Re: [COUPERS-TECH] Prop-er dimensions revisited again

[Dr. R. B.]

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Burkhead, you never post a syllable on this forum but that I don't learn something from it about my own flying.  Thank you for taking the trouble all these years.  I benefit from your remarks every time I go aloft, and so does every Ercoupe pilot who reads you.

 

You're a mark to shoot for, sir.  My greatest appreciation for sharing your knowledge with me.

 

Dr. Robert Beeman

----- Original Message -----

From: Ed Burkhead

To: alan1 ; Coupe-Tech

Sent: Monday, May 02, 2005 1:53 PM

Subject: RE: [COUPERS-TECH] Prop-er dimensions revisited again

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Alan,

 

Prop choosing is a white-magic art.  You’ve got to be an engineer who has studied the subject to do it by calculation.

 

For us mostly-ignoramuses, here’s some discussion.

 

In order for your engine to develop its rated horsepower, it must be able to develop the rated RPMs.  If the prop is so steeply pitched or too long, the engine may strain for all its worth but not turn enough RPMs to get enough cylinder firings per minute to get rated power.

 

There are lots of considerations in prop design. 

 

1.  The airfoil chosen – is it good at high speed or low – often (?always?) the airfoil shape changes from the tip to the hub so it’s in the ballpark of optimum for each part of the prop blade.  A prop designed for a 200 mph aircraft would not be appropriate for a 70 mph aircraft.

 

2.  The length of the prop.  A too-long prop can’t be swung by a small engine – so you may not get rated horsepower.  A too-short prop can’t effectively convert the power of the engine into the flinging-backward of the air (propulsion).

 

3.  The pitch of the prop.  The pitch of the prop is the number of inches the prop would move forward on each rotation if it had perfect efficiency (no slippage).  i.e. a 46” pitch prop would pull the plane forward exactly 46” in one full turn of the propeller.  In reality, there is slippage – about 3%-10% by our casual measurements.  The McCauley 1B90 seems to have about a 5% slippage on a Coupe with a C-85 engine, for example.

http://edburkhead.com/Ercoupe/enginepropeller_efficiency.htm

 

4.  The aerodynamic cleanness of the airframe.

 

5.  The weight of the aircraft.

 _____________________________________

 

Static RPM is the RPM indication you get stationary, at full power, in zero wind.  (A close approximation is sideways to the wind.)  Aircraft Specification A-718 and A-787 both specify, for McCauley 1A90/1B90 that static RPM must be in the range of 2025-2225.  If your static RPM is lower than this, you can’t develop proper horsepower for takeoff.  If your static RPM is greater than this, your climb could be great but you might over-rev the engine in level flight.

 

 _____________________________________

 

Sensenich’s website lists the 76AK-2-46 as being a “standard” propeller for the Ercoupe 415-C, D, E & G (as opposed to a climb or cruise prop).  What you get in performance is way beyond my ability to calculate.  Just guessing, based on the more common McCauley props, a “standard” prop gets you cruise speeds in the range of 108 mph, not that good of climb and a ceiling of maybe (just guessing) 10,000’-11,000’ density altitude.  But this is speculation.

http://www2.sensenich.com/direct/miscprop.htm

 

Because of the mix of variables, it takes pretty good analysis to compute performance without going up and flight testing it to find out what happens.

 

Fred Weick wrote a widely used design guide for small aircraft propellers back in the ‘1925s and it was widely used for a long time (and still has some use).  I found it on the Internet and tried understanding it but it was beyond me.  I’ve learned that really understanding this subject is beyond me.  Knowing the basics, above, and doing some flight testing may give you enough information to make intelligent choices of propellers for your Coupe.

http://www.wood-carver.com/articles.html scroll down for links to Weick’s article.

 

Hope this helps.

 

Ed Burkhead

http://edburkhead.com

ed -at- edburkheadQQQ.com (change -at- and remove the QQQ)

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