Random Aerodynamic Oddity of the Day Thread

[freebeard]

An interesting edge case. How do you handle 200MPH tail winds?

Dominator 3 in Kansas City for extra coating before heading back into storms

I thought it would have suction fans to hold it down, but it doesn't just lay frame, it can drive spikes into the ground!

The biggest development from Dominators 1 & 2 is the gable roof, presumably to kill lift compared to a flat roof. Cybertruck would be a good base vehicle for this, it would need just skirts and a bed-liner coating.

[cRiPpLe_rOoStEr]

Now look at the rear section...

[kach22i]

Post #198
Ducktail Critique from an Industrial Design Perspective - Page 10 - Pelican Parts Forums










If anyone ever finds some wind tunnel testing of this creation, please post it.

[freebeard]

https://www.mt-propeller.com: Another World’s First Takes First Flight – The 11-bladed Propeller!

On a PA31T1 equipped with the P&W PT6A-135A turbines, the World’s first 11-bladed propeller took the sky with an impressive noise and sound signature.
The very promising results in static thrust, 15% increase over the standard certified 5-bladed propeller, and the jet-noise signature showed another time what could be possible in the propeller developments.

They've worked their way up through 7- and 9-blade versions.

[Bicycle Bob]

Multiple blades help with the very coarse pitch used to keep the tip speeds subsonic. On windmills, a single blade is sufficient to extract almost all the possible energy, but it needs a counterweight, and adding a second blade causes weird balance issues, so three are used.

[freebeard]

I see a lot of Prandtl influence, with lots of pitch at the hub with washout to zero at the tips.

More blades allow slower rotation? They really sell the sound signature.

[Bicycle Bob]

Originally Posted by freebeard

I see a lot of Prandtl influence, with lots of pitch at the hub with washout to zero at the tips.

More blades allow slower rotation? They really sell the sound signature.

Slower rotation keeps the tips subsonic and quiet, but it does not chop up the column of air passing through the prop disk finely enough to push on all of it with only two or three blades. Using such a coarse pitch puts a lot of totally wasted spin on the propwash - this would be a good place for contra-rotating props, if they don't have new sound issues.
Fan jets are basically multi-bladed turboprops with tip shrouds. Every time they brag about better mileage, the size goes up, and the wasted momentum in the exhaust is reduced.

Attached is a picture of a prop showing the correct pitch all the way across. The rate of change is highest at the hub, and it never goes flat at the tips.

[Piotrsko]

Pitch is measured in distance per revolution>>>at the tips<<< As you progress towards the hub, in order to have the same thrust and prop load, pitch gets higher until its so high that part stalls. The insane pitch shown is what it takes to run that prop at the speed they want at the rpm the prop is operating at. According to Irv Culver, when the tips go sonic all that thrust is turned to noise and does not contribute to thrust. One of the multitude of reasons commercial aircraft don't fly supersonic.

Btw: on that non operating turbine, the prop is "feathered" or in low drag mode

[Bicycle Bob]

Originally Posted by Piotrsko

Pitch is measured in distance per revolution>>>at the tips<<< As you progress towards the hub, in order to have the same thrust and prop load, pitch gets higher until its so high that part stalls. The insane pitch shown is what it takes to run that prop at the speed they want at the rpm the prop is operating at. According to Irv Culver, when the tips go sonic all that thrust is turned to noise and does not contribute to thrust. One of the multitude of reasons commercial aircraft don't fly supersonic.

Btw: on that non operating turbine, the prop is "feathered" or in low drag mode

Pitch is the same at any diameter. The propeller blade can twist to match forward and rotational speed all the way to the hub and across, as illustrated. There is no stalled zone, but the high angle near the hub is just unable to do much work so it is often neglected, especially if it is not needed to cool the following motor, or needs more physical strength.
A variable-pitch propeller is only correct at one setting - the effective angle of attack will vary at different diameters at other settings.
It is quite common for prop tips to go supersonic during take-off, and they get loud, but they don't stall any more than supersonic aircraft fall out of the sky. That much power would create very destructive noise if it was all converted.

[cRiPpLe_rOoStEr]

Most of the propellers I see are still no more than 3-blade, and that's not considering the fixed-pitch 2-blade propellers that I see more often on cropdusters.