"Similarly, if you have air being blown into the wing and pressurizing it a bit, or if the external skin surface (or gear door) otherwise sees a pressure lower than inside the wing, you'll have a greater problem keeping the gear doors closed but you won't change the overall lift that the wing produces.

 

  Cheers,

  - Kyrilian"

 

Good post, Kyrilian.  As you note, if you want to know where the loads actually act on the structure, the devil is in the details.   The aero guys could care less because the airplane is one giant control volume for analysis purposes.

 

Your last comment above reminded me of a story appropriate to this group of speed freaks and has some interesting aerodynamic lessons taught by a master.  

 

I think it was in 1976 that I made my first trip to Oshkosh in my straight leg 182 with Jim Long, retired Navy jet jock and CFI, and Russ Kirk, a fellow mechanical engineer.  (Palo Alto to OSH was 12:30 actual flying time.  It is half that in a Lancair IV.  But I digress.)

 

Tired of fast food at the air show, we went to the Roxy in downtown OSH for a real meal.  After a two hour wait, we were directed to a table for four, and the waitress asked if we would mind providing the fourth seat to a single gentleman sitting in the corner nursing a beer.  Jim chimed in without hesitation "If he speaks airplanes, he is welcome."  The waitress responded "This week EVERYBODY in Oshkosh speaks airplanes!" and showed the gentleman to our table. 

 

The fellow who approached was very thin, wore a white shirt with a very narrow black tie and a denim jacket with the elbows worn through.  "Hi," he said.  "Thanks for having me.  My name is R.T. Jones."

 

I looked at Russ and both of us had to lower our eyebrows from our scalp line. 

 

“Where are you from, R.T.?”

 

“A little town you never heard of named Mountain View in California.”

 

“We know it well since we all live next door.  Do you work at NASA Ames?”

 

“Yes, I do.”

 

So here we had R.T. Jones, the world’s most famous aerodynamicist, inventor of the swept wing and theoretical genius, sitting at our table. 

 

The restaurant finally threw us out on the street 6 hours later when they closed.

 

Lessons for this group gained during that fascinating, beer soaked evening:

 

“Why are these Vari-Ezes going so fast, R.T.?  Is it because of the canard configuration?”

 

“No, it is because they are so slick.  Theoretically canards are inferior to conventional layouts for speed, but for this class of airplane, speed is dominated by friction drag, and the antidote for friction drag is slickness.”

 

In cruise conditions, 90%+ of the drag we experience comes from friction drag, not the induced drag associated with lift and wing plan form.  (Not so for heavy airliners in the thin air of the stratosphere where it is more like 50-50, the point of minimum overall drag.)

 

This led to a discussion of laminar flow boundary layers and the following interesting tidbit. 

 

During WWII, R.T. spent most of his time with full size fighter aircraft in giant wind tunnels testing them to see how to make them faster.  Laminar boundary layer flow as known but not well understood on actual aircraft.  

 

All that was known at the time was put into the P-39 Aerocobra (and the later P-63).  They did the best they could to build laminar flow wings and the engine was put mid-ships behind the pilot to allow the largest part of the fuselage to be farther back to help laminar flow persist along the fuselage as long as possible.  (See the Piaggio Avanti which puts this same principle successfully put to work.)  But the airplane was a major disappointment.  In short, it was a dog.

 

Why?

 

Two reasons were the major contributors.  First, the manufacturing technology of the period (aluminium sheet and rivets) could not construct a surface that was consistently slick enough to maintain laminar flow.  We now know that the surfaces must be very slick and smooth.  On our class of airplane, this means that wavy-ness must be reduced to less than 2-3 mills per inch (that is 0.002-0.003 inches per inch, better than one part in 300) and there must be no steps or disturbances.  The sensitivity to disturbances is maximalized at the leading edge where the boundary layers are thinnest and declines as you move back where boundary layers get thicker, and finally transition to turbulent flow.  

 

This group has seen the proof of step sensitivity with leading edge tape.   Even 0.003 inch thick tape is enough to trip boundary layers unless the step is smoothed out.  Hand painting clear coat along the step and then carefully wet sanding and polishing it after it cures is allegedly good enough to prevent boundary layer tripping with the tape.

 

The second factor was found to be increased internal pressure within the wing and fuselage (“wing pressure,” get it?).  The P-39 had under-wing radiator pods to cool the Allison engine located amidships.  The radiator pods had inlet scoops and diffusers in front of the radiators that converted a large fraction of the ram pressure into elevated static pressure.  The ducts were not well sealed, and a portion of this pressurized air leaked into the inside of the wing and into the fuselage.  And these pressures are significant, about 1 psi at 200 KIAS, and 4 psi at 400 KIAS.

 

The result? Little geysers of air squirting out of every gap, leaky rivet, sheet metal seam, and hole on the aircraft.  Every little jet and leak hurts twice: first the momentum loss of the air scooped up and then squirted out sideways, and second, the disruption of the boundary layer flow over all the aircraft.  Not only do boundary layers become highly turbulent (increasing the skin friction factor dramatically), but the intensity of the turbulence is exacerbated by the energy added by all the little fountains adding further instability into the flow.

 

So what does it mean for us?  It means that as the airplanes get slicker and slicker, it becomes more and more important to prevent air from entering at pressurized locations, and then squirting out in an uncontrolled fashion through holes and seams.  Being composite aircraft, the inherent construction methods yield smooth, stiff, seam-free surfaces.  But consider air leaking out of cowls, from cockpits, from landing gear doors, etc. 

 

Further speed increases will come from continuing attention to detail.  Think about how air can move around inside wings (along pushrod passages, exiting out around flaps and control surfaces or out landing gear doors) and inside the fuselage (door or canopy leakage can be substantial) and then figure out how to minimize it.

 

Fred AKA

Captain Tuna, Chicken of the Skies