The effect disappears at low A normal cruise or high-speed flight. They do not carry around with them always, the tremendous drag of the vortices.
I get that, which is why I said "unless the wing-loading is very light". That's why I also put a weight comparison (The Nemeth Parasol/Umbrella Wing, however, for reasons I explained, was largely guesswork).
The Zimmerman/Vought/Sikorsky design did not need the outward-turning props to counter it for cruise. They knew this, it's a misconception of everyone after who's looked at it.
The simple fact is that the other planes which have used various shapes of low aspect-ratio planform, achieved the same slow speed flight, without worry about shaping the slipstream.
There was an
NACA report that was on
Reply #22 in this thread, which covers the matter in decent detail.
Regardless of Charles Zimmerman's intentions, the outward-rotating props did, indeed, have little effect on cruise performance: They however, did have an effect at higher AoA and lower-speed (which is the point of this design). Basically, the NACA report states that the direction of propeller-rotation affects lift as much as 33-
1/3% to 50% (which is quite substantial), with the remaining 50% to 66-
2/3% coming from the velocity of the slip-stream alone.
Here's a quote from the report...
NACA RM No. L6I19 said:
The effects of propeller operation on the left of the model are presented in figure 42 at angles of attack ranging from about 0-degrees to 30-degrees. At angles of attack of -0.5 degrees and -0.6 degrees for propeller blade angles of 20-degrees and 30-degrees, respectively, increases in coefficient of lift amounting to between 0.2 and 0.3 were measured for the propeller advance-diameter ratio ranges investigated. This change in lift coefficient is caused principally by the change in the local angles of attack of the wing induced by the slipstream rotation.
As the angle of attack is increased the change in lift-coefficient at a given propeller advance-diameter ratio increases. Calculations showed that about one-third to one-half the total increase in lift due to propeller operation at the high angles of attack results from the lift component of the propeller resultant force. Most of the remaining increase is attributed to the increased slipstream velocity over the wing.
I'm not pulling numbers out of my ass.
Zimmerman was after something else, not connected straightforward with the low aspect-ratio slow flight.
I think he also had an idea of being able to have enough thrust to go vertical. Not sure if he succeeded in that department, but that was one of his goals. That would be desirable for any fighter-plane assuming fuel burn wasn't affected badly enough.
The Navy ignored the real promise which the Arup/Nemeth/Eshelman demonstrated, to let Zimmerman explore his quirky exaggerated props, and after the war continued on as if those other planes never happened to amply provably demonstrate superior flight qualities.
While I don't have any data on Eshelman's Flying Flounder (NX28993 and NC22070), I do have data on the Eshelman FW-5.
- Weight: 2650 lb. (GTOW)
- Wing-Area: 232 ft^2
- Aspect-Ratio: 3.8793
- Wing-Loading: 11.4224
It's wing-loading is higher than the Arup S-2/S-4 (3.6967/4.3956), the Vought V-173 (5.2881), but so is its aspect ratio at 3.8793 (
Arup S-2: 1.7109;
Arup S-4: 1.7729;
Vought V-173: 1.2750), and the somewhat lower aspect-ratio compensates to a degree for the heavier wing-loading.
The Arup S-2 (1.7109) and S-4 (1.7729); the V-173 (1.2750), and; the Wainfan FMX-4 Facetmobile (1.0514) all have aspect ratios under 2:1. The XF5U-1 also has such an aspect ratio in the same range (1.1462), but has a wing-loading that is in the double-digits (35.2042), whereas all the others are not.
If the V-173's wing-loading was brought as high as the XF5U-1, you would see stall speed go up by 2.58. So the V-173 would go from having an enviable stall-speed of 40 knots to the very unenviable 103 knots. Yet the stall-speed of the XF5U-1 was around 20-25 knots by some estimates, around 40 knots for others.