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AeroElectric-List Digest: 7 Msgs - 05/22/06

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Lee Logan

Joined: 10 Jan 2006
Posts: 86

Posted: Tue May 23, 2006 6:12 am Post subject: AeroElectric-List Digest: 7 Msgs - 05/22/06

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
Guest

Posted: Tue May 23, 2006 6:40 am Post subject: AeroElectric-List Digest: 7 Msgs - 05/22/06

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
Guest

Posted: Tue May 23, 2006 7:27 am Post subject: AeroElectric-List Digest: 7 Msgs - 05/22/06

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
Guest

Posted: Wed May 24, 2006 5:47 pm Post subject: AeroElectric-List Digest: 7 Msgs - 05/22/06

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?


	


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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