Quote:
Originally Posted by Randy
The rear prop blades had to be cut down about 1.5 inches each . . . tip speeds were too high because the airflow was faster through the rear prop than the forward one.
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Here' a little side note on propeller physics to explain this comment...
As the tip speed of the propeller nears the speed of sound it stops producing thrust. "Compression" acts on the propeller just like on the wing itself (a propeller is a rotating wing). Generally to get around this problem, they add more propeller blades to reduce the diameter (and tip velocity) instead of chopping the propeller blades. It was probably an expedient to avoid designing a new 4 blade variable pitch propeller (the Do 335 had 3 bladed variable pitch propellers...) and installing it.
However, you can't just keep adding blades because the prop wash from the previous blade will disrupted the laminar flow (create turbulence) of the air and once again cause the subsequent propeller blades to lose thrust. 4 blades was the practical limit in WWII (modern turboprop planes sometimes have more then 4 blades due to superior design).
The 4 blade limit also constrains the propeller size for a given engine horse power and aircraft speed. For example, early models of the F4U Corsair had 3 blades, but when the engine horse power was significantly increased (500+ h.p.), they had to use a 4 blade prop. At some point, adding more horse power to a WWII engine is useless because the prop size limited by the size of the aircraft and the engine r.p.m. That' where twin engine planes come in.
One post WWII method around this problem of tip velocity is to use two counter rotating props on one massively powerful engine. This method has the added advantage of eliminating the torque and P-factor (has to do with the asymmetric 'angle of attack' of the propeller blades in non-level flight). However, these complicated transmissions were beyond the technology of WWII.
For all these reasons, having two inline engines with counter rotating propellers is the next best thing, and to Do 335 incorporates these propeller physics concepts into it's design. As an added bonus, the inline engine design also reduces drag compared to a two engine fighter plane like the Me-110 or P-38.
These are some of the balance factors for an aerodynamic engineer when designing aircraft and all of these factors (and much more) were taken into account in the Do 335. It was an excellent and innovative aircraft in it's time and one of my long time favorites!!!
