|
|
Ernest, Marc & Ed.
On reading the info Ed put up, there was a chap by the name of Bill, he quite obviously knew what he was talking about as he mentioned turbulence of exit air causing drag, as well as exit shape (sharp edge separation) causing drag. I didn't quite understand his solution - perhaps someone on here will. So I guess the bigger the outlet the bigger the potential drag as Ernest points out - Tracy was alluding to this as well.
When inlet size is limited, by design or other, I do believe benefits can be achieved with exhaust augmentation, especially needed for climb. As Ed said it's hard to do as climb is slow, as is ground running. You get the best outcome from cruise with either exhaust or outside air augmentation. more in climb with exhaust augmentation, naturally.
However the all important ground running is still the problem.
I guess there might be some help from the prop blast in ground running, the question is, is it enough for a marginal system. I believe it might be.
George (down under)
Ed Anderson wrote:
Interesting, Marc
Certainly seems logical that exhausted aided ejection would aid most at low
airspeeds. That is certainly a tough cooling scenario - with little air
flow. At higher airspeeds (in flight) there should be sufficient air mass
flow to cool - just a matter of taking full advantage of it.
The aided ejection is another path to getting at the same goal ... a localized low pressure area at the cowling outlet.
I was initially very surprised when playing with JavaFoil to see that most airfoils have a high pressure area at the tail end. It is supposed to be high pressure on the bottom and low pressure on the top. What was that high pressure right at the end. Turns out that is where air stagnates. It stops flowing and forms a high pressure area. The rudder cable exit points are known to be an area were air can flown into the airframe on a Delta. That are very near the tail end of the airfoil. This never made sense to me until I realized that stagnation causes high pressure.
What has this got to do with cooling? We want low pressure on the back side of our rads. This requires exit air to exit...fast. It can be made to exit faster if the exhaust opening is to small. But get this, making the opening to large can cause the air to slow more than the opening being to small!! It's about localized flow. If the ambient air is flying smoothly past the exhaust opening, it is likely to create a low pressure area right where you want it (Bernoulli), but if you create a larger and larger opening, at some point you're going to cause the air to trip and go stagnate. You will create a high pressure vortex right at the exhaust opening.
The engine exhaust can be used to push this vortex in right direction to get everything flowing smoothly again. That will allow you to have a larger opening without the stagnation. You can also reshape the airframe around the exhaust opening to insure a smooth flow. The hard part is, you can't really know what you need to do without some sort of wind tunnel. Dang, this stuff is hard!!
My advice, FWIW, if your flying and have marginal cooling...tuft around the cowling exhaust and fly formation with someone to get video of it. If the airstream is smooth, increase the exit size. If it is stagnate, try to add some exhaust augmentation, decrease the exit size, clean up the airflow around the exit, or maybe even open up the inlet to allow more flow. In any case, for the lowest exit pressure and the least drag, you most definitely want the airflow around the rear of the cowling exhaust to be smooth.
--
Homepage: http://www.flyrotary.com/
Archive and UnSub: http://mail.lancaironline.net:81/lists/flyrotary/List.html
|
|