RV8Squaz
Well Known Member
OK, before you come to the conclusion that I've fallen off my rocker or been taking a bit much from the bottle, hear this...
While my airspeeds will never match, come close, or even remotely be in the same league as the aformentioned aircraft, my 200 hp, constant speed prop bird does exhibit a similar characteristic. That characteristic my RV-8 and all RVs have in common with the Concorde and SR-71 is... aerodynamic heating! Due to high airspeed, the friction of the air on the aircraft skin will cause a temperature rise. I know what your thinking... "that Jerry must be delusional."
So how did I come to this silly conclusion? Well I noticed my OAT never seemed accurate. I know folks have reported higher temps than actual and I know in some cases it was caused due to poor placement of the probe. However my probe is on the underside of my right wing in the NACA scoop for the copilot vent. It is well away from any potential exhaust. So why did my OAT consistently read about 4 deg C higher than what I though it should read even though I had calibrated it?
On one of those nice flying days (really, aren't they all nice), I noticed my OAT rose about 3 deg C while accelerating from 70 KIAS to about 170 KIAS despite maintaining level flight. Hmmm. I slowed down, the temp went down. Naahh, that couldn't be. It must be some weird air mass thing. Everytime I changed airspeed regardless of altitude, the temp followed. Now, that got my attention and got the electrons in my brain stirring.
So I turned to my favorite information resource... the internet! There I found calculators and formulas that reinforced what I was seeing. I figured I should have been seeing a 4-5 deg C rise? Why the difference? 4-5 deg C rise is from 0 to 170 KIAS, I was going from 70 to 170 KIAS. Thus at 70 KIAS I was already seeing a 1 deg C temp rise due to aerodynamic heating!
OK, sounds great. Can this really be true? Well today I was on final approach into ATL in one of my company's Boeing 757s. Our aircraft and most high performance aircraft have a SAT (static air temp) gage and a TAT (total air temp) gage. The TAT is the sum of the SAT (the true OAT) and the temp rise due to aerodynamic heating. As we approached the final approach fix, we maintained 170 KIAS and 3500' MSL and guess what... The TAT read 4 deg C higher than the SAT!
I can now consider my hypothesis to be correct... At our typical RV cruise speeds, our OAT gages will read about 4 deg C higher than the actual OAT and thus I'm sure my fuselage will stretch a few millimeters as I accelerate to 170 kts! I don't think I 'll be needing any of the space shuttle's thermal tiles or exotic metals, but I do find 4 deg C to be significant if not interesting. The 4 deg C difference will have a minor effect on true airspeed calculations and anything else where temperature is a factor. While saying my RV-8 is like the Concorde and SR-71 is a "strectch" (pun defintely intended), I am astonished that our little airplanes can go fast enough to cause a temp rise! I hope the next time you observe your OAT rise due to increasing airspeed it puts a smile on your face like it does me.
Jerry
N84JE
While my airspeeds will never match, come close, or even remotely be in the same league as the aformentioned aircraft, my 200 hp, constant speed prop bird does exhibit a similar characteristic. That characteristic my RV-8 and all RVs have in common with the Concorde and SR-71 is... aerodynamic heating! Due to high airspeed, the friction of the air on the aircraft skin will cause a temperature rise. I know what your thinking... "that Jerry must be delusional."
So how did I come to this silly conclusion? Well I noticed my OAT never seemed accurate. I know folks have reported higher temps than actual and I know in some cases it was caused due to poor placement of the probe. However my probe is on the underside of my right wing in the NACA scoop for the copilot vent. It is well away from any potential exhaust. So why did my OAT consistently read about 4 deg C higher than what I though it should read even though I had calibrated it?
On one of those nice flying days (really, aren't they all nice), I noticed my OAT rose about 3 deg C while accelerating from 70 KIAS to about 170 KIAS despite maintaining level flight. Hmmm. I slowed down, the temp went down. Naahh, that couldn't be. It must be some weird air mass thing. Everytime I changed airspeed regardless of altitude, the temp followed. Now, that got my attention and got the electrons in my brain stirring.
So I turned to my favorite information resource... the internet! There I found calculators and formulas that reinforced what I was seeing. I figured I should have been seeing a 4-5 deg C rise? Why the difference? 4-5 deg C rise is from 0 to 170 KIAS, I was going from 70 to 170 KIAS. Thus at 70 KIAS I was already seeing a 1 deg C temp rise due to aerodynamic heating!
OK, sounds great. Can this really be true? Well today I was on final approach into ATL in one of my company's Boeing 757s. Our aircraft and most high performance aircraft have a SAT (static air temp) gage and a TAT (total air temp) gage. The TAT is the sum of the SAT (the true OAT) and the temp rise due to aerodynamic heating. As we approached the final approach fix, we maintained 170 KIAS and 3500' MSL and guess what... The TAT read 4 deg C higher than the SAT!
I can now consider my hypothesis to be correct... At our typical RV cruise speeds, our OAT gages will read about 4 deg C higher than the actual OAT and thus I'm sure my fuselage will stretch a few millimeters as I accelerate to 170 kts! I don't think I 'll be needing any of the space shuttle's thermal tiles or exotic metals, but I do find 4 deg C to be significant if not interesting. The 4 deg C difference will have a minor effect on true airspeed calculations and anything else where temperature is a factor. While saying my RV-8 is like the Concorde and SR-71 is a "strectch" (pun defintely intended), I am astonished that our little airplanes can go fast enough to cause a temp rise! I hope the next time you observe your OAT rise due to increasing airspeed it puts a smile on your face like it does me.
Jerry
N84JE