From: brndlfly@athena.mit.edu (Matthew T Velazquez)
Subject: Re: Past the speed of sound...
Organization: Massachusetts Institute of Technology
Distribution: usa
Date: Mon, 14 Dec 1992 22:54:44 GMT

L697187@LMSC5.IS.LMSC.LOCKHEED.COM writes:

>I have a friend here at work who wants to know why propeller-driven
>aircraft cannot go past the speed of sound?  Or can they?  Feel free
>to get as technical as you like in your explaination...we'll just
>pretend to understand.

The big problem with propeller-driven a/c is that the propeller is necessarily
going faster than the aircraft (in some cases a lot faster).  That means that
the blade tips will approach the speed of sound a lot earlier than the rest of
the airplane.  Lots of problems stem from this.  As compressibility (i.e., high
subsonic Mach #) comes into play, you begin to develop buffeting (everybody 
remember their Right Stuff?).  Propeller buffet is B-A-D.  If you can make the
propeller shaft survive the transonic buffeting, you get this neat little
phenomenon called shock waves.  Now shock waves aren't all that bad by them-
selves; they're just a necessary consequence of going mother fast.  When you
put them on a propeller, you get all sorts of nice problems.  The wave drag
associated with a supersonic airfoil is really big, like maybe half again as
much as the viscous drag, depending on the Mach No.  That means that in 
addition to overcoming the drag of the aircraft, the propeller also has to
overcome its own wave drag.  What does that mean?  Well, the wave drag of the
propeller is opposite the rotation direction (about).  So you've got all of
these little shocks trying to pull the propeller the wrong way around the
shaft.  Not very economical.  
	To answer your question, there is nothing stopping propeller-driven
a/c from busting the sound barrier.  In fact, I think an easy formula for
accomplishing it would be:

		BD-5 + Rare Bear engine = supersonic prop plane.

	Yeah, yeah, so I'm getting a little out of hand, but you get the idea.
It takes a LOT of power to get there.
	There are ways to get around it, though.  Sweeping the prop tips is one
way.  You see swept tips on some helicopters (Apache comes to mind).  Sweeping
the tips decreases the velocity normal to the leading edge, and therefore
decreases the normal Mach Number.  It turns out that in all supersonic flow the
normal direction is the only important one for determining pressure drops, 
thrust coefficients, etc.  So now the supersonic prop problem becomes a 
structures problem.  How do you make a propeller that's going in excess of Mach
1 around a seven, eight, ten foot disk with swept tips and not have it fly
apart?  A whole new ball game...
	OK, tradeoff time.  Another way to get around the transonic problem is
to make transonic airfoils.  The reason you get shocks on the suction surfaces
of airfoils is the pressure gradient there.  You can design airfoils to reduce
the pressure gradient across the upper surface and thereby delay the onset of
the shock wave by quite a bit.  The airfoils come out looking lilke guppies.  
The problem is that said airfoils suck at lower speeds (just like low-speed
airfoils suck at high speeds, imagine that:o)#.  Note that these airfoils will
not help you ONE BIT once you go supersonic, 'cause the whole idea was to allow
the airplane to get closer to Mach 1 without having the prop go supersonic.

Whew! Time to quit.  Hey Mark, did I just mislead the entire Net?

	Hope that helps, or at least lets you get some sleep in front of your
	terminal:o)#

			-T
				T Velazquez
				MIT Aero/Astro
				brndlfly@athena.mit.edu
"Crayolas are one of the few things the human race has in common."
				-Robert Fulghum