thvgjp
Topic Author
Posts: 147
Joined: Fri Mar 01, 2002 8:17 am

### Max Trust Vs Mtow

Please explain to me how a airliner can move based on thrust alone? By this I mean if for example a 717 ay a mtow of 110000 lbs can move on the ground and in the air with 2 engines giving a max trust of 18500lbs each for a toatl of 37000 lbs of thrust. This may be a stuipid question but I realise that a force applied to any object gives an opposite and equal force. Thus 37000 lbs of thrust would be hard to move 110000 lbs of aircraft, it is obvious I am missing something here, but exactly what I dont know

Thanks
Glen

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Max Trust Vs Mtow

The force is always equal to the mass times the acceleration.

Or, to put it another way: The acceleration is always equal to the mass divided by the force.

You can push a 1300 kg car, right? Just leave it in neutral and release the handbrake. But I don't think you can benchpress 1300 kgs...

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

### RE: Max Trust Vs Mtow

The same way I (a fat old man) can push my 4000 lb car.

The weight is straight down, it is gravity. Moving it along a horizontal surface is not gravity, it is rolling resistance based on a number of things including the energy required to flex the sidewalls of the tires.

You are correct in assuming that the B-717 would not take off from a standing start going straight up.

In the first day of ground school we learn about the "four forces" acting upon an airplane.

1. Weight or gravity.

2. Lift generated by wings or rotors (or a shaped body) to oppose gravity.

3. Thrust generated by engines and applied by propellers, rotors or jet exhaust/fan efflux.

4. Drag The resistance of the air flowing over the parts of the aircraft.

The engines do not make the thing fly. Wings or rotors do that. The engines exist to push the wings or rotors through the air. Sailplanes fly better, in some respects, than powered airplanes.

Happiness is not seeing another trite Ste. Maarten photo all week long.

QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

### RE: Max Trust Vs Mtow

As Slamclick and FredT have said, you're not counteracting the weight with the thrust. The lift produced by the wings of the a/c does that, while all that the thrust is overcoming is friction between the landing gear and the runway (which, incidentally, is a function of the weight of the a/c), and the total drag of the airplane.

qantasA332

lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

### RE: Max Trust Vs Mtow

Most folks confuse this, they think the wing holds the fuel and the engines hold the plane in the air. Considering how often regular folks spend looking at an airplane, I think most forget the wing altogether cuz they always see it from the side profile...

To summarize what everyone said:

Wing lifts plane, engine thrust pushes the plane along.

The meaning of life is curiosity; we were put on this planet to explore opportunities.

sovietjet
Posts: 2592
Joined: Sat Mar 15, 2003 12:32 am

### RE: Max Trust Vs Mtow

Basically when the engines push a plane they first have to overcome the static friction to get the plane moving and then make a force bigger than the normal friction to keep the plane moving. If the force applied is exactly equal to the force of friction the plane would have a constant velocity, if the force is bigger it accelerates.

Ff = M(Fn)

M=coefficient of friction between tires and runway
Fn= Normal force, mass times the acceleration due to gravity(-9.8m/s2)

Therefore a plane weighing 110000(B717) lbs(49830 kg) would have an Fn of 488334 N acting on it. If the coefficient of friction is 0.3 then

Ff=0.3(488334)
Ff=146500 N of friction resistance

The engines apply 164500 N of force(max thrust of B717 engines) then

164500 - 146500 = 49830(a) a= acceleration

And the plane would accelerate at 0.36m/s2. If my coefficient of friction is right that would be how fast a B717 accelerates at but I don't know the coefiicient of friction which depends on tires, runway surface, etc...

QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

### RE: Max Trust Vs Mtow

Approximate coefficients of friction between a/c landing gear and runway are as follows:

Dry Concrete: 0.7
Light Rain: 0.5
Heavy Rain: 0.3
Snow or Ice: 0.1 to 0.2

Sovietjet, as I said in my first post, you musn't forget that friction isn't the only thing the thrust is counteracting -- it has to overpower total drag of the airplane as well...

qantasA332

sovietjet
Posts: 2592
Joined: Sat Mar 15, 2003 12:32 am

### RE: Max Trust Vs Mtow

The total drag of the airplane is the friction plus the air resistance isn't it? Anyways the air resistance in the beginning of the takeoff run is very little and I don't think it does too much to slow the airplane down while it takes off. Headwinds and tailwinds would also affect the acceleration I think. I have to correct my above post because i forgot to multiply the force of the engines by two. Therefore the total thrust isnt 164500 but 329000

329000 - 146500 = 49830a

a= 3.66m/s2

And with that acceleration it would need about 1100m(about 3650 feet) of runway on a day with heavy rain

KGAI
Posts: 118
Joined: Tue Dec 23, 2003 5:35 am

### RE: Max Trust Vs Mtow

Sovietjet and QantasA332

I think you 2 are talking about the wrong friction. You don't need to break the friction between tires and ground (unless you want the plane to skid aorund). You're trying to break the friction of the gear axles to get the wheels turning. I won't even venture to guess how much that is.

The other thing the plane has to overcome is momentum. That's about it at low speeds.

KGAI
Posts: 118
Joined: Tue Dec 23, 2003 5:35 am

### RE: Max Trust Vs Mtow

This thread has got me thinking, how *do* airplanes fly with less thrust than weight?

A plane flies by turning air downwards with the wings. So, let's say that a 10,000lb plane has 3,000lbs total thrust. And let's assume there is no form drag.

So, you have 3000 lbs force going into the wing, which in turn generates 10000 lbs force to lift the plane. Where is the extra force coming from?

I think if nobody answers I will start a new thread, as this seems important.

KGAI
Posts: 118
Joined: Tue Dec 23, 2003 5:35 am

### RE: Max Trust Vs Mtow

A wing generates lift by turning air downwards. This process creates drag. So how is it that the drag is not so great that it overcomes the force of the engine?

Looking at the drag equation, http://en2.wikipedia.org/wiki/Drag_equation I see that it depends on velocity (of course). So can it be that by giving a lot of air a little bit of speed in the y-axis, you end up with more force than by just pushing with the engines?

That scenario seems similar to turbofans having more thrust than turbojets, even when using the same core. ie, it's more efficient to get your thrust by move a lot of air slowly than by moving a little bit of air quickly.

I know there are aerospace engineers on this forum. Care to enlighten me?

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Max Trust Vs Mtow

KGAI,
hardly AE, more like physics.

Go back to the car example. You are able to push the car up a slope without being able to lift the car.

The aircraft is effectively pushing the air downwards as it passes by, with a force equal to the weight of the aircraft. The airflow is turned slightly down, but there is really no requirement per se for the aircraft to exert a force pushing the air forward to generate lift.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

### RE: Max Trust Vs Mtow

So, you have 3000 lbs force going into the wing, which in turn generates 10000 lbs force to lift the plane. Where is the extra force coming from?

Depending on the total wing area of the a/c, 3000 lbs X S (total wing area) would always be greater than the weight. It isn't just 3000lbs going into the wing...

Also, KGAI, keep in mind that there are a few different types of drag:

--Skin friction drag, or the drag due to the frictional force of air flowing across the various surfaces of the wings, tail, fuselage, etc.
--Pressure drag, which arrises due to a wake of seperated flow behind the airplane and its various parts. Because this wake is relatively unmoving, it has a lower pressure than the appropriate air in front of the aircraft, causing front to rear drag.
--Induced drag, or drag due to lift, basically. Air from the bottom of the wing flows around the tip to the lower pressure region on top. This essentially "tilts" the lift vector back by pushing the downstream air down, causing it (the lift) to be less efficiently cancelling out weight.

Those are the main three. Other types of drag include Interference drag (from the juncture of two bodies on the aircraft), Cooling drag (associated with engine cooling), and Profile drag (a seperate category for the wing's parasite drag).

Thus, drag doesn't just depend on velocity, as you thought, KGAI. The equation for total drag is:

D=(CDp X q X S) + (CDi X q X S)

where D is total drag, CDp is the coefficient of parasite drag, q is dynamic pressure (1/2 X density X V squared), S is the total wing area, and CDi is the coefficient of induced drag. With that you can see that a number of things affect drag on an airplane, and if you were to formulate accurate figures to plug into the equations for both lift and drag, you would see that lift should be greater...

