Anyone care to comment?

 

Indeed I would.  This was a topic at the Lancair Oshkosh get together where I spoke on the topic.  I spent some time years ago on the topic of aerodynamic heating which becomes an issue as our speeds rise above Mach numbers of about 0.3 which corresponds to about 200 knots IAS at higher altitudes.  It results in SR-71’s being made of  titanium in order to survive at Mach 3, and the effects on an ICBM re-entry at Mach 18 are mind bending in the extreme.

 

Jeff wrote:

 

I assume my OAT probe, located forward of the baggage door (fuselage) was affected by compression friction and engine heat,

 

There are three SEPARATE effects going on here.  Engine cowl heating is very small except at air discharge points.  It can be appreciable in some of the Cessnas that have a cowl that floats away from the fuselage allowing air to leak all around, but should be negligible otherwise.

 

Most of us know about compression heating.  It is maximum at the leading edge stagnation point where the flow stops (“stagnates”) to zero velocity converting the kinetic energy of air motion to pressure and heating.  This heating raises temperature as the square of the velocity (and more at higher Mach numbers), velocity here meaning true air speed.  As the flow moves away from the stagnation point, it accelerates, pressure falls, and temperature falls.

 

Except ---   in the boundary layer.  There the flow rubs against the wall and generates FRICTIONAL heating (to be confused with compression heating).  The amount of heating due to friction depends on the fluid and there are other secondary effects having to do with compressibility and the local shape, but a very good rule of thumb is that the FRICTIONAL heating will be about 80-82% Of the COMPRESSION heating.  This number is called the recovery coefficient or recovery factor, but is known by other names as well.

 

So what happens is, the airframe is heated by compression alone at the nose of the spinner and leading edges, by a mix of compression and friction as the flow accelerates away from the impact point (as in along the wing or fuselage) and then by friction heating alone where the flow reattains the free stream velocity.  Above the wing, the pressure is BELOW ambient (necessary for lift) meaning the compression heating is negative, but the local velocity is higher than the free stream velocity (Bernoulli), so there is MORE frictional heating on the top of the wing.  The negative compression heating and increased friction heating virtually cancel out.

 

The result is that the entire airframe is elevated to a temperature above ambient, including your OAT probe.  The OAT probe is reading some temperature above the OAT, always, regardless of its location. 

 

Note the Piper chart.  It uses 80% of ram temperature rise for the frictional heating.  You apply this to your OAT reading (subtract it) to get the REAL OAT which is lower.

 

UNLESS you are flying with a Chelton and perhaps some other EFIS.   The Chelton corrects for heating and give you the true OAT.  I have both Chelton OAT and a second conventional OAT for my engine monitor (and an induction air temperature) since the engine is taking in compressed air and I want to know that temperature.  They differ, sometimes significantly, and the temperature difference is the aerodynamic heating effect shown in the.

 

When I was racing with Brent Regan in his LIV, we were flying at 27,000 feet, OAT was forecast 30 C above standard, and the total TAS was 320 knots (burning a lot of fuel).  Using standard airspeed corrections you would get 345-350 knots, a big error.  I made up a set of charts to correct.  Now it is done in some of the black boxes we fly.   Check yours to see what it is displaying.

 

Final caution: if you use your E6B to calculate the TAS from IAS, or use that ring on the outside of your conventional steam gage airspeed indicator, the TAS reading you get is WRONG because are failing to compensate for pitot tube compressibility and aerodynamic heating.  The correction used is for incompressible flow.  For a Lancair IV flying at 285 KTAS, using the incompressible corrections yield an incorrectly calculated TAS of 305, a twenty knot error.  The error is about two thirds due to failure to compensate for aerodynamic heating, and one third for compressibility effects in the pitot tube.   The Chelton system compensates for all of this, and thus the TAS shown on the lower screen of the Chelton is much closer to truth with the primary error being static probe error.  Flight testing with four way GPS calculations and modifications of the static probe will correct this final error, but it takes a lot of very careful work. 

 

Message: distrust your calculated TAS.  It is probably erroneously high.  (Sorry.)  And your OAT is probably lower than what you see on your instruments.

 

Fred