Author Topic: Speaking of staying at Holiday Inn...P-47M Best Glide Speed Testing and Results  (Read 2491 times)

Offline FLS

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Re: P-47M Best Glide Speed Testing and Results
« Reply #15 on: October 29, 2009, 05:36:57 PM »
Rough estimate is a 20 MPH difference in best glide speed between an empty and full fuel state based on speed difference percentage being half of the weight difference  percentage.

Very interesting to see boomerlu figure out that best glide AOA stays the same for a particular aircraft. I suppose that was more fun for him than looking it up. :headscratch: I don't enjoy math but I'm enjoying this thread.  :aok

Offline boomerlu

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Re: Speaking of staying at Holiday Inn...
« Reply #16 on: October 29, 2009, 05:56:51 PM »
You're on the money FLS - I like to figure out how things work and if it takes some math to do it, so be it.
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Offline Stoney

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P-47M Best Glide Speed Testing and Results
« Reply #17 on: October 29, 2009, 10:05:51 PM »
Using the formula posted above, best glide angle for the P-47M will be -4.3 degrees.  I'm still working up the Cd figure.
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Offline boomerlu

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Re: Speaking of staying at Holiday Inn...
« Reply #18 on: October 29, 2009, 11:59:09 PM »
Stoney, if by Cd you mean coefficient of drag... I'm not sure I see the point?
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Offline Stoney

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Re: Speaking of staying at Holiday Inn...
« Reply #19 on: October 30, 2009, 12:03:03 AM »
Stoney, if by Cd you mean coefficient of drag... I'm not sure I see the point?

Well, I stumbled upon that website I linked above doing a google search for thrust equations.  Most of the ones I've seen get pretty heavy with multiple variables and such, especially since they all offer different types of approximations for propellor efficiency.  Anyway, these guys sort of take a backdoor by figuring out the Cd first, through the glide angle, and then use that Cd to help determine the thrust numbers, without needing to know the prop efficiency. 
"Can we be incorrect at times, absolutely, but I do believe 15 years of experience does deserve a little more credence and respect than you have given from your very first post."

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Offline boomerlu

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Re: Speaking of staying at Holiday Inn...
« Reply #20 on: October 30, 2009, 12:35:51 AM »
Still don't quite see why you want Cd - are you calculating it to find the difference between windmilling and stopped?

Edit: I kind of see it now, it is for the windmill/stopped difference.

Aside: The difference between multiple variable calculations and this one is that the complicated ones are theoretical - trying to conclude the most from the least amount of input information. Since you can directly calculate Cd from experimental data, you can shortcut the theoretical numbers. The catch is that you probably can't generalize from it.
« Last Edit: October 30, 2009, 12:54:25 AM by boomerlu »
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Offline FLS

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Re: P-47M Best Glide Speed Testing and Results
« Reply #21 on: October 30, 2009, 06:54:59 AM »
Using the formula posted above, best glide angle for the P-47M will be -4.3 degrees.  I'm still working up the Cd figure.

Shouldn't this be a positive value?

Offline Stoney

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Re: P-47M Best Glide Speed Testing and Results
« Reply #22 on: October 30, 2009, 07:26:44 AM »
Shouldn't this be a positive value?

Delta H is negative from losing 1000 feet.  Maybe I set my formula up wrong in Excel.  From the aircraft, it appears to have a nose down attitude.  The math is not my strong suit.

@ Boomerlu:  If we find the Cd at this condition (basically a clean aircraft), we can then create accurate thrust numbers without having to worry about prop efficiency calculations, which are difficult to create, or we can actually derive prop efficiency numbers, should we so choose, because Thrust = Drag in unaccelerated flight.  If you look through that website I posted above, there's a whole battery of performance data we can create once we find some of these flight test numbers.  We could also do glide tests with different loads on the plane (drop tanks of different sizes) to determine their contribution to drag.  For example, do the 75 gallon DTs create less drag than the 150 gallon DTs?  How much?  We could also compare the different P-47s to each other.  The D-11 in-game is always said to have less drag than the bubble tops.  How much?  What impact does the bubble top have on drag.  Anyway, tons of potential to do some research on the different aircraft in-game.  I happen to be a P-47 fanboi, so I started with the Jug.
"Can we be incorrect at times, absolutely, but I do believe 15 years of experience does deserve a little more credence and respect than you have given from your very first post."

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Offline FLS

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Re: P-47M Best Glide Speed Testing and Results
« Reply #23 on: October 30, 2009, 08:58:17 AM »
Glide path angle makes sense. I thought you meant  AOA for best glide. 4 degrees is ballpark for best glide AOA. Wing incidence can give you nose down at positive AOA.

Offline hitech

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Re: Speaking of staying at Holiday Inn...
« Reply #24 on: October 30, 2009, 02:19:47 PM »
Well, I stumbled upon that website I linked above doing a google search for thrust equations.  Most of the ones I've seen get pretty heavy with multiple variables and such, especially since they all offer different types of approximations for propellor efficiency.  Anyway, these guys sort of take a backdoor by figuring out the Cd first, through the glide angle, and then use that Cd to help determine the thrust numbers, without needing to know the prop efficiency. 

Can you give me that link?

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Offline FLS

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Offline colmbo

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Re: P-47M Best Glide Speed Testing and Results
« Reply #26 on: October 30, 2009, 02:42:27 PM »
Glide path angle makes sense. I thought you meant  AOA for best glide. 4 degrees is ballpark for best glide AOA. Wing incidence can give you nose down at positive AOA.

Don't forget that AOA is based on relative wind.  You're descending in the glide so the glide angle will always be negative while AOA can be quite high if you get slow enough.
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Offline FLS

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Re: P-47M Best Glide Speed Testing and Results
« Reply #27 on: November 01, 2009, 11:31:06 PM »
Don't forget that AOA is based on relative wind.  You're descending in the glide so the glide angle will always be negative while AOA can be quite high if you get slow enough.

Yes that's what I said.