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  #1  
Old 02-25-2007, 04:29 PM
mh53j mh53j is offline
 
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Posts: 29
Default Equivalent flat plate area

Does anyone know what the equivalent flat plate area of the RV-3 is at 200 mph? Any idea for a midget mustang?

Thanks,

Brett
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  #2  
Old 02-25-2007, 05:46 PM
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gmcjetpilot gmcjetpilot is offline
 
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Default There is some good info

From cafe foundation has done reports on RV-4, RV-6a and RV-8a

The only one that list flat plate area and drag estimates in detail are for RV-6a; granted that its not a RV-3 they have identical span and the flat plate ignored gear.

RV-4 (Dave Anders highly modified and arguable fastest RV)
http://cafefoundation.org/v2/pdf/RV-4.pdf

RV-6a (typical built by individual)
http://cafefoundation.org/v2/pdf/RV-...inal%20APR.pdf

RV-8a Factory Demo plane
http://cafefoundation.org/v2/pdf/RV-8A%20APR.pdf


From the RV-6A calculated data:

-6A N157ST Serial Number 20075, power 180hp O360, Prop Fixed pitch, Bernard Warnke, Maple, 52, laminations, 70 x 74 in
Flat plate drag area** 2.32 sq ft
Oswald's e, span efficiency** .851
Cdo, zero lift drag coefficient** .021
Cl max, maximum lift coefficient** 2.136
Propeller efficiency, η, ** not available
Thrust horsepower @ 7079 ft.** 126 hp @ 199.7 TAS
Thrust horsepower @ 9058 ft.**111 hp @ 191.6 TAS
Carson's speed, V for best speed/drag** 121.1 kt/139.5 mph
V for max lift to drag** 92.0 kt/106 mph
V for minimum sink** 69.9 kt/80.5 mph
Maximum lift to drag ratio (glide ratio)**
11.39Minimum glide angle** 4.7 degrees
Minimum sink rate** 749 fpm
Minimum drag** 134.3 lb
CAFE Challenge score @ 176.6 TAS 687,385. (neglecting climb/descent legs)
CAFE Triaviathon score 148.8
For the full report go to Cafe foundation.
From Van's Specs:
The RV-3 has a wing area of 90 sq-ft, 19ft 11in span, wing loading 12.22 lbs/sq ft, power loading 11.0 - 6.9 lb/hp

The RV-6(a) wing area is 110 sq-ft, 20ft 2in span, wing loading 14.5 lb/sq-ft, power loading 10 lb/hp
From Vans specs a 150 hp RV-6a top speed is 196 mph. A 150 hp RV-3 top speed is 207 mph. As a rough estimate, the RV-3 has about 25 hp less drag near 200 mph than a RV-6a. That is shown by Vans spec top speed for a 125 hp RV-3, 195 mph, about the same as a 150 HP RV-6a.

EAA magazine had an interesting article on drag and front plate area of different homebuilts in the last few years. I recall the little w seat retract Glasairs and Lancairs are in the low 2's of sq-ft. However the new Lancairs sit taller for comfort, since many did not like laying on their back to fly. Also they sacrifice wing area (like 25% less) for less frontal area but higher stall and sink. Of course its way more than just frontal area or flat plate area equiv. Variez or Longez is way more efficient than a RV but than you will not fly them in and out of a 1000 foot grass strip, or carry as much payload or cargo as comfortable. Original 2 seat Glasair, Lancairs can be a few mph faster than a RV, but you give up short rough runway performance and simple fixed gear.
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Raleigh, NC Area
RV-4, RV-7, ATP, CFII, MEI, 737/757/767

Last edited by gmcjetpilot : 02-25-2007 at 06:03 PM.
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  #3  
Old 02-25-2007, 06:27 PM
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Kevin Horton Kevin Horton is offline
 
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Location: Ottawa, Canada
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Default

The CAFE Foundation report on the RV-6A that George quoted gives the equivalent flat plate drag area for the parasite drag only. The total drag is higher, as there is also induced drag on top of that.

If you want the flat plate drag area based on the total drag, you can come up with an approximate value based on performance data. The equivalent flat plate drag area, in square feet, is equal to:

348.7 * power * propellor efficiency/ (air density * speed ^ 3), with the following units:

power in units of horsepower
air density in units of slugs/ft^3
speed in units of mph

At sea level, the air density is 0.0023769. You can get values for other altitudes on the web.

If we take our 195 mph at sea level 150 hp RV-6A (numbers from Van's site), and assume a propellor efficiency of 0.85, we calculate an equivalent flat plate drag area of 2.52 square feet. This is a bit more than the CAFE Foundation number (2.32), but our number includes induced drag, and their number doesn't.

