steady level flight, it's much easier to reason with weight acting down, and lift, thrust and drag acting with reference to the flight path, i.e., lift upwards, thrust forwards and drag backwards, in this case.

Dragging induced drag into the discussion only confuses matters (pun intended).
Not in the least because the induced drag is only one of the components of the total drag of an aircraft, and not quantified that easily.

AvEnthusiast,
To be fully correct, your FA book should have stated that thrust & drag are much smaller than lift & weight for conventional aircraft.

By moving a 'typical' wing through the air with a 'normal' angle of incidence, you can produce far more lift than drag - just as well, or aviation would never have gotten where it is now!
Even adding a fuselage, etc., you still can for a given amount of drag easily generate ten times as much lift. So a 100,000lbs aircraft will need only 10,000lbf thrust.

For some sailplanes this lift/drag ratio can be up to 20:1 and even much more.

For unconventional shapes, such as "lifting bodies" (and to some extent the space shuttle), you're at the other end of the range, with lift/drag ratios as low as 3:1.

And for an extreme case, take a toy balloon, fill it with helium, hang a small weight on it until it doesn't rise or sink: lift = weight. Now tow it behind a car. Pull=thrust and drag will now be much more than the lift and the weight!

All this to stress that there is no direct relation between lift and drag. It totally depends on the shape and design of the craft you're talking about.

CJ

PS1 : Some wing shapes (deltas, for instance) can be dragged through the air at very high angles of attack and still produce lift. However, by that time the drag becomes of the same order as the lift.

PS2 : One red herring I noticed earlier. The lift we are talking about here is the sum total of the lift of the wing and the lift or downforce of the tail. In steady level flight the resultant force passes through the centre of gravity.

[Oktas8]

I resisted the urge to say something last night, but now I have to post...

I think you are confused about some basics FGD135.

ALL 4 FORCES ARE UNRELATED

No. Lift and drag are two components of the total aerodynamic force acting on the various parts of the aircraft. It is nonsense to suggest that two components of the same force can be independent. To put it another way, the lift & drag [edit] quoted in textbooks are merely a convenient way to describe the real world, and the two cannot be separated in the real world.

By "induced drag" you do, of course, mean that component of lift that is aligned with the drag vector. (The lift vector is inclined "rearwards" and can be resolved into components that are, 1, opposite the weight vector and, 2, in alignment with the drag vector).

A common definition of lift is "the aerodynamic force acting perpendicularly to the relative air flow". The definition of drag is "the force opposing motion".

It is a contradiction in terms to have a "rearwards component of lift". By definition, lift is perpendicular to the direction of travel. It is convenient to describe induced drag by imagining a localised lift vector being inclined rearwards, so the student can "see" the drag increasing. But remember that a force inclined rearwards is actually a vector sum of lift & drag.

Any component of any force that acts rearwards (parallel to the airflow) is by definition drag. Labelling it "lift dependent drag" or "lift independent drag" is just assigning a cause or a reason for the force, not changing the force.

Regards to all,
O8

[freshy1234]

What happens to those forces with the flap application?

When the flaps are out there is an increase in drag therefore slowing the aircraft down if the same amount of thrust is still being produced. But flaps allow more lift to be produced at lower speeds (i.e. landing)

[FGD135]

Oktas8, on the question of whether the 4 forces are unrelated:

... the lift & drag [edit] quoted in textbooks are merely a convenient way to describe the real world, and the two cannot be separated in the real world.
If you are talking about components of forces, then yes, of course they are related and cannot be separated. And, you are talking about components when you adopt this kind of definition:
A common definition of lift is "the aerodynamic force acting perpendicularly to the relative air flow".

But, we are not, strictly speaking, talking about the components. We are talking about the forces themselves (refer to the title of the thread) and whether they are unrelated.

By definition, they have to be unrelated. This is because the definition of a force stipulates that it acts at a point, and in one particular direction. So it can't be acting in two directions!

The definition of drag is "the force opposing motion".
A definition of convenience - but not a satisfactory definition of the drag force. By the definition I gave above, the drag force cannot include other forces (or their components).

But remember that a force inclined rearwards is actually a vector sum of lift & drag.You're putting the cart before the horse to express it that way round. The lift force "comes first" - so to speak, and is inclined rearwards.

This means that the "total drag" then, is the sum of the drag force and the components of any of the other 3 forces.

Yes, I know the mathematics makes no such distinctions, but the mathematics deals exclusively with the components of the forces.

Semantics? Splitting hairs?

[ChristiaanJ]

.... Semantics? Splitting hairs?

Not at all, IMHO.

Most of these discussions, using "word images" only, lead to confusion, unless the terms used are first defined clearly and unequivocally.

And of course, in this given case: level unaccelerated flight, the vector sum of all the forces, however defined, is... yes, zero.

CJ

[Oktas8]

Hi FGD135,

I'll stick by what I said, although I will say that words are subject to misunderstanding. Following ChristiaanJ's post, I'll suggest that Lift is always perpendicular to the relative airflow - vertically up in S&L, and drag is always parallel to the relative airflow - horizontally rearwards in S&L. That said though...

However, it's almost impossible to have this kind of discussion without using pictures - which I can't put on here... So perhaps it's best not to continue arguing about the independence of lift & drag - at least, unless we have some pictures!

Cheers,
O8