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  #31  
Old 11-14-2011, 08:09 AM
RV8R999 RV8R999 is offline
 
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Quote:
Originally Posted by hevansrv7a View Post
I'm sorry but this is just wrong.

Vy is best rate of climb. Best rate of climb is achieved when available horsepower is at its maximum. By definition, this would be at the speed which requires the least power for level flight and that is about 76% of L/D max. For a graphical understanding, please see: http://home.cogeco.ca/~n17hh/Models/ClimbGraph.jpg


But let's be clear that these are theoretical and often have nothing to do with our actual numbers, especially on FP but even with CS props. Engine power is not a constant. In reality, if Vy and L/D max are equal it is a co-incidence. On my airplane, Vx and Vy are pretty much the same at 110 kts indicated, which is well above my minimum power speed of around 75 kts indicated. And don't forget the weight affects these numbers.

sorry but this is just wrong too...

Vy occurs at Maximum Specific Excess power and rarely occurs at minimum power required because of prop efficiency losses at lower airspeeds.
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  #32  
Old 11-14-2011, 09:14 AM
AlanTN AlanTN is offline
 
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I agree that Vy and max L/D are not necessarily the same. 'Stick and Rudder' page 364 paragraph 1 makes the statement, "Of all the working speeds of the airplane this is the one whose exact location on the speed scale is hardest to fix; too much depends on the exact characteristics of the power plant---r.p.m., propeller pitch and diameter and so on."

That whole section on page 363 and 364 is worth reading.

My original idea was that the RV-12 POH defines the speed of best glide as 85 knots, and the speed of best distance (most miles per gallon) should occur at the speed of best glide or possibly slightly higher because you don't have the prop drag that you would have in a glide. Vy is much more complicated.

I was attempting to discover under what conditions Van's was getting 614 miles on 20 gallons of fuel. (Actually I think it is 19.8.)

The comments here have been quite helpful.

Last edited by AlanTN : 11-14-2011 at 09:18 AM.
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  #33  
Old 11-14-2011, 01:19 PM
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hevansrv7a hevansrv7a is offline
 
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Default Can we admit we agree?

Quote:
Originally Posted by RV8R999 View Post
sorry but this is just wrong too...

Vy occurs at Maximum Specific Excess power and rarely occurs at minimum power required because of prop efficiency losses at lower airspeeds.
We seem to agree that in real life, the maximum available power over that required to hold the airplane up is what makes it climb and that the rate of climb is the weight times the rate divided by the numbers for unit of power.

For a theoretical engine, my statements are correct because they are based on airframe behavior (curves) and an assumed constant thrust HP. That is what the graph shows.

As I pointed out in the example of my airplane, the various factors (rpm, torque curve, high AOA, altered cooling, prop efficiencies (advance curve, etc.) and others) in our engines mean that the actual V speeds for climbing can be just about anywhere with no way to know except to experiment. My minimum power speed for level flight is about 75 and my best climb (x or y) is about 110. That's even higher than my best L/D of about 96. If I had a constant speed prop I could probably get my best ROC well below 96.

The only important take-away here is that neither can be determined from the other.

The nice thing about EFIS's such as my GRT is that Vx is easy to find. Anyone with a VSI can find Vy.
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  #34  
Old 11-14-2011, 01:28 PM
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hevansrv7a hevansrv7a is offline
 
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Default Oh, btw, one more thing...

Quote:
Originally Posted by AlanTN View Post
I agree that Vy and max L/D are not necessarily the same. 'Stick and Rudder' page 364 paragraph 1 makes the statement, "Of all the working speeds of the airplane this is the one whose exact location on the speed scale is hardest to fix; too much depends on the exact characteristics of the power plant---r.p.m., propeller pitch and diameter and so on."

That whole section on page 363 and 364 is worth reading.

My original idea was that the RV-12 POH defines the speed of best glide as 85 knots, and the speed of best distance (most miles per gallon) should occur at the speed of best glide or possibly slightly higher because you don't have the prop drag that you would have in a glide. Vy is much more complicated.

I was attempting to discover under what conditions Van's was getting 614 miles on 20 gallons of fuel. (Actually I think it is 19.8.)

The comments here have been quite helpful.


I don't know who said that the best glide for the -12 is at 85, but it's important to understand what is being said.

The speed at which the airplane will get its best mpg in zero wind is unlikely to be the same as its best glide with a zero power engine and also unlikely to be the same as a glide at minimum idle. Further, zero power on the engine can be with stopped prop or windmilling. The simplest way to understand this is to understand that a zero power engine means there is prop drag whereas your best mpg will be with the engine running and thus no prop drag. In the case of the C-152 that Jack Norris tested, the difference between a prop-less glide and a glide per the POH was very significant.

If you want to know your best glide with a dead engine you have to accept the risk, do it carefully, and just do it (hopefully over an airport).

But if you want to know your best speeds with power on there are many methods and my two Oshkosh presentations (on the website) explain that in detail.
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  #35  
Old 11-14-2011, 01:41 PM
RV8R999 RV8R999 is offline
 
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Quote:
Originally Posted by hevansrv7a View Post
We seem to agree that in real life, the maximum available power over that required to hold the airplane up is what makes it climb and that the rate of climb is the weight times the rate divided by the numbers for unit of power.

For a theoretical engine, my statements are correct because they are based on airframe behavior (curves) and an assumed constant thrust HP. That is what the graph shows.

As I pointed out in the example of my airplane, the various factors (rpm, torque curve, high AOA, altered cooling, prop efficiencies (advance curve, etc.) and others) in our engines mean that the actual V speeds for climbing can be just about anywhere with no way to know except to experiment. My minimum power speed for level flight is about 75 and my best climb (x or y) is about 110. That's even higher than my best L/D of about 96. If I had a constant speed prop I could probably get my best ROC well below 96.

The only important take-away here is that neither can be determined from the other.

The nice thing about EFIS's such as my GRT is that Vx is easy to find. Anyone with a VSI can find Vy.
But in real life the THP is not constant throughout the operating envelope
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  #36  
Old 11-14-2011, 07:23 PM
DaveLS DaveLS is offline
 
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The RV-12 Pilot Operating Handbook on page 6-10 published by Vans states the the RV-12 best glide speed is 85 kts.

'Aerodynamics for Naval Aviators' states that the best rate of climb (Vy) for JETS is L/Dmax. This is not necessarily true for propeller aircraft. I just double checked this.

Alan, I think your math works well, starting at 116 mph true and ending up with 88 KIAS, where the other example started at 85 KIAS and ended up with 112 mph true, all at 7,500 feet.

-Dave
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  #37  
Old 11-14-2011, 07:55 PM
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Geico266 Geico266 is offline
 
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I went flying tonight and set the throttle to 97 mph. Knots are for ships.

Fuel burn was 3.2 gph.
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  #38  
Old 11-14-2011, 09:00 PM
AlanTN AlanTN is offline
 
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Thanks, Larry. Do you have any idea what your RPM was? It is interesting that Peter used 3.8 gph and you only used 3.2 gph. It must be those rivets that you filled in! :-)

Last edited by AlanTN : 11-14-2011 at 09:07 PM.
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  #39  
Old 11-14-2011, 09:50 PM
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DonFromTX DonFromTX is offline
 
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I would be very proud of a plane that does that well, congratulations!

Quote:
Originally Posted by Geico266 View Post
I went flying tonight and set the throttle to 97 mph. Knots are for ships.

Fuel burn was 3.2 gph.
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  #40  
Old 11-15-2011, 06:23 PM
DaveLS DaveLS is offline
 
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In the intrest of setting the record straight, I posted information that is just plain wrong. Thanks 'heavensrv7a' for pointing out my error. After rereading (for the third time) here is what I should have said:

L/Dmax is not equal to Vy for a jet, it is equal to Vx, and maximum endurance for jets only. P94 and p154 AforNA

L/Dmax is best glide (max distance in a glide) for both jets and props (if prop drag is ignored, a big if). p94

And,

L/Dmax is the max range speed for props. P94

L/Dmax is a great number because going faster or slower will increase drag. Flaps, retractable landing gear, and weight will all change the speed at which L/Dmax is found. Increasing drag will lower it, decreasing weight will increase it. When flying slower than L/Dmax, raising the nose will eventually lower the aircraft.

-Dave
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