Using Van's top speed data for the RV-3, and assuming a prop efficiency of 0.85, I calculate a drag area of 2.1 square feet.
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  #4  
Old 02-25-2007, 06:33 PM
mh53j mh53j is offline
 
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Default GMC & Kevin

Thanks for all the detailed info so fast. This group boggles my mind with the amount of information that can be dispursed in such a short amount of time.

Thanks again,

Brett
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  #5  
Old 02-25-2007, 07:04 PM
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hevansrv7a hevansrv7a is offline
 
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Location: Detroit, MI
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Default Problems with the numbers?

George, maybe it's just my not understanding, but here are my questions about some of the CAFE numbers you cited (I'm not picking on you, just asking for help understanding it). Kevin and others, please feel free to help out.

It describes a triangle for which the acute angle is 4.7 degrees, for which the hypotenuse is 92 kts or 106 mph and for which the base is 11.39 times as long as the height.

The tangent of 4.7 degrees is .0822 and the inverse of that is 12.1632. So why is the glide ratio only 11.39? Should it be 12.1632? In other words, the glide ratio does not agree with the angle given. Or does it?

If 92 or 106 mph kts is the hypotenuse and the base is 11.39 times the height, then the sink at best glide should be 815 ft/min. I get this by finding the proportional length of the hypotenuse as the square root of ((11.39 squared) plus 1)=11.4338. Then I divided 106 mph by 11.4338 to get the vertical speed and converted to ft/min.

But, using a glide ratio of 12.1632, the sink would be 766 f/m or much closer to their numbers. Of course, I'm mixing up minimum sink with sink at best glide, but there's not enough information.

Wondering..
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  #6  
Old 02-26-2007, 05:30 AM
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Kevin Horton Kevin Horton is offline
 
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Location: Ottawa, Canada
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Default

Quote:
Originally Posted by hevansrv7a
[font=Franklin Gothic Medium]George, maybe it's just my not understanding, but here are my questions about some of the CAFE numbers you cited (I'm not picking on you, just asking for help understanding it). Kevin and others, please feel free to help out.

It describes a triangle for which the acute angle is 4.7 degrees, for which the hypotenuse is 92 kts or 106 mph and for which the base is 11.39 times as long as the height.

The tangent of 4.7 degrees is .0822 and the inverse of that is 12.1632. So why is the glide ratio only 11.39? Should it be 12.1632? In other words, the glide ratio does not agree with the angle given. Or does it?
I agree that there appear to be inconsistencies in the CAFE glide ratio numbers for the RV-6A.

Looking at page 4 of the report, we see that the minimum drag was 134.3 lb, at a weight of 1650 lb. This gives a best lift/drag ratio of 12.29. If draw up a vector triangle from this data, the 1650 lb vector is vertical, as it is coming from the earth's gravity. The 134.3 lb drag vector is aligned with the flight path, but pointing backwards. Thus the 1650 lb is on the hypotenuse, and we can get the glide angle by taking the inverse sin of (134.3/1650) = 4.67 degrees, which looks like where the 4.7 degrees value came from. The glide ratio is 1/tan(4.67) = 12.25.

I can't explain where the 11.39 value came from for the glide ratio. That question should go to the CAFE Foundation. Maybe they applied a correction for the drag from a windmilling prop - if so, they should have told us about it.
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  #7  
Old 02-26-2007, 12:11 PM
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gmcjetpilot gmcjetpilot is offline
 
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Default Corrections and assumptions

Quote:
Originally Posted by Kevin Horton
I agree that there appear to be inconsistencies in the CAFE glide ratio numbers for the RV-6A.
I see that, but the devils in the details, note all the asterisks.
"The confidence factor for this data is about 4%, principally
due to less than ideal atmospheric conditions during the tests.
The calculated values noted with ** in this report are derived
from this data. "
Even though the title says calculated, it is from empirical or measured data and analytical (my opinion). The estimates we make by assuming 0.85 prop efficiency, for example, are pretty close but can easily be off by 4% or more. In fact 0.85 prop efficiency would be very good, especially for a wood prop.

Also glide ratio may be affected by residual thrust with idle power and fixed prop.
"The zero thrust glide information is considered only an approx-
-imation on this aircraft due to atmospheric disturbances and
technical problems during the glides. The zero thrust data in
calculated data is thus marked with an **."
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RV-4, RV-7, ATP, CFII, MEI, 737/757/767

Last edited by gmcjetpilot : 02-26-2007 at 12:23 PM.
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