A while  back I proposed using excess energy in the exhaust stream to
 drive a  turbine which would provide boundary layer control (suction)
 for  better takeoff and landing performance.  I still think this  isn't
 a bad idea as it might be quite a bit simpler than a turbo  compound to
 construct and implement.  I admit that it would be  important to think
 out what would happen if there were an engine  failure while utilizing
 such a system..

 Similarly, I  think it would be worth exploring what cruise drag
 reduction might  be afforded by applying active boundary layer control
 for parts of  the airframe that are difficult to streamline properly.
 We were  looking at a pusher design recently on here which had the prop
 mounted too close to the widest part of the fuselage for airflow
 without separation.  Active boundary layer control could improve  the
 ability of the air to stay attached as it follows the contours  of the
 fuselage.

 The consequences of a turbo failure  would be poorer cruise efficiency,
 but nothing  dangerous.


 Regards,

 Matt-

 You  can do the same thing by adding Fowler flaps, leading edge
 slats and  cutting down the wing area. Safer as they will not go
 out if the  engine dies. The best example is the Mitsubishi Mu-2.
 http://en.wikipedia.org/wiki/File:Mitsubishi_Mu-2.jpg


 The  ultimate is VTOL. No wing area to speak of.
 http://www.rotaryeng.net/VTOL.html Least safe :)

 Paul  Lamar

 Fowler flaps don't help cruise speed except in that they  might allow
 for lower overall wing area.

 The MU2 doesn't exactly  have an exemplary safety record..  Part of the
 reason is that it has  such low wing area that it's easy to get behind
 the power curve - where  induced drag is rising rapidly.  I recognize
 that part of the MU2's  problem has been pilot training - I'm not
 calling it unsafe outright, just  unforgiving.

 Fowler flaps add significant weight and complexity to wing  design.
 However, if there were a targeted location on the airframe that  would
 be difficult to clean up (fair) but which would benefit from  boundary
 layer control, driving it with the turbo gives you the opportunity  to
 use some of the horsepower in the exhaust stream without having  to
 design a complex reduction unit.


 Matt-

 I understand  Matt. Historically suction devices on wings get
 plugged up. Blowing is  another option and the F104 had that.

 Paul Lamar

 Jim Bede With the XBD-2 and others have done the suction wing idea for
 boundary layer control. The plane also had twin engines driving a ducted
fan
 and  fiberglass main gear legs

 _http://www.bedecorp.com/designs/xbd2.htm_


 The F-4 used compressor bleed air blowing from manifolds exposed when
 leading edge droops extended.  Also from the trailing edge, blowing down
 the
 back of the flaps that extended to almost 90 degrees. So if you are  short
 on
 wing area, and weigh 74,000 pounds, its a good idea.

 Lynn E. Hanover


Again, I'm not interested in high lift.  I'm looking for drag reduction.

Matt-

I understand Matt.
If you break an aircraft down into its component parts I think you will find
the wing drag is the major culprit. The wing drag is mandated by the FAA
61 knot stall rule. At high speed the lift falls to near zero so it is
primarily skin friction drag on the wing. Cut the wing area and cut the drag
hence high lift devices and VTOL.

I recall reading somewhere the percentages of component drag offenders but I
don't remember where.  "A Practical Guide to Airplane Performance and
Design"
by Donald R Crawford could be the place. Do you have Hoerner Fluid Dynamic
Drag?
I think the fuselage is down there.

Yes there are certain fuselages that have been designed that are high
drag. The Glass Goose comes to mind. Suction applied in the right
place might fix it.

The big picture must be looked at in the light of turbo compounding.

Paul Lamar


Don't let me get away from the big picture...  :)


Matt-