I have been meaning to post this for a while. This is the proposed fuel layout for the Whirlwind Mk.II which as the document makes clear was substantially improved over the Mk.I. I would also note that this is also not the final configuration, that had a further revised nose with the 25 gallon tank replaced with a 30 gallon tank so the total in that case would have been 199 gallons.
So, I'd say this guy behind DAWA has identified an interesting aspect of Whirlwind performance, and maybe one where the propeller the series-production Whirlwinds received did in fact harm the type's performance, but my preliminary impression is that the detailed reasons for this are probably not reflected perfectly accurately on the DAWA page.
Furthermore, wildly varying dynamic pressures would be passed into the ram-air intakes, which sit immediately behind the blades.8 The system becomes what a physicist would call “chaotic”. The intermittently windmilling prop produces fluctuating boost pressures on top of reduced r.p.m.
Richard Turner
10/11/2023 04:08 PM
A recent exchange of letters published in "Aeroplane" magazine also referred to a whirlwind that was returned to Rolls Royce in an attempt to improve the power of the peregrine engines. Rolls Royce found that the air intakes to the engines were too small and followed a convoluted path , they modified the aircraft sent to them and gained a considerable increase in power. Apparently the modifications were dismissed by Westland because Rolls Royce were just car engine people and knew nothing about aeroplane construction.
Usually used in the context of a transmission. I had an old Chevy with automatic transmission that would "hunt" right about 35mph. Couldn't decide which gear it wanted, and would cycle between them seemingly endlessly until you added speed or slowed to make the decision easier on it.It seems to be a more conventional explanation this time around, though I'm not entirely sure of this as the use of the term "hunting" and the mention of an "intermittently windmilling" propeller sound a bit odd to me:
Less bite, less drag says the PCU, and it takes it all the way to flat. If it goes flat, no thrust is produced.
No thrust, slower speed, less drag, results in a faster shaft.
Drag is going to change instantaneously as the bite changes across the entire prop because the bite/thrust/"lift"(if you will) creates it's own drag.Well, I mostly agree with your description, but the act of the aircraft slowing down is comparatively slow while the pitch change is quite rapid, so I don't think that the cycle you describe would appropriately be described as "hunting", and with the engine running at full speed the entire time, there wouldn't be any boost pressure fluctations either
Yes, but also the tip has the fastest rotational speed. If the propeller tip reaches the critical mach number for it's airfoil, massive drag, massive thrust loss with the shockwave. Not sure how the rest of the slower prop evens that out. And we seem to be in an area of performance where the critical number is creeping up the span of the blades. The main airfoil is curved less aggressively across the middle. If you get supersonic flow across that, you could have a large section affected fairly rapidlyProblem with that is that's not going to be the entire propeller that undergoes the sudden drag rise, as local angle of attack and Mach number are different everywhere along the radius
Compiling data from official flight test reports I came up with this........View attachment 748290
From the Pilot's Notes for the type, it seems that the "All-out level" rating was 3000 rpm, +5 3/4 lbs/sqin [though 5 and 6 look almost the same on the poor scan I have, and there's another, more general limit statement that I'd read as "+6 3/4 lbs/sqin"], but the Pilot's Notes also state that with 100 octane fuel, it was permissible to use up to 3000 rpm/+9 lbs/sqin "for short periods when necessary in operations".
Hi Del,
Well, I mostly agree with your description, but the act of the aircraft slowing down is comparatively slow while the pitch change is quite rapid, so I don't think that the cycle you describe would appropriately be described as "hunting", and with the engine running at full speed the entire time, there wouldn't be any boost pressure fluctations either.
Considering that there's often some kind of hysteresis effect to compressiblity phenomenons, maybe that could induce actual hunting? The propeller speeds up, increases pitch, the critical Mach number drops, drag increases in a leap, the propeller slows down, drag decreases but stays high, propeller decreases pitch and slows down further, suddenly angle of attack decreases critical Mach number below the lower hysteresis threshold, drag decreases in a leap, propeller speeds up, cycle repeats.
Problem with that is that's not going to be the entire propeller that undergoes the sudden drag rise, as local angle of attack and Mach number are different everywhere along the radius. And if the process is more gradual, the leap is averaged out, so I'm skeptical it could happen like that in practice. (And no boost pressure fluctuations with my explanation, either.)
What do you think?
Regards,
Henning (HoHun)
I think also there is a misunderstanding resulting from the community focusing on the Propellers.
I had a look at the Whirlwind airfoils (wing) and the thickness ratio should give us a simple answer. I don´t think there is much to look past it:
View attachment 766671
At 17%, the Whirlwind is certainly on the better par of the end of 1930´s era but certainly not aligned with the NACA 5 digit airfoils known with later props fighters that achieved 400+mph. 15% was an average maximum (Corsair, FW-190, BF-109, Tempest...).
Notice also that the dash 630 mod does not make much sense isolated from the context. It might reflect an alteration along the span of the location of maximum thickness.
I think also there is a misunderstanding resulting from the community focusing on the Propellers.
I had a look at the Whirlwind airfoils (wing) and the thickness ratio should give us a simple answer. I don´t think there is much to look past it:
![]()
At 17%, the Whirlwind is certainly on the better par of the end of 1930´s era but certainly not aligned with the NACA 5 digit airfoils known with later props fighters that achieved 400+mph. 15% was an average maximum (Corsair, FW-190, BF-109, Tempest...).
Yes. That exactly my take too. That's why I suggested that dashing the Naca number was the result of a geometrically varying maximum thickness location. You can see the tip being at 35% already.
@HoHun : Corsair is a 23015. 15%