Prop produces most power when aircraft is stationary?

Question

Ive read that airplanes make the most power when stationary. I dont mean the engine makes more horsepower but the prop makes more push. Then Ive read that helicopters rotors are more efficient when the helicopter is moving. Source says that after moving at about 20mph you lose ground effect but then gain transient lift?. Can someone explain this to me? is the prop/rotor producing more thrust while stationary or moving?

Thank you guys for the lengthy replies, Im a private pilot, I do understand the basic concepts. Maybe its just the wording but read this here.

"The power is all dissipated in the propwash. So what's really going on here is that a prop engine delivers maximum thrust at standstill." http://home.earthlink.net/~x-plane/FAQ-Theory-Jets-Props.html

Now if that is true, wouldn't a helicopter need less power to take off since it has max thrust at standstill?

I think its just the way im comprehending what Im reading. But anyway, back to what I first asked, which rotor/prop makes more thrust, the stationary one or the moving one?

Translational lift is what I meant, not transient, sorry for this.

Answer 1

I see where you are going with the Helo comparison. The fault in your logic is that the Helo doesn’t so much rely on displacing air downwards to fly as a fixed wing displaces air aft. The rotary wings (as Igor Sikorsky called them, a more accurate term than rotor blades) work more like the wings of your light a/c than the propeller. They generate lift by moving air over them as they rotate. This is why translational lift is greater. The speed of the air over the rotary wings increases with increased forward velocity. Therefore more lift is generated.
Notice we call it lift not thrust.

Answer 2

The prop produces the most thrust at its maximum RPM without the tips breaking the sound barrier, and when the blades are at the best lift/drag angle of attack.

If you add any more horsepower at this point, the prop will go supersonic and create more drag or the blades have to move to higher pitch which creates more drag. Whether it is stationary or moving for the most thrust depends on the type of propeller/engine combination.

Helicopters are totally different. In a low-altitude hover, a cushion of air builds up under the rotor which helps it stay airborne. When the helicopter starts to translate, it moves off the cushion of air so much more power needs to be applied to maintain altitude. After the helicopter is moving at a certain speed, the ram effect of air going into the top of the disc offsets the loss of the cushion of air.

A helicopter aways uses the most power when translating to forward flight.

Answer 3

Ground effect is not all that well understood. However it goes something like this As the rotor pushes air downward, the air hits the ground and resists changing direction due to inertia. This causes a weak high pressure area under the rotor disk. A small air cushion similar to that made by a hovercraft is formed, which boosts the lift created. It should also be noted that proximity to the ground hampers the formation of wingtip vortices, which are a major source of drag. This also adds to the bonus. This effect dissipates very rapidly with altitude. As the other answer says, at 1.5 rotor diameter (or wingspan) it is unnoticeable.
20 Mph is as good a number as any to describe when the helo will ''fall of the cushion'', but that depends on rotor design. Somewhere during acceleration, forward speed will help feed air to the rotor disk and thus create lift more efficiently, without the rotor having to work to suck air to itself. This increase in efficiency is described as translational lift.
Fixed wing craft experience the same effects. No prop is fully efficient with no forward speed. All designs have a ''sweet spot'' of RPM and Airspeed they were designed for, so that speed varies. Obviously, variable and controllable pitch props have the upper hand here, as they can remain efficient over a wider range of speeds. Blade profile, tip design, thickness, length, all have an effect here.

Answer 4

Like with many of these questions, it depends, there's no simple or universally correct answer.

A fixed pitch prop is a compromise that produces acceptable thrust for takeoff where revs may actually be below optimum, and allows reasonable cruise power at an efficient altitude without over revving the engine. The higher and faster you fly the more pitch you need. With less pitch the prop has to turn faster to provide the same thrust. At lower altitude you need less pitch to allow the prop to spin at the same speed and priduce good thrust. Too little pitch and the engine just revs without producing thrust. Does that make sense?

Constant speed propellors try to match the pitch to altitude and speed automatically.

So given an ideal prop you produce maximum power where the engine can produce maximum power, that may or may not be at ground level. Turbo charged engines may be able to produce more power where they can get more charge weight into the engine, that may be where the intercooler is getting a good supply of cold air which allows a turbo charged engine to produce a denser charge than it could on the ground.

