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  #1  
Old 02-28-2017, 06:39 PM
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SMO SMO is offline
 
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Default Speed differences

When I do a speed test I use the 3 leg process and putting the results into a spreadsheet to determine the accurate speed.

Last October I got TAS of 197 knots.

Today I got TAS of 193 knots.

The differences were - I was 100 PA feet lower today (7980), and it was 17* C colder (30.6* F colder) today. I was also slightly lighter today.

My question: Is it possible to calculate how much of the airspeed difference can be explained by the lower temperature and slightly lower elevation? Or should these things make a difference in airspeed?

My thinking: Colder air is denser therefore increases drag so I would go slower, but by how much?. Colder air is denser therefore should increase power (however MAP was the same - 22.6") so I would go faster, but by how much?

Note: My CAS error in both cases was less than a half knot (0.2 in Oct and 0.0 today).
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  #2  
Old 02-28-2017, 07:34 PM
frghtdg frghtdg is offline
 
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Temp difference might be part of it, but you fall close to a 2% difference between the two flights. Well within +- expected analyst. A tighter squeeze would require more data from very calibrated instruments to find the reason to why 197 vs. 193.......knots.....at 8,000'.
Wow.....
I think you be bragging, hoss.
One Stallion u got there. I would not geld that Spitfire.
Just sayin.
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  #3  
Old 02-28-2017, 07:43 PM
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flyboy1963 flyboy1963 is offline
 
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Default one silly idea

Mark, you are no doubt a highly experienced stickhandler, so just a crazy idea here....you say you were 'lighter'.
if you can quantify how that affected the CG, I know when I am tail heavy, the elevator is lifting just slightly more, and the counterbalance horns are up in the slipstream.
....or some such similar changes could induce a knot of drag I'm guessing!

Do you block off your oil cooler in winter? less air thru the cowling should == a wee bit less drag there.
Did you stick your arm out.... when turning left?

ah, such problems, trying to figure why you are going really fast , or really Really fast!
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  #4  
Old 02-28-2017, 07:46 PM
BobTurner BobTurner is online now
 
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Well,
If you just look at he drag side, it goes like rho v squared. A 17C drop in T is about 6%, so rho should go up 6% at the same pressure.
The engine is trickier. The intake charge scales with density and temperature in real engines, as the speed of sound changes with the square root of T and that influences how fast the air can flow thru the intake port. Bottom line: if you assume the power held constant and just drag changed, then you'd expect about a 2% drop in TAS which is just what you observed. To see if this makes sense we need to know the temperature in the intake manifold.
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  #5  
Old 02-28-2017, 08:32 PM
BillL BillL is offline
 
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With Bob's comment, the engine will be making a little more power, but is adversely influenced by cooler oil temperature. With all the equations and influences, I would suspect you are dead on. Well - +.5kts
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  #6  
Old 02-28-2017, 10:24 PM
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Quote:
Originally Posted by frghtdg View Post
197 vs. 193.......knots.....at 8,000'........I think you be bragging, hoss..........
In both cases I am running lean of peak at ~11.7 gph, 2300 RPM. These are pretty standard Rocket speeds, lots are a fair bit quicker than mine.

Quote:
Originally Posted by BillL View Post
With Bob's comment, the engine will be making a little more power, but is adversely influenced by cooler oil temperature....
Didn't think about OT - was 153*F today and 174*F in October.
FYI average CHT today was 270*F and was 334*F in October - been told I need to throttle the cowl exit a bit to get those temps up, maybe some kind of cowl flap system like others have done.
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  #7  
Old 02-28-2017, 11:33 PM
BobTurner BobTurner is online now
 
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Ah, much simpler now. Operating LOP engine power is pretty much determined by fuel flow. So engine power about the same in both cases. (You should have noticed fuel flow at peak egt was slightly different in the two flights). Thrust equals power over velocity, thrust is equal to drag which equals rho v squared. Or v cubed is proportional to 1/rho. Rho up 6%, V down 2%. Exactly what you observed.
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  #8  
Old 03-01-2017, 07:20 AM
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bret bret is offline
 
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At first glance I was questioning if you meant MPH vs Knots? then I saw your sig, Rocket.....never mind.....jealous!
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  #9  
Old 03-01-2017, 07:47 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by SMO View Post
Didn't think about OT - was 153*F today and 174*F in October.
FYI average CHT today was 270*F and was 334*F in October - been told I need to throttle the cowl exit a bit to get those temps up, maybe some kind of cowl flap system like others have done.
153 OT and 270 CHT? You are definitely a candidate for one.
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  #10  
Old 03-01-2017, 08:42 AM
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SMO SMO is offline
 
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Thanks for all your insight, Bob.

Quote:
Originally Posted by BobTurner View Post
(You should have noticed fuel flow at peak egt was slightly different in the two flights)
I normally do the Big Mixture Pull to a target fuel flow so don't monitor the peak EGT. Would be interesting to see data relating peak EGT and OAT. Perhaps my target fuel flow needs to be adjusted based on OAT - more things I never thought about! Guess I could create the data over time, but might need to wait till next winter now to get some colder days. Warmed up here overnight - now about 31*F on the ground (and snowing) and forecast for warmer.
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