XP-55 as tractor?

[Tophe]

In the past, I thought that the pusher propeller of the XP-54/55/56 family* was, compared to the classical tractor propeller of the P-40/47/51:
- faster because the propeller was not blowing on the dragging fuselage, reason for its proposal
- less safe in case of bailing out (requiring ejector seat or discardable blades etc.), reason for its refusal
Though, I have read somewhere that pusher propellers have the deep disadvantage of "traînée de culot"  (in French, meaning something as bottom-drag or tail-drag, the device reducing it on shells being called "base-bleed" according to the Web's Wiktionnaire – please could you help me translating this tail-drag phenomenon?) and I would like to understand...    Is this like the vortex on wingtips that is dragging unless ducted by a winglet, so the fuselage (and/or fin) aft of the propeller would reduce the vortex drag?

(from http://www.dg-flugzeugbau.de/Data/winglet-...enuntergang.jpg )

This is important to understand the XP-54/55/56 fate, so books presenting them should explain... May our technical experts tell us what the authors have not understood?

That would explain also why some muliple-engined planes (B-36 etc) have been designed with pusher-propellers instead of the usual tractor-propellers... Understanding would be great, to explain if you ever build such a what-if pusher Mosquito...   As far as I am concerned, I consider building my old MPM 1/72 kit of XP-55 as 'improved' what-if tractor Ascender, turned classical, P-51B like...  

(*: XP-54 1/72 at http://www.hannants.co.uk/search/?FULL=PLA07272
XP-55 1/48 at http://www.hannants.co.uk/search/?FULL=CM4806
XP-56 1/72 at http://www.hannants.co.uk/search/?FULL=MPM72098
XP-56 1/48 at http://www.hannants.co.uk/search/?FULL=CM4808 )

[elmayerle]

Tophe, it sounds like the term you're looking for is "boat-tail drag" and it's something generally evaluated quite carefully in wind tunnel testing (though if you do your data reduction wrong, you get a problem like the initial prototype F-111s had where they had excessive boat tail drag because incorrect data reduction led to doing just about everything that subsequent exhaustive wind tunnel testing showed would increase boat tail drag - compare the first prototype with what went into production after they'd done all the mitigation they could).  Basically, the flow around the fuselage wants to swirl into the low-pressure area immediately behind the fuselage and you get drag causing turbulence (look at the patterns in the water behind the back end of a rowboat).  One approach is careful design of the aft fuselage to minimize this low-pressure area; another used on projectiles is base-bleed.  Base-bleed works by generating high-pressure gas that fills in this volume and reduces the vortex flow.  Something like winglets wouldn't work here due to the differences in flow between here and what you have at the wingtips.

A pusher propeller at the back of the fuselage would simply add extra dynamic forces into this evaluation.  In the case of wing-mounted pusher propellers, there's not the same drag from the wing to interact with so you don't have as many problems.  You still will have some interactions though with the airflow around the wing and that will need careful evaluation, first in the wind tunnel and then in carefully calibrated flight test.

If anyone else wants to add in, feel free.  This is just off the top of my head based in aerodynamics courses taken some time ago.

[Tophe]

Thanks, dear engineer, for the translation and explanation.
I think I understand partly: the drag due to bad tail design may be worse than the one avoided by not blowing on one's face, while this is not systematic, everything can be right (even pusher) or wrong (even tractor). That is why wind-tunnels are for, a simple idea is not enough... and we dreamers won't steal your job, dear engineer, our World needs you...
As a smile before making twins, here is the pusher XP-55 becoming tractor as YP-55 (keeping a rear engine, P-39-like), and the final P-55A, without even canard foreplanes anymore (just the originality of swept wings and fins at mid-span)...

[elmayerle]

Tophe, there an article on the SP-55 in the new issue of Flight Journal and they report that the test pilots at Wright-Patterson were really scared of the XP-55's severe and viscious stall characteristics.  Given the stability and control analysis I did for a graduate course using the published NACA wind tunnel test reports on this aircraft, I can believe they were scared since I'd be scared to try flying it.  As built and flown, you really, really needed a stability augmentation system for good handling qualitites as the tunnel data shows it quite unstable.  *sigh* That might be why two out of three crashed.

PS.  If they did convert it to a tractor propeller, they'd like relocate the radiator from the top of the rear fuselage to somewhere that made more sense and allow better shaping of the rear fuselage.

[Tophe]

Thanks for these further explanations, Evan.
While... I guess the stall characteristics were related to the bad control of the canard layout, rather than to a boat-tail drag affecting mainly maximum speed... And the XP-54/56 may have been ordered instead, with pusher propeller and no foreplane - the flying wing was not easy to control either, and that lead us to... TWIN-BOOM  (XP-54), yes! You may trust me, I am completely unbiassed on such a subject... ^_^  

[elmayerle]

I couldn't be entirely certain, but it looked to me like Curtiss didn't properly evaluated the interactions of canards and wing as the numbers were better without the canards. Fascinating what you can find in old NACA reports.