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  #11  
Old 10-28-2010, 06:34 AM
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N941WR N941WR is offline
 
Join Date: Jan 2005
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Quote:
Originally Posted by nucleus View Post
My plane goes faster if the extra weight is in the baggage compartment.


Hans
This is true and the reason many planes fly faster after they are painted. The reason being is that most of the paint is aft of the CG and that aft weight helps unload the HS.
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  #12  
Old 10-28-2010, 06:38 AM
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Ironflight Ironflight is offline
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Engineering school was a long time ago, but the formula I always use for Rate of Climb (ROC) is:

ROC - 33,000 x (Excess HP/Weight) with ROC in fpm and weight in lbs. A little algebra can be used with the formula to compare the ROC's for two different weights. While you should know the HP(required and available) from your Phase 1 flight testing, the actual values of Excess HP and the constant drop out when you are looking for Delta ROC.

As my professors used to say, "The derivation is left as an exercise for the student."

Paul
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  #13  
Old 10-28-2010, 07:15 AM
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Nick Nick is offline
 
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Quote:
Originally Posted by wrongway john View Post
For estimating rate of climb, I use Dave Anders very simple rule of thumb that he used for his RV-4, that for ever pound less, it increases the roc by 3 fpm.
That's the same thing Dean Wilson (designer of the Avid Flyer) told me many, many years ago. Is a rough estimate, but works!
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  #14  
Old 10-28-2010, 08:08 AM
Steve Hall Steve Hall is offline
 
Join Date: Oct 2010
Location: Cambridge, MA
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Quote:
Originally Posted by Ironflight View Post
Engineering school was a long time ago, but the formula I always use for Rate of Climb (ROC) is:

ROC - 33,000 x (Excess HP/Weight) with ROC in fpm and weight in lbs. A little algebra can be used with the formula to compare the ROC's for two different weights. While you should know the HP(required and available) from your Phase 1 flight testing, the actual values of Excess HP and the constant drop out when you are looking for Delta ROC.

As my professors used to say, "The derivation is left as an exercise for the student."

Paul
I would modify your formula a bit:

ROC = 33,000 x (Excess HP/Weight) x eta

where eta is the propulsive efficiency of the propeller. For a propellor at optimum efficiency, eta is typically about 85%. But if you are climbing at an off-design speed (i.e, climbing with a cruise prop), efficiency can be significantly lower.
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