The transition speed issue is completely different. A helicopter rotor produces thrust which lifts the aircraft by brute force, but it is also a wing, so as you accelerate forward you reach a point where the rotor starts to lift as a wing. That's called transition. Many early helicopters could barely hover but flew fine at normal forward speeds.

Answer 5

The prop will produce maximum thrust and power when it achieves maximum rotations.
This is irregardless of being stationary or airborne.
The term maximum power on the ground refers to a take off procedure rather than actual power of the engine and prop.
It is used for short runway take off. You hold the brakes to achieve maximum rpm's and release the brakes.
The prop produces more thrust in the air as more air is pulled through the prop due to forward movement. This is why aircraft only need to fly at a percentage of maximum power to attain cruise speed.
Ground effect occurs on aircraft at 1.5 times the wingspan. This refers to a height above ground where the vortexes from lift begins and creates drag. It does not refer to speed as different craft need varying speeds for lift.
Helicopter rotors develop thrust and torque which has many effects on flying. Yes the rotors are more efficient when moving as the torque from the pitch angle is lessened. Helicopter rotors are complex as the rotating blades use speed, pitch angle and torque to develop lift.
This is a little long winded but there is a lot more than a simple explanation.
Hope I answered what you required.

Answer 6

Well, not really.
First you have to separate fixed pith props, and variable pitch (like helis)
Pitch being the angle of the blade in relation to the prop axel.

If the pitch is fixed: the efficiency of the airflow will be established at a certain air speed. More or less air speed will make the prop less efficient.
Therefore if the plane has to travel 500 miles compared to a 3 miles take off. It is logical that props are calculated for better efficiency at cruising speed. Not take off.

With variable pitch you can have 100% efficiency at any speed.

Think of it like a car with only one gear versus a car with all the gears. It all depends which gear and at what speed.

The inertia and lift of a plane or spinning rotor of a heli, in forward flight makes the aircraft glide, thus the energy need for maintaining speed is lowered.

With helis:
When hovering the lift is produced by the rotor speed moving air over and under the blade, generating lift.
The forward speed of the aircraft adds to the speed of the rotor blade, generating more lift.
So when airspeed increases = you get more lift.

The same happens with planes: no speed= no lift


Answering your question: in general terms, the power generated by a prop depends on the pitch/speed ratio. So you can have both.

Indeed, you have several other factors that influence the efficiency of a prop. Ground effect, variable airfoil blades…

I hope it helps

Answer 7

Great Question, Think about the amount or air molecules available for the prop to "bat" backwards. When the engine is running up, stationary, it has to pull molecules and doing so will have some voids or micro vacuum areas, so the molecule count going rearward will be less, by the way we are talking about a fixed angle prop at this time. Stress on the blades are max at this time, but it just seems to be more push especially if you are standing behind it. To my knowledge, all modern day helicopters are variable pitch(collector) so to test the rotor by holding it down, would be tough, but do-able. I think my molecule theory would hold up in this case also. Jet engines move lots of air when they are chained down, but working power is zip, of that I am positive, of the prop theory, I am guessing!

Answer 8

Most propeller aircraft make more power at ground level because the air is more dense, But many aircraft have normalized turbochargers, which add boost as the air becomes thinner in order to maintain power at higher altitudes.

I don't see how a helicopter can have transient lift, because a rotor turns at about 450rpm, and has a radius of 27ft on a large chopper such as the black hawk. That means that the rotor turns just shy of 400mph, way short of being transient.

Answer 9

value. preliminary value of a jet is considerably bigger than that of a turboprop of comparable length. Jets will additionally consume gasoline at a lots bigger value than turboprops. at the same time as jets will grant bigger speeds at bigger altitudes and oftentimes longer tiers than turboprops, those comprise a exceedingly hefty value ticket. Aviation oftentimes is an costly industry, and while picking between a turboprop or a jet, evaluate what you will using the airplane for. funds, time standards, and distances mandatory to holiday are in straightforward terms some issues to think approximately while procuring an airplane. in case you look tricky adequate you will locate the airplane it quite is best for you.

Answer 10

Yes, because piston engines produce almost constant power, so lower the speed is higher the thrust is, because power=speed*thrust