Quote:
Originally Posted by Crumpp
Come on man....
You understand the basic's of rotational mechanic's right?
On any radius of the circle, the point closest to the origin travels at a slower velocity than a point distal to the origin.
That is why as a generality, a smaller disc is better for Vmax performance.
|
1.jpg
There are two shock wave areas in propeller.
One is near tip, the other is around root.
Ma----compound speed, =squareroot of (rotating speed^2+ TAS^2), Mach
Mak----critical shock wave stall speed for a certian airfoil, Mach
The propeller portion near root is usually thick and not very streamline, so Mak is quite low which means easily render shock wave. The tip portion, on the other hand, has a very high(near sonic)Ma, so shock wave inevitable although this portion is quite thin and streamline.
Afterall, there is a trade off upon propeller's diameter above Vmax, if you use a bigger one, the shock wave area near tip is quite big, bad thing. But you get a smaller advance ratio, that's a good thing.The art is to find a optimum point where whole propeller reaches maximum efficiency at a certain speed above Vmax.
Speed is an important concept in combat, just like altitude. Pilots know what's the best altitude for their aircraft, eg, P47D, are willing to fight fw190/bf109 above 6000m altitude. If fly a La7, the lower altitude, the better. Why altitude is so important? one reason is "engine output".So is speed.
If your opponent will lose 500HP at a certian speed between Vmax amd Vne due to lower propeller efficiency, you also wanna drag him to such high speed and beat him in an energy fight style.
The samller aircarft, the lower drag coefficent and smaller weight, thus easier reach high speed and better output/weight ratio. One couldn't have it both ways.Shouldn't those tiny soviet/German aircrafts pay the price during high speed dive?
