What is the wind speed behind prop in cruise flight?

beestforwardspeed

beestforwardspeed

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If you are in cruise flight at lets say 2500 RPM and 110 KIAS, how would you calculate what the wind velocity would be directly behind the prop (say on the windscreen)?

I figure it must be faster than the speed of the plane, but just how much faster, I don't know.
 
beestforwardspeed said:
If you are in cruise flight at lets say 2500 RPM and 110 KIAS, how would you calculate what the wind velocity would be directly behind the prop (say on the windscreen)?

I figure it must be faster than the speed of the plane, but just how much faster, I don't know.
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When the brakes are set and the prop up to speed it is like a fan (slippage). When the aircraft is moving it is like a screw(less slippage). In a well designed system the propwash is the airspeed with a slightly different direction and some small venturi effect (free air in front of prop squeezed into a smaller area behind the prop.


I stand to be corrected.
 
Maybe someone with an open cockpit could test this with a pitot tube and airspeed indicator. Good question. My guess...125 kias.
 
beestforwardspeed said:
I figure it must be faster than the speed of the plane, but just how much faster, I don't know.
Click to expand...

Yeah, it does have to be faster than the speed of the plane, and I also don't know how much faster. I assume a full aero engineering treatment could get you a real answer, but just looking at the relatively simpler physics version can get a few general rules:

Consider a frame of reference where the airplane is still, and the "airspeed" is the air moving towards/through the airplane at the airspeed (110 kts). Just like we learned in private pilot 101, gravity = lift and thrust = drag to maintain level cruise flight.

In this frame of reference (airplane still, 110 kt "wind" blowing by), the drag is anything that makes the air passing by the plane go slower than 110 kts (form drag in the form of rivets and the like), and/or diverts the air from traveling directly from front-to-back on the plane (induced drag in the form of angle of attack). The thrust is anything that makes the air go faster than 110 kt, ie the prop. So yes, in cruise flight the air behind the prop is definitely moving faster than the true airspeed.

The "dirtier" the plane is, the bigger the difference between airspeed and behind-the-prop speed (more drag means more thrust needed).

The bigger the prop circle area is, the lower the difference between airspeed and behind-the-prop speed (moving more air, don't have to move it as fast for the same thrust).
 
mcmanigle said:
Yeah, it does have to be faster than the speed of the plane, and I also don't know how much faster. I assume a full aero engineering treatment could get you a real answer, but just looking at the relatively simpler physics version can get a few general rules:

Consider a frame of reference where the airplane is still, and the "airspeed" is the air moving towards/through the airplane at the airspeed (110 kts). Just like we learned in private pilot 101, gravity = lift and thrust = drag to maintain level cruise flight.

In this frame of reference (airplane still, 110 kt "wind" blowing by), the drag is anything that makes the air passing by the plane go slower than 110 kts (form drag in the form of rivets and the like), and/or diverts the air from traveling directly from front-to-back on the plane (induced drag in the form of angle of attack). The thrust is anything that makes the air go faster than 110 kt, ie the prop. So yes, in cruise flight the air behind the prop is definitely moving faster than the true airspeed.

The "dirtier" the plane is, the bigger the difference between airspeed and behind-the-prop speed (more drag means more thrust needed).

The bigger the prop circle area is, the lower the difference between airspeed and behind-the-prop speed (moving more air, don't have to move it as fast for the same thrust).
Click to expand...

I think you are close; one may be able to use the actuator disk theory to get a crude estimate. Using parameters that one can get reasonable estimates for, my quick bit of math (meaning it may be in error) gets me:

Ve = sqrt(V^2 + (P*q/(V*A*rho/2)) )

Where:
Ve = Velocity of the air relative to the airplane as it exits the prop disk. (m/s)
V = Velocity of the air relative to airplane. (m/s)
P = Power being generated by the engine. (W)
q = Prop efficiency (swag 0.8?).
A = Area of the disk made by the propeller. (m^2)
rho = density of air. (kg/m^3)

I'll leave it as the proverbial exercise for the reader to compute estimates for some common aircraft. May prove my math wrong, too.
 
It's noticeable if you're jumping out, that's bout' all I have.
 
The prop on my Jodel used to be a 76-44. 76" diameter, 44" pitch. At a cruise RPM of 2250, the no-slip forward speed should be 93.75 MPH. The airplane cruised at 95. It made no sense. The chord line on that prop is essentially the bottom face.

An article by Don Downie said that an airplane that had a slightly higher cruise speed than what the pitch would give by calculation indicated that the diameter was too large. I shortened that prop to 72" in the hope I would get more RPM and therefore more HP, and gained nothing but lost a bit of cruise speed.

The propeller is a flat-bottomed airfoil. Such airfoils, used as wings, will often generate lift at angles of attack as low as -4°. The propeller might do the same and create its thrust not simply by reaction (Newton) but also by pressure differential (Bernoulli). So the prop blast need not necessarily be a bunch faster than the cruise speed.

A rocket in space will travel far faster than its exhaust velocity.

I Dunno. I think it's one of those things that's not intuitive.

Dan
 
"But at the exit, the velocity is greater than free stream because the propeller does work on the airflow."
I think this means "immediately" behind the propeller disk. The airflow may practically be slower on a blunt nose Mooney with the nearly vertical windscreen redirecting flow ....(which could actually measure less than airspeed as the vector of the airflow would be a different direction than impact air parallel to line of flight) vs a swept windscreen. It is known that the propwash and air velocity from forward motion can be reversed in an open cockpit airplane behind the windscreen.



[FONT=Arial, Helvetica, sans-serif]
propanl.gif
Most general aviation or private airplanes are powered by internal combustion engines which turn propellers to generate thrust. The details of how a propeller generates thrust is very complex, but we can still learn a few of the fundamentals using the simplified momentum theory presented here.[/FONT]
[FONT=Arial, Helvetica, sans-serif]Propeller Propulsion System[/FONT]
[FONT=Arial, Helvetica, sans-serif]On the slide, we show a schematic of a propeller propulsion system at the top and some of the equations that define how a propeller produces thrust at the bottom. The details of propeller propulsion are very complex because the propeller is like a rotating wing. Propellers usually have between 2 and 6 blades. The blades are usually long and thin, and a cut through the blade perpendicular to the long dimension will give an airfoil shape. Because the blades rotate, the tip moves faster than the hub. So to make the propeller efficient, the blades are usually twisted. The angle of attack of the airfoils at the tip is lower than at the hub because it is moving at a higher velocity than the hub. Of course, these variations make analyzing the airflow through the propeller a verydifficult task. Leaving the details to the aerodynamicists, let us assume that the spinning propeller acts like a disk through which the surrounding air passes (the yellow ellipse in the schematic).[/FONT]
[FONT=Arial, Helvetica, sans-serif]The engine, shown in white, turns the propeller and does work on the airflow. So there is an abrupt change in pressure across the propeller disk. The propeller acts like a rotating wing. From airfoil theory, we know that the pressure over the top of a lifting wing is lower than the pressure below the wing. A spinning propeller sets up a pressure lower than free stream in front of the propeller and higher than free stream behind the propeller. Downstream of the disk the pressure eventually returns to free stream conditions. But at the exit, the velocity is greater than free stream because the propeller does work on the airflow. We can apply Bernoulli'sequation to the air in front of the propeller and to the air behind the propeller. But we cannot apply Bernoulli's equation across the propeller disk because the work performed by the engine violates an assumption used to derive the equation.[/FONT]
 
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Dan Thomas said:
A rocket in space will travel far faster than its exhaust velocity.
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A rocket in space does not experience significant drag. Taking the same vehicle-is-stationary frame of reference on a rocket in space, *any* amount of thrust will speed it up.

In similarly unrelated situations, a twin with both props feathered and both engines off in a nose dive will be picking up speed, but indeed the air behind the prop is moving at the same speed as the rest of the air.

But the basic idea that to overcome drag, a prop has to push air by, and the "push" critically means it has to be forced to move by at more than the airspeed, is correct. From subsequent posts, looks like smarter people than I gave a more detailed treatment :-)
 
Aren't there aerobatic airplanes that can "hang on the prop"? If so, speed behind prop definitely greater than speed in front of prop.

Or helicopters, if you think the rotor is similar enough to a prop.

F = m(dP/dt). In order for the momentum to change, you either added mass or made it go faster.
 
Mistake Not... said:
Aren't there aerobatic airplanes that can "hang on the prop"? If so, speed behind prop definitely greater than speed in front of prop.

Or helicopters, if you think the rotor is similar enough to a prop.

F = m(dP/dt). In order for the momentum to change, you either added mass or made it go faster.
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Hanging on the prop or hovering has reduced the propeller back to no forward airspeed. Just as holding the brakes and running up to max RPM.
 
Sleepingsquirrel said:
Hanging on the prop or hovering has reduced the propeller back to no forward airspeed. Just as holding the brakes and running up to max RPM.
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And that's equivalent to saying thrust goes to zero when you're flying.

Exercise: What happens to pitch when you suddenly pull power in a near-centerline-thrust aircraft with a conventional elevator or stabilator (i.e., most of them)? Can you explain that with zero or near zero excess slipstream?
 
Basically a propeller is an airfoil so just like the wing there must be greater pressure on the back side (bottom) than on the forward side (top) If not then you will not move forward or remain in flight.
 
Here's my answer: In straight and level flight at constant cruise speed, the airspeed behind the prop is 112% of the aircraft's airspeed.

I have only a vague idea as to why I think that figure is correct based on the efficiency of props.
 
tinerj said:
Here's my answer: In straight and level flight at constant cruise speed, the airspeed behind the prop is 112% of the aircraft's airspeed.

I have only a vague idea as to why I think that figure is correct based on the efficiency of props.
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Well, at least we are close. Your guess is 123.2 kias and mine was 125 kias.
 
What is the windspeed behind the prop when an airplane is taking off from a treadmill?
 
Since every force creates and equal and opposite force, and since the force of the propellers thrust is sufficient to move several thousand pounds if aircraft at 100+ mph, wouldn't the air speed immediately behind the prop have to be much higher than the tas?


The speed of the air hitting the windshield of a pusher prop plane would be tas. But the thrust coming off the back of the plane would be much greater wouldn't it?
 
MAKG1 said:
Exercise: What happens to pitch when you suddenly pull power in a near-centerline-thrust aircraft with a conventional elevator or stabilator (i.e., most of them)? Can you explain that with zero or near zero excess slipstream?
Click to expand...

But is that that circular slipstream just the relative wind vectored in different directions because of the prop, or is it actual air thrown back by the prop? Combination of the two? I'm not trying to argue with you, that's an honest question which I don't know the answer to.
 
The important thing is not speed - it is the mass of the air being moved.

Move more mass for a given weight of aircraft, and you go faster and accelerate faster.

Such is why the initial full throttle acceleration in an aircraft has a higher 'g' than the feeling later in the take off roll because the difference in the relative mass of air being moved by the propellor - the energy needed to maintain the existing speed of the aircraft as it accelerates takes a larger fraction of the mass of air being moved by the prop rendering the acceleration reduced. It is why there is basically no feeling of acceleration when you push the throttle in all the way if in cruise - it is why with a constant speed prop you can feel the push when you increase RPM from 2400 to 2500 - you are moving more air = greater mass of air = more speed ultimately.

Simple stuff folks if you consider it. Speed is not important - mass of air is.
 
Tflhndn said:
Since every force creates and equal and opposite force, and since the force of the propellers thrust is sufficient to move several thousand pounds if aircraft at 100+ mph, wouldn't the air speed immediately behind the prop have to be much higher than the tas?


The speed of the air hitting the windshield of a pusher prop plane would be tas. But the thrust coming off the back of the plane would be much greater wouldn't it?
Click to expand...

In straight-and-level flight, the prop is not moving thousands of pounds of aircraft...momentum is doing that.

In straight-and-level flight, lift=gravity and thrust (prop)=drag.
 
JeffDG said:
Actually, they're both important.

You can increase thrust by either moving more air, or by accelerating it to a higher speed.
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and, you're moving more air because it's moving faster through the prop. Unless your blue knob makes the prop longer....

I'm not sure it's the best idea to be throwing equations at lawyers.
 
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