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X-15 Lift and L/D at high-mach and high-altitudes


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Feb 13, 2008
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I remember it being said that the X-15 at high altitude and high mach actually only needs it's chines to provide sufficient lift and that the wings are actually (or at least virtually) unnecessary.

This strikes me as quite confusing as while the plane is moving very quick from a groundspeed and mach-number standpoint, it's equivalent airspeed is quite low at altitudes of over 150,000 to 250,000 feet let alone 300,000 or more feet which would mean very little air is going over the plane...

Also the higher the mach number, the lower the L/D ratios get, so instinctively you'd be flying at very low equivalent airspeeds due to the thin air and at high mach numbers where you'd have bad L/D ratios...

So how does the plane get enough lift just on it's chines?

I just don't get it...

KJ Lesnick


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Senior Member
May 28, 2007
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Lift (and drag) is roughly dependent on the square of airspeed. Hence usually with high speed designs the wing size is set by landing or takeoff speeds - it would do fine with smaller wings at high speed. This is actually true even of light aircraft.