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02-28-2017, 06:39 PM
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Join Date: Oct 2007
Location: Salmon Arm, BC
Posts: 933
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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).
__________________
Mark Olson
1987 RV-4 Sold
2003 Super Decathlon - Sold
F1 EVO Rocket, first flight May 31/14
First in line for the Sonex JSX-2T kit
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02-28-2017, 07:34 PM
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Join Date: Feb 2016
Location: Boynton Beach, Florida
Posts: 62
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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.
__________________
Hank
N14HN Phase 1
140268
B744
Sailplane Racing
Dues
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02-28-2017, 07:43 PM
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Join Date: Oct 2008
Location: Lake Country, B.C. Canada
Posts: 2,416
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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! 
__________________
Perry Y.
RV-9a - SOLD!....
Lake Country, BC
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02-28-2017, 07:46 PM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
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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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02-28-2017, 08:32 PM
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Join Date: Sep 2007
Location: Central IL
Posts: 5,516
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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
__________________
Bill
RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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02-28-2017, 10:24 PM
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Join Date: Oct 2007
Location: Salmon Arm, BC
Posts: 933
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Quote:
Originally Posted by frghtdg
197 vs. 193.......knots.....at 8,000'........I think you be bragging, hoss..........
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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
With Bob's comment, the engine will be making a little more power, but is adversely influenced by cooler oil temperature....
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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.
__________________
Mark Olson
1987 RV-4 Sold
2003 Super Decathlon - Sold
F1 EVO Rocket, first flight May 31/14
First in line for the Sonex JSX-2T kit
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02-28-2017, 11:33 PM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
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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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03-01-2017, 07:20 AM
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Join Date: Feb 2010
Location: Gardnerville Nv.
Posts: 2,828
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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!
__________________
7A Slider, EFII Angle 360, CS, SJ.
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03-01-2017, 07:47 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by SMO
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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153 OT and 270 CHT? You are definitely a candidate for one.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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03-01-2017, 08:42 AM
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Join Date: Oct 2007
Location: Salmon Arm, BC
Posts: 933
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Thanks for all your insight, Bob.
Quote:
Originally Posted by BobTurner
(You should have noticed fuel flow at peak egt was slightly different in the two flights)
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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.
__________________
Mark Olson
1987 RV-4 Sold
2003 Super Decathlon - Sold
F1 EVO Rocket, first flight May 31/14
First in line for the Sonex JSX-2T kit
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