Propeller Exit Airflow

[GeneWeber]

Hi Folks,

Does anyone have, or know of, any diagrams that illustrate the airflow exiting a propeller? Specifically: How uniform is the volume and velocity across the area? Are there areas of greater and less turbulence? How much does the “cone” of air shrink in diameter after it exits the prop? I realize that the airframe and relative wind have some effect on the exit flow, but I’m just interested in what the flow looks like to a first order.

Thanks & Regards,

Gene

[Vance]

Hi Gene, You ask a great question very well. Thank You, Vance

[rwr]

Lonnie Prince (Prince P-tip), Stu Gort (Powerfin), or Daryl Heineman (Warp Drive) are all very knowledgeable and are quite willing to discuss these things.

Some vague answers to your questions:

Yes, there are diagrams. Do Google searches.

The volume and velocity is not uniform.

The turbulence varies, but incoming airflow is a huge factor here.

The cone does shrink. Lonnie Prince explains that his P-tip design has less shrinkage, and therefore acts with the efficiency of a larger-diameter prop.

[GeneWeber]

Thanks Vance. Sometimes I think it takes a few interested responses to get these conversations going ;)

Hi Bob,

I’ve been doing Google searches. Lots of hits, but no diagrams. Perhaps I just haven’t put together the proper search terms. I’ll keep searching.

Stuart Gort was extremely helpful answering some questions I had in the past. I thought about contacting him. Guess I will.


Lonnie Prince explains that his P-tip design has less shrinkage, and therefore acts with the efficiency of a larger-diameter prop.
Hmmm, my jaded Dilbert marketing sensors are flashing. :rolleyes: My knowledge is obviously very limited or I wouldn’t be asking these questions. But, in Gordon Leishman’s “Principles of Helicopter Aerodynamics” he discusses Rotor slipstream applying the Laws of Fluid Mechanics. He states that, “Because the flow velocity increases in the wake below the rotor, continuity considerations require that the area of the slipstream must decrease.” He does the math to derive the far wake slipstream diameter of a hovering rotor. The blade is inconsequential in the derivation. Again, this topic is beyond me, so the P-tip may indeed reduce the shrinkage. Did he give you any technical details as to how it does this?

Best Regards,

Gene

[rwr]

Ask Lonnie about how his design has less shrinkage. As I remember, it has something to do with the bent tip design creating less tip vortex, and therefore less induced drag, less noise, more thrust, yada yada yada.

The few guys out here that use Prince P-tips swear by them. Lots quieter, and moderately more efficient. One ultralighter claimed he felt as much improvement going from a warp to a P-tip as he did going from a 503 to a 532.

[Doug Riley]

Gene: You can map out the flow pattern (qualitatively) yourself by tying some yarn tufts to the grille of an ordinary household fan and turning it on. Tie another tuft to a thin stick and move it around within the flow for a "portable" indicator. Using this experiment, you can see the taper, helical swirl and airspeed distribution (including a "dead zone"' in the middle of the slipstream). Hold a second fan so that it blows into the intake of the original one, move it around and watch the slipstream move in response.

I can't imagine that the P-Tip actually reduces the downstream "taper," since that taper is a function of the acceleration of air that we all want. End cuffs on airfoils do restore some aerodynamic "length" to the airfoil, so the slipstream may be a bit wider from that point of view.

[hiphop]

According to my Rotary-Wing Aerodynamics book by Steoniewski (Dover, about $10), in the section on Momentum theory, the ratio of the downstream radius to disk radius:

R_u/R = sqrt((0.5*Vc + sqrt(0.25Vc^2+1))/(2*sqrt(0.25Vc^2+1))

where Vc is the normalized velocity (ie: Vc = 0, propellor not moving, Vc >> 1, moving very fast).

Vc = Velocity/sqrt(w/(2*rho)) where w is the propellor disc loading (thrust/area) and rho is the atmospheric denisty.

Hope this helps :)

--Gabriel :cool:

[BenMullett]

Without wishing to contradict anybody, ;) we have seen some variations from theoretical convergence that we think are explained by Fan theory. It's so long ago that I don't recall the reference, but the Vortex flow of the fan is important to the downstream wake.

To simplify it, one extreme case of vortex flow would be a centrifugal fan - all the air goes 'sideways' rather than along the axis of rotation.

The other extreme would be a high tip-speed low-solidity (small chord, few blades) prop, which mostly shoves air back rather than spinning it round and centrifuging it away from the axis of rotation.

The torque transferred to the air is greater in the first case than in the second.

Years ago, (in racing hovercraft) we needed to recover the thrust we lost by spinning the air with a high-solidity low-tip-speed fan that met our noise goals. So we would use flow-straighteners downstream of the fan. These amounted to foils whose downwash effect negated the swirl in the duct, and recovered some torque which improved handling trim in roll.

Better, they added thrust if set correctly - like the NASA winglets do, but only over a restricted range of operation. This helped us to optimise them for static thrust by measuring the force dragging these things forward as we tweaked 'em.

I spent a lot of time with a pitot-static and wool tuft in the breeze from 100+bhp motors, back in the 1970s & 80s. It's pretty easy stuff to do if you have a gyro or trike! I went on to design props for our trike thrust packs, and we shipped many thousands of them before the microlight bubble burst, and pretty much everybody here who wanted one had one. Replacement market was kinda slow.....

The bottom line is that the flow swirl is dependant on the torque applied - so large, low tip speed props eject more swirl than faster-spinning ones with the same power - the gear reduction multiplies the torque.

This swirl can cause the flow to spread out - and the 'hole in the middle' can become an interesting rope of twisting turbulent air - but all the above neglects prop blade tip vortices, quasi-elliptical blade loadings and hub effects!

Yes, the jet area downstream probably does reduce in area according to theory, but an arbitrarily high vortex flow can mask this by causing an initial divergence of the flow - the jet area may then be effectively reduced by a growing cone of turbulent but slower air within the main jet. With good mixing this might improve the momentum transfer?

All the best, Ben