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Old Apr 05, 2006, 12:33 PM
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Best L/D Ratio

Correct me if im wrong, but if you look at wind tunnel tests and take down a number of cl points with their corresponding cd values in excel, then in the next row divide cl/cd, you will come up with L/D ratios. Right? I know these are not corrected values for wing planform, but to me the best L/D ratio for any airfoil I have looked at comes at a cl of at least 0.6. The value of the CL goes even higher when correcting for wing planform and aspect ratio... and those high CL's correspond to angles of attack at about 4-6 degrees. Am i totally off base on this?
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Old Apr 05, 2006, 12:52 PM
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What are some examples of numbers you have for C_l and C_d? Where are you getting these numbers? From somebody's wind tunnel data?
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Old Apr 05, 2006, 02:07 PM
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Here is a sample of my results. You can see the best L/D ratio is about 0.9 in this case, but it goes down with corrections. It was my understanding that gliders are trimmed to fly at the best L/D ratio... the high cl is produced at high angle of attack
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Old Apr 05, 2006, 02:36 PM
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You have to look at the polars to see how the airfoil performs..
Seling 3021...

Where the drag is the least...
from:http://www.nasg.com/afdb/index-e.phtml
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Old Apr 05, 2006, 02:49 PM
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"Here is a sample of my results. You can see the best L/D ratio is about 0.9 in this case, but it goes down with corrections. It was my understanding that gliders are trimmed to fly at the best L/D ratio... the high cl is produced at high angle of attack"

I think you mean lift coefficient 0.9. Some trim conditions are near L/D in cases. Other trim conditions are for minimum sinking speed conditions. Some trim conditions are for high speed conditions.

For a whole glider, L/G= Cl/(Cdo + Cdi + Cdp).

Cdo is for the airfoil.

Cdi is for the planform including: sweep, aspect ratio, taper ratio, twist, angle of attack. See:
http://aero.stanford.edu/WingCalc.html

Cdp is for fuselage, vert. tail, horz. tail and etc. Cdp=Cdf(Af/Aw) + Cdv(Av/Aw) +
Cdh*Cdhi(Ah/Aw) + etc.
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Old Apr 05, 2006, 02:55 PM
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Quote:
Originally Posted by gsr916
Here is a sample of my results. You can see the best L/D ratio is about 0.9 in this case, but it goes down with corrections. It was my understanding that gliders are trimmed to fly at the best L/D ratio... the high cl is produced at high angle of attack

Am I blind, or am I reading L/D's in the 20-30 range right on your spread sheet? I must be missing something.
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Old Apr 05, 2006, 02:56 PM
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Quote:
Originally Posted by space_case
Am I blind, or am I reading L/D's in the 20-30 range right on your spread sheet? I must be missing something.
0.90 ? You must be looking after the decimal point? Look further to the left!
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Old Apr 05, 2006, 03:12 PM
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I know the L/D's are high, but they are highest when cl is 0.9. that is damn near the stall angle of the wing though. With cl of 0.2 or 0.3, the profile drag is the least, but induced drag isnt necessarily the least. Maybe model aircraft designers dont look at the actual L/D ratio, but rather just the area in the bucket when the cd is lowest. In our senior project report, we can certainly pick a smaller angle of attack (corresponding to lower cl) but the speed will be higher... Not neccessarily what we want but... I have just read in many locations that gliders are normally trimmed at best L/D Ratio, which corresponds to 7 degrees incidence in my case.
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Old Apr 05, 2006, 03:19 PM
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I take it you're saying that the max Cl/Cd on the table (37.50) occurs at Cl of 0.9 and that this corresponds to an angle of attack of about 4 to 6 degrees? Do I understand that correctly?
If so that doesn't seem impossible. What's the aerofoil you're getting these numbers from and what sort of range are the Reynolds numbers for these tests? (are we talking 10,000 or 20,000,000?)

Aidan

P.S.
you seem to have cleared up the first part while I was typing
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Old Apr 05, 2006, 03:59 PM
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Uh... bear in mind that you're not taking into account the airframe parasitic drag in all this. Add in that and you probably end up with a more typical Cl where the L/D is optimized.

Best airframe L/D is always shown in those typical little graphs where the Cl/Cd and the airframe drag rise from opposite sides of the chart and intersect at the crossover where the best AIRFRAME L/D is located. This typically happens at a far lower Cl than the Cl/Cd occurs.

Besides, I was under the impression that L/D is only a whole airframe sort of parameter. Cl/Cd is the airfoil only and is not the same thing as the L/D.
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Old Apr 05, 2006, 05:47 PM
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actually I did take account for parasitic drag of the airframe, just not in those numbers. Regardless, i get results that neccessitate high angles of attack. The wind tunnel drag coefficient was corrected for aspect ratio and parasitic drag. Even if the best L/D is at around 0.8 or so, the angle of attack that would induce stall isnt too far off the angle of attack imposed by the wing incidence... Anyone know of any wing incidences for full size sailplanes? Maybe those are higher. Wikipedia says that an average incidence is around 6 degrees, but I don't know if that is 100% trustworthy. the reynolds # for the wind tunnel test i looked at was for Re = 100,000. Re=200,000 was not far off, and our plane will operate inbetween those values.
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Old Apr 05, 2006, 05:49 PM
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oh, also... I looked at wind tunnel test data in the back of the book: Model Aircraft Aerodynamics, by Martin Simons
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Old Apr 05, 2006, 06:27 PM
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Quote:
Originally Posted by gsr916
Am i totally off base on this?
I don't think you are.

I don't know why around 6 shouldn't be correct?
Those figures seem all reasonable to me.
What exactely makes you doubt them so much?
Well, besides the fact, that one generally should't trust blindly in figures, of course.

biber
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Old Apr 05, 2006, 06:50 PM
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A wing has angle of attack is for summing two parts. The angle of attack for the airfoil and induced angle of attack. The induced angle attack is proportional to Cl^2 and inversed to aspect ratio.

Incidence is for rigging on the ground or into the shop. The angle of attack of the airplane is measured to the air speed in the direction the path of the aircraft from the air mass. See:
http://www.av8n.com/how/htm/aoa.html
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Old Apr 05, 2006, 08:36 PM
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Quote:
Originally Posted by biber
I don't think you are.

I don't know why around 6 shouldn't be correct?
Those figures seem all reasonable to me.
What exactely makes you doubt them so much?
Well, besides the fact, that one generally should't trust blindly in figures, of course.

