Does the Thrust need to be greater than the weight?

[xFlankerx]

I have to say I only know the basics of how a plane works. But one thing I'm still wondering about; Does the Thrust generated by a plane's engines need to be greater than the weight of the plane, for it to move, and thus take off?

Looking at the specs for a Boeing 747-400, I see that it weighs 800,000, but the engines produce 63,300 lb of thrust (is this for one engines, or all 4?).

This may be a "Duh!" moment for you guys, but bear with me :)

[TheOddOne]

The only time you need the thrust to exceed the mass (better term than weight) is if you want to go vertically upwards, i.e. like a space rocket or a military jet at an air display.

In steady flight, mass is balanced by LIFT not thrust. The opposing force here for thrust is DRAG

Hope this helps a little.

Try searching this site for 'Principles of Flight' or Google for oddles of good stuff.

Cheers,

The Odd One

[xFlankerx]

Awesome, thanks. Yea, my problem came from the military display example, where the Thrust to Mass ratio came into play.

Thanks again.

[Milt]

For the high performance aircraft climbing vertically at constant IAS the thrust has to be equal to the weight PLUS the drag!!

[TheOddOne]

Milt,

Nice one, but it'll only be form & parasitic, shouldn't be any induced drag as no lift is being generated (if the Zero Lift Angle is being 'flown')

Cheers,
The Odd One

[dartman]

.... hmmm, without giving it too much thought, I think the wing is rather indifferent to the path of the a/c relative to the earth, so will still be generating lift, whether or nor the a/c is straight and level, inverted, vertical, etc. (assuming a positive A of A) So say an a/c in a vertical trajectory would all things being equal, have the nose slightly off vertical so the 'weight' could counteract the lift vector, or be flying with a A of A of exactly zero. Where the hell is my copy of Kermode when I need it.
Anyone actually flown vertical have a better answer?

[hawk37]

I don't see why the wing would be producing any lift at all. Use the horizontal stab/elevator to reduce aoa to zero at the 90 degree point.

For Milt's response, I think he means TAS, as IAS would mean TAS increases in the climb.

Hawk

[Wizofoz]

You fly a "Zero Lift" A of A which would be zero on an aircraft with a symetrical wing, or slightly less than zero on (i.e. a slight negative A of a) on an aircraft with a cambered wing.

As most aircraft have a positive angle of incidence, the deck angle would be slightly nose down, rather than pure vertical.

as IAS would mean TAS increases in the climb.

Yes it does, because drag decreases with altitude due to lower air density. Thus for a constant thrust, a constant IAS but increasing TAS would result. (However, as most types of engine reduce in power with altitude, this doesn't seem likeley!!)

[hawk37]

Wizofoz, the IAS cannot cannot remain the same. Here is Milt's quote

“For the high performance aircraft climbing vertically at constant IAS the thrust has to be equal to the weight PLUS the drag!!”

If the thrust is equal to weight plus drag, then there is no excess thrust to accelerate the aircraft. Because to keep the same ias, then the TAS would have to increase. It is, of course, climbing up, into less dense air.

My post was to suggest that instead of “IAS”, that the use of “TAS” would be correct for Milt's post.

In Milt’s case, I believe the thrust would have to be greater than the weight plus the drag, in order to accelerate and keep the same IAS. Again, his applies to Milt's scenario.

Hawk

[Wizofoz]

thrust has to be equal to the weight PLUS the drag!!”

Drag then decreases as the air density reduces, thrust is now greater than drag so the aircraft accelerates.

It is exactley the same theory as a normal, constant IAS climb in an aircraft with a flat rated engine. For the same thrust and IAS, TAS will increase with altitude.

[error_401]

Ohhh - give me these engines to fly vertically
:E


I'd love to try how far I could go...
:ok:

[Gargleblaster]

I can fly vertically without any engines at all :8

At least for some time...

[Phil Hudson]

Ohhh - give me these engines to fly vertically

http://img514.imageshack.us/img514/4262/737sp20gn.jpg

:ok:

[hawk37]

Oz, reference your quote:

"Drag then decreases as the air density reduces, thrust is now greater than drag so the aircraft accelerates.

It is exactley the same theory as a normal, constant IAS climb in an aircraft with a flat rated engine. For the same thrust and IAS, TAS will increase with altitude"

If it is a constant ias climb (can we talk cas?) as you say, then how can drag decrease (compressibilty aside)?

Secondly, I know engines can be flat rated with respect to temperature. Can you explain this flat rating with altitude and speed? New to me.

Hawk

[FCS Explorer]

if u wanna go straight up, when u can not use lift created by wings u will need more thrust then weight. like rockets. u sure have seen pics of jet fighters going straight up. some of them can do the trick.

[XPMorten]

I have to say I only know the basics of how a plane works. But one thing I'm still wondering about; Does the Thrust generated by a plane's engines need to be greater than the weight of the plane, for it to move, and thus take off?

Good question.

The answer is YES, that is if you want to take off with brakes ON!
The static friction coeff on a locked tire on dry asphalt is Cd=1.
This means "breakaway" thrust needed is the same as the Gross weight.

Well, this is "almost" the truth since some of the acf
weight is on the nose gear which does not have brakes :O

Cheers,

M

[pstaney]

Wiz. Here's another way to consider it. Newton and F=ma.
You've said true airspeed will be increasing. Thus aircraft is accellerating. thus unbalanced force on the aircraft. And simply since the case is going straight up, the up force greater than the down force. thus Thrust greater than weight plus drag. Thus if thrust equals weight plus drag, aircraft cannot maintain same ias.
'm with Hawk on this on
Stan

[XL5]

F=ma with F being the resultant force is the only way to consider it and says it all. Newton didn't spend all that time sitting under apple trees so that future generations could invoke witchcraft and mumbo-jumbo to explain physical systems.

[barit1]

I know engines can be flat rated with respect to temperature. Can you explain this flat rating with altitude and speed?

Up to a point, this is possible. I've seen engines with an "altitude bump" to provide (nearly) the same static thrust at 5K (Denver or J-burg) as at SL.

But the engine has less air to work with, of course, so a full-rated takeoff at this altitude is naturally a bit harder on the hot section.

[FakePilot]

Somehow you can get 100,000 pounds of vertical force by only supplying 30000 pounds of horizontal force. There's gotta be a way to make perpetual motion machine out of this. :)

However, one fact is helping me understand:
Notice helicopters always have engines that can produce as much thrust as they weigh. If I'm wrong on this, please correct.

I'm guessing the difference is taken from the energy in the air itself. Lower energy air (turbulent) is produced and remains until heat and gravity restore said energy. Helicopters can't benefit from this in a hover.

Hey, I can guess, can't I?

[barit1]

Notice helicopters always have engines that can produce as much thrust as they weigh. If I'm wrong on this, please correct.


The helo's "engines" (by this I surmise you mean the turbine/s, gearbox, and rotor) create much more thrust (actually lift, since it's vertically oriented) than they weigh. The excess lift carries the fuselage, landing gear, fuel, payload etc. plus some margin for maneuvering.[I]

And just to show that the basic fundamentals of lift are really no different from airplanes, Helos are oft referred to as "rotary-wing aircraft", vs "fixed-wing" for conventional planes.

[Wizofoz]

then how can drag decrease

Chicken and egg thing here Hawk. The drag does not decrease because the TAS increases, thus keeping a constant IAS.

If TAS was constant, then drag would decrease, the aircraft would accelerate until drag plus weight equalled thrust once again.

By "Flat rated" I meant "Maintains power with altitude" as in a turbo-charged piston,or de-rated turbo prop.

[FakePilot]

barit1,

Thanks for understanding my poor aviationese. :)

Consider this then:

The helo can fly horizontally (IAS) at altitudes HIGHER than it can hover (at a given weight, conditions, etc). So when the helo is moving, why does it get extra lift???

This is what I've trying to figure out.

One physics instructor told me that it had to do with heat in the air, something about "adibatic (?) and Pascal."

Hmmm, can anybody tell me if two identical aircraft in identical conditions (weight, alt, etc etc) EXCEPT for temp, does the one in colder air require more pitch up to maintain altitude????

[barit1]

Helos in forward flight gain "translational lift" but someone else will have to give a good entry-level explanation.

And (neglecting Mach effects) if the two aircraft at the same altitude are at the same INDICATED airspeed they'll have the same angle of attack (AOA). But the one in warmer air will travel faster (true air speed).

[glekichi]

Okay... I admit it... Im confused!!!
And for the first time since I started aviation.
Nobody has tackled the original question od thrust vs lift.

An aircraft sitting on top of a pole will still be experiencing xxx pounds of force due to gravity, but no work is being done, right?
Same as the walls holding up your roof.

So.. how much work is required to keep an aircraft in flight?

Not the full amount required if it were not moving foward, obviously.
(ie. helicopters, rockets, etc.)

But how much? The faster you go horizontally the less work required to keep it up there?
Technically.. if it isnt getting any higher or lower then there shouldnt be any need for energy to hold it up there at all...

If you went fast enough you would be in the earths orbit... but im guessing it has nothing to do with this, either.

Seems to me its somewhere in between being suspended in the air and being held up there due to thrust....

Is the wing acting almost like a parachute, slowing the fall towards earth to the point where very little energy is required to hold it up there?

Can anyone help me out here?

[barit1]

Nobody has tackled the original question of thrust vs lift...

...So.. how much work is required to keep an aircraft in flight?


Every airplane has a lift/drag characteristic (l/d ratio). It's actually a curve, with a maximum value perhaps slightly below cruising speed, but falling off at higher or lower speeds.

It can be determined experimentally by setting the engines to zero thrust and measuring the glide ratio, which is the same thing as l/d.

The work done by the engines is simply F x V (i.e. lbf x ft/sec), where F = drag for stable flight.

Are we getting there? :8

[glekichi]

barit>

Thanks for the input.

I know exactly what you mean about l/d and that... im just thinking conceptually here... high school science style.

What if we ignored parasite drag and consider purely lift and induced drag.

The faster you go, for some reason, the energy required to fly becomes less?
Why is that?

Pointless perhaps, but im just curious.

[chornedsnorkack]

barit>

Thanks for the input.

I know exactly what you mean about l/d and that... im just thinking conceptually here... high school science style.

What if we ignored parasite drag and consider purely lift and induced drag.

The faster you go, for some reason, the energy required to fly becomes less?
Why is that?

Pointless perhaps, but im just curious.
Well, let us have a look at a wing.

Assuming a symmetrical airfoil, you could fly it unloaded. Angle of attack zero. Or in a case of asymmetrical, cambered wing at some angle of incidence to the fuselage, fly slightly nose-low so that there is no lift. There would be relatively small drag, but the drag would be still there. Lift/drag would be zero.

Alternatively, you could fly an airfoil with angle of attack near 90 degrees. A flat foil at a right angle to the airflow creates huge drag, but very little lift. The "leading" and "trailing" edges normally have a somewhat different shape from each other, so the angle of attack where exactly zero lift is produced is slightly different from, but close to 90 degrees.

Now what happens if a wing flies at an angle of attack which is large, say 45 or 60 degrees? Then the wing has huge drag, but also creates a lot of lift. This happens because it deflects a lot of air downwards, rather than up.

And what about smaller angles of attack? Well, what happens is that the wing deflects air slightly, generating moderate lift - at the cost of drag much lower than the lift. L/D gets to the ranges of 10, 20, 50...

So what about speed?

The lift has to equal weight in sustained level flight.

At certain speed, suitable for cruise, the lift equals weight at the angle of attack giving best L/D.

If the aircraft slows down, then to continue supporting the weight, the craft must increase the coefficient of lift by means like adopting high AoA and extending flaps - which increase lift at the cost of a drag which is high relative to the lift.

If the aircraft tries to speed up above the cruise speed then the drag increases with the square of speed (assuming low Mach numbers) while the coefficient of drag cannot be decreased much - the minimum at zero AoA is not far below the drag coefficient in cruise, and cannot be attained because that would mean no lift.

So, this is why airframes have a certain optimum speed.