That scenario seems similar to turbofans having more thrust than turbojets, even when using the same core. ie, it's more efficient to get your thrust by move a lot of air slowly than by moving a little bit of air quickly.

The reason turbojets are less efficient is not drag related -- in the process of accelerating a small amount of air to great KE, a lot of energy is lost. That's it, I think...

Hope that helps,
qantasA332

KGAI
Posts: 118
Joined: Tue Dec 23, 2003 5:35 am

### RE: Max Trust Vs Mtow

So FredT, you're basically saying that at speeds where a plane is able to generate enough lift to fly, the air is basically acting like a solid?

At least, solid enough that the plane can't just fall through it. I guess that makes sense. The air was already there before the plane arrived. All the plane has to do is keep moving to find some more air.

BTW, do you know what the rough percentage is of drag that is induced drag? At cruise configurations for a commercial aircraft.

KGAI
Posts: 118
Joined: Tue Dec 23, 2003 5:35 am

### RE: Max Trust Vs Mtow

QantasA332,

Nice post. Very comprehensive.

It's just that I already ruled out all the other types of drag in my theoretical example. And your explanation of turbofan efficiency is basically the same as mine.

I was questioning why it appears that the wing puts out more work than goes into it. But I think I was approaching the question wrong.

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Max Trust Vs Mtow

KGAI,
no, in that respect the ramp example is poor. A ramp will not move away. However, the physics are similar. The wing exerts a force on the air equal to the weight of the aircraft, and the air keeps the wing up in return.

A better similarity would be pulling one of those rolling kid toys on a string over the keys of a piano. Picture a quite heavy toy, so that it will press the keys down if left standing at one point. But if you pull it along the keys fast enough, it will not push each key to the bottom but depress them all slightly as it goes along. That is just what the wing does to the air, it pushes every air "parcel" it passes down slightly and in return does not sink.

Further, do you think you would have to pull the toy with as much force as you would need to use to lift it? I think not, if it is equipped with the right wheels.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

KGAI
Posts: 118
Joined: Tue Dec 23, 2003 5:35 am

### RE: Max Trust Vs Mtow

very nice explanation FredT, thanks

sovietjet
Posts: 2592
Joined: Sat Mar 15, 2003 12:32 am

### RE: Max Trust Vs Mtow

KGAI - but friction is what makes a car move. If Ff is bigger than the engine force it wont move. My physics teacher has explained this to us thoroughly.

KGAI
Posts: 118
Joined: Tue Dec 23, 2003 5:35 am

### RE: Max Trust Vs Mtow

Sovietjet,

Friction is what makes a car move. That's why you don't break it. If you overcome the friction between tire and road, that means the car is skidding.

The friction we are talking about here is friction within the plane's mechanism, that prevents the wheels from turning. Break that friction and the wheels roll. Break friction between wheel and ground and the wheels skid.

timz
Posts: 6323
Joined: Fri Sep 17, 1999 7:43 am

### RE: Rolling Vs Skidding

Sovietjet, you said thrust had to be 0.3 times the plane's weight just to overcome friction. If that were true, you could park the plane on a 25% grade, forget to set the brakes, and it wouldn't roll-- right?

lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

### RE: Max Trust Vs Mtow

Remember there is static friction and dynamic friction. The static prevents motion from zero and the dynamic is the resistance once the thing you're pushing starts to move.

Which is why takeoff use more fuel per interval that actual flight.
The meaning of life is curiosity; we were put on this planet to explore opportunities.

sovietjet
Posts: 2592
Joined: Sat Mar 15, 2003 12:32 am

### RE: Max Trust Vs Mtow

Yes, sorry. I must've gotten confused or something. My teacher sucks at teaching and i got my semester final tommorow :-(

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Max Trust Vs Mtow

"Which is why takeoff use more fuel per interval that actual flight."

Er, no. You use more fuel during takeoff as you are running the engines full shaft to get the aircraft accelerated and climbing.

Whatever you use all that thrust for won't change the fuel consumption. However, fuel consumption goes down as the air density goes down.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

### Who is online

Users browsing this forum: Dispatcher9999, drdisque and 22 guests

### Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos