4 forces acting on unaccelerated, straight&level flight?

[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 unrelated

Someone once said you should make everything as simple as possible, but no simpler.

[Lasiorhinus]

Originally Posted by muffman

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.

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.

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

[CaptainInsaneO]

Thrust/Drag & Weight/Lift

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]

KIS

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]

Originally Posted by FGD135

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]

Originally Posted by FGD135

.... 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