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Max Trust Vs Mtow  
User currently offlineThvgjp From Ukraine, joined Mar 2002, 158 posts, RR: 0
Posted (10 years 6 months 3 weeks 3 days 9 hours ago) and read 2319 times:

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

22 replies: All unread, jump to last
 
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 1, posted (10 years 6 months 3 weeks 3 days 9 hours ago) and read 2304 times:

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...  Big grin

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineSlamclick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 2, posted (10 years 6 months 3 weeks 3 days 9 hours ago) and read 2301 times:

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.
User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 26
Reply 3, posted (10 years 6 months 3 weeks 3 days 7 hours ago) and read 2258 times:

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


User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 4, posted (10 years 6 months 3 weeks 3 days 1 hour ago) and read 2221 times:

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.
User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2578 posts, RR: 17
Reply 5, posted (10 years 6 months 3 weeks 3 days ago) and read 2219 times:
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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...


User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 26
Reply 6, posted (10 years 6 months 3 weeks 2 days 20 hours ago) and read 2198 times:

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


User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2578 posts, RR: 17
Reply 7, posted (10 years 6 months 3 weeks 2 days 13 hours ago) and read 2171 times:
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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  Smile/happy/getting dizzy


User currently offlineKGAI From United States of America, joined Dec 2003, 120 posts, RR: 0
Reply 8, posted (10 years 6 months 3 weeks 2 days 11 hours ago) and read 2161 times:

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.


User currently offlineKGAI From United States of America, joined Dec 2003, 120 posts, RR: 0
Reply 9, posted (10 years 6 months 3 weeks 2 days 11 hours ago) and read 2153 times:

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.


User currently offlineKGAI From United States of America, joined Dec 2003, 120 posts, RR: 0
Reply 10, posted (10 years 6 months 3 weeks 2 days 10 hours ago) and read 2150 times:

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?


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 11, posted (10 years 6 months 3 weeks 2 days 10 hours ago) and read 2140 times:

KGAI,
hardly AE, more like physics.  Smile

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.
User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 26
Reply 12, posted (10 years 6 months 3 weeks 2 days 10 hours ago) and read 2145 times:

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



User currently offlineKGAI From United States of America, joined Dec 2003, 120 posts, RR: 0
Reply 13, posted (10 years 6 months 3 weeks 2 days 10 hours ago) and read 2138 times:

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.


User currently offlineKGAI From United States of America, joined Dec 2003, 120 posts, RR: 0
Reply 14, posted (10 years 6 months 3 weeks 2 days 9 hours ago) and read 2128 times:

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.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 15, posted (10 years 6 months 3 weeks 2 days 7 hours ago) and read 2109 times:

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.
User currently offlineKGAI From United States of America, joined Dec 2003, 120 posts, RR: 0
Reply 16, posted (10 years 6 months 3 weeks 2 days 7 hours ago) and read 2101 times:

very nice explanation FredT, thanks

User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2578 posts, RR: 17
Reply 17, posted (10 years 6 months 3 weeks 2 days 7 hours ago) and read 2098 times:
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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.

User currently offlineKGAI From United States of America, joined Dec 2003, 120 posts, RR: 0
Reply 18, posted (10 years 6 months 3 weeks 2 days 6 hours ago) and read 2091 times:

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.


User currently offlineTimz From United States of America, joined Sep 1999, 6798 posts, RR: 7
Reply 19, posted (10 years 6 months 3 weeks 2 days 5 hours ago) and read 2075 times:

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?

User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 20, posted (10 years 6 months 3 weeks 2 days 3 hours ago) and read 2059 times:

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.
User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2578 posts, RR: 17
Reply 21, posted (10 years 6 months 3 weeks 2 days ago) and read 2052 times:
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Yes, sorry. I must've gotten confused or something. My teacher sucks at teaching and i got my semester final tommorow :-(

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 22, posted (10 years 6 months 3 weeks 1 day 22 hours ago) and read 2050 times:

"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.  Smile

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.
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