Does the Thrust need to be greater than the weight?

[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]

Re: Does the Thrust need to be greater than the weight?

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]

Re: Does the Thrust need to be greater than the weight?

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]

Re: Does the Thrust need to be greater than the weight?

Originally Posted by glekichi

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?

[glekichi]

Re: Does the Thrust need to be greater than the weight?

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]

Re: Does the Thrust need to be greater than the weight?

Originally Posted by 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.

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.