Prop disc airspeed

[Bevan]

Has anyone here measured, or can make an educated guess what the ratio of airspeed is between that measured at the typical pitot location and inside the propeller disc area (preferably at the point of greatest thrust, ie outer 3rd of the prop’s radius) at cruise speed?

In other words, how much faster does the “thrusted” air (relative to some point on the airframe) than the undisturbed air, at RV cruise speed? 10%, 25%, 50%, 100%?

Since it is disturbed (by the prop) air, it would have to be an average air speed.

Bevan

[scsmith]

Here is what actuator-disc theory gives.

First estimate the thrust:

eta P = T x V

assume eta = 0.85 P = 180 hp = 99,000 ft-lb/s V= 190 mph = 280 ft/s

So T = 300 lb.

Next estimate the pressure jump across the disc:

T = A x delta_P

assume 72" prop, so A =28.25 sq ft

so delta_P = 9.9 psf

actuator disc theory says that half of that pressure differential is on the front of the disc, where we know the stagnation pressure is the same as free stream.

Bernouli's equation gives us the velocity at the disc face:

p_fs + 1/2 rho V_fs^2 = p1+ 1/2 rho V1^2

assume sea level values

2116 + 1/2 (0.002377) (280)^2 = (2116-9.9/2) + 1/2 (0.002377) V1^2

solving for V1: Velocity at the propeller disc face =287 ft/s.

So there is a 7 ft/s (4.77 mph) rise in velocity at the disc face. The velocity increase in the slip stream far downstream is twice that, so the slip stream velocity would be 294 ft/s.

[Taltruda]

Not really sure what you are trying to figure out, but I believe hit can calculate the tip speed (or any location that you want) by figuring out the circumference of the circle, converting it from inches and rpm into miles per hour. Then take that speed, call it A, and your aircraft speed, call that B, and using A squared plus B squared equals C squared, with C being the speed of the tip (or whatever location you used for A.

[scsmith]

Tom, we are calculating the average inflow velocity of the air flow into the propeller.

What you are describing is the helical speed of the propeller itself.

[Bevan]

I’m looking at my plane and theoretical drag reduction due to protusions such as antennas and exhaust pipes. I always assumed that reducing protrusions on the fuselage (within the propeller arc) would provide greater benefit than on the wings (outside the prop arc) such as pitot tube, landing light lenses etc. because the airspeed in this area is faster than at the wings. But then I got to thinking there’s not much thrust produced within first 1/2 of the prop’s radius anyway and therefore possibly not much difference in drag reduction by reducing any protusions along most of the fuselage compared to most of the outer wing. There would be a more critical area of the wing (within the outer 1/3 of the prop radius) that would benefit the most for attention to protusion detail. But RVs typically don’t have any protusions in this area anyway. Thanks Van!

In other words, if one must mount a hard point for some external load, avoid putting it under the wing near the outer arc of the prop. Tight to the fuselage or under the outer part of the wing would be better and possibly no significant different between the two.

Recently I’ve been experimenting with an ash dispensing tube I built for scattering ashes. It’s 4” in diameter aluminum tube with flat front and back doors, draggy. The main attach point is the existing tiedown hardpoint so out of the propeller arc. I expected some yaw (only one tube is mounted under the right wing) but can’t seem to notice any yaw at all. Since there is so little, if any yaw (drag) from a 4” blunt tube under one wing mid span, there must be very little drag but why? And if so, why would I be concerned about a bit of drag from something protruding from the fuselage like a skinny antenna, exhaust pipe etc.?

I’m thinking these protusions produce less drag than first thought.

Bevan

[plehrke]

Originally Posted by Bevan

I’m looking at my plane and theoretical drag reduction due to protusions such as antennas and exhaust pipes. I always assumed that reducing protrusions on the fuselage (within the propeller arc) would provide greater benefit than on the wings (outside the prop arc) such as pitot tube, landing light lenses etc. because the airspeed in this area is faster than at the wings. But then I got to thinking there’s not much thrust produced within first 1/2 of the prop’s radius anyway and therefore possibly not much difference in drag reduction by reducing any protusions along most of the fuselage compared to most of the outer wing. There would be a more critical area of the wing (within the outer 1/3 of the prop radius) that would benefit the most for attention to protusion detail. But RVs typically don’t have any protusions in this area anyway. Thanks Van!

In other words, if one must mount a hard point for some external load, avoid putting it under the wing near the outer arc of the prop. Tight to the fuselage or under the outer part of the wing would be better and possibly no significant different between the two.

Recently I’ve been experimenting with an ash dispensing tube I built for scattering ashes. It’s 4” in diameter aluminum tube with flat front and back doors, draggy. The main attach point is the existing tiedown hardpoint so out of the propeller arc. I expected some yaw (only one tube is mounted under the right wing) but can’t seem to notice any yaw at all. Since there is so little, if any yaw (drag) from a 4” blunt tube under one wing mid span, there must be very little drag but why? And if so, why would I be concerned about a bit of drag from something protruding from the fuselage like a skinny antenna, exhaust pipe etc.?

I’m thinking these protusions produce less drag than first thought.

Bevan

A big driver in protuberance drag is how far back on the fuselage or from leading edge it is. The thicker the boundary layer the less drag the protuberance causes.

Hanging stuff adds another type of drag. Interference drag caused by the interaction between the fuselage or wing with the thing you are hanging can cause extra drag if the spacing between the two is not optimized.