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Lee Logan
Joined: 10 Jan 2006
Posts: 86
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 Posted: Tue May 23, 2006 6:12 am Post subject: AeroElectric-List Digest: 7 Msgs - 05/22/06 |
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Not to jump into the middle of so delicious a cat fight, but you guys might
be interested in this data. I have an insurance company database with
manufacturers specified horsepower, max speed, and 75% cruise speed numbers
for 440 (mostly American) light aircraft. The average horsepower is 220.5,
average top speed is 164.4 knots, and the average manufacturers claimed 75%
power cruise speed is 153.5 knots. That works out to 93.4% of the average
top speed.
Regards,
Lee...
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frank.hinde(at)hp.com
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| Posted: Tue May 23, 2006 6:40 am Post subject: AeroElectric-List Digest: 7 Msgs - 05/22/06 |
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Aren't catfights great?
I too have been on the receiving end of massively overinflated speed
claims by kit manufacturers. However if there is one thing everyone says
in their first flight reports of a Vans aircraft is..."Performance as
advertised"...More in some cases.
Other kit's false advertising drove me straight into the arms of the RV
fraternity...Now if only I could get my 7a finished!
Frank
--
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Hopperdhh(at)aol.com
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| Posted: Tue May 23, 2006 7:27 am Post subject: AeroElectric-List Digest: 7 Msgs - 05/22/06 |
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Ah! I agree, I'm not a cat either, but a simple engineer. Let's see, drag
goes up as the square of speed so power required goes up as the cube of speed.
(Power=force x velocity) Force in this case is the thrust required to
overcome the drag. Now, the cube root of .75 is 0.90856. So we should expect
speed at 75 percent power to be 91 percent of the speed at 100 percent power.
Isn't math beautiful?
Dan Hopper
RV-7A Flying 164 hours now. What a wonderful airplane!
In a message dated 5/23/2006 10:43:33 A.M. Eastern Daylight Time,
frank.hinde(at)hp.com writes:
Not to jump into the middle of so delicious a cat fight, but you guys
might be interested in this data. I have an insurance company database
with manufacturers specified horsepower, max speed, and 75% cruise speed
numbers for 440 (mostly American) light aircraft. The average
horsepower is 220.5, average top speed is 164.4 knots, and the average
manufacturers claimed 75% power cruise speed is 153.5 knots. That works
out to 93.4% of the average top speed.
Regards,
Lee...
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khorton01(at)rogers.com
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| Posted: Wed May 24, 2006 5:47 pm Post subject: AeroElectric-List Digest: 7 Msgs - 05/22/06 |
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On 23-May-06, at 11:22 AM, Hopperdhh(at)aol.com wrote:
| Quote: |
Ah! I agree, I'm not a cat either, but a simple engineer. Let's
see, drag
goes up as the square of speed so power required goes up as the
cube of speed.
(Power=force x velocity) Force in this case is the thrust
required to
overcome the drag. Now, the cube root of .75 is 0.90856. So we
should expect
speed at 75 percent power to be 91 percent of the speed at 100
percent power.
Isn't math beautiful?
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Math is beautiful, but it can mislead you too.
For an aircraft with a normally aspirated engine, it can only produce
100% power at sea level. So, that means that the max speed with 100%
power is at sea level. The speed with 75% power at sea level should
be about 91% of the speed with 100% power, as you calculated.
However, if we keep the power level the same (i.e. 75%), and we
increase the altitude, the true airspeed will increase, as the air
density is lower, and thus the drag is lower. So, the speed at 75%
power will be highest at the highest altitude at which the engine can
produce 75% power. This will probably be somewhere around 8,000 ft,
and the speed at 75% power at this altitude will be more than 91% of
the speed with 100% power at sea level.
Kevin Horton
RV-8 (Finishing Kit)
Ottawa, Canada
http://www.kilohotel.com/rv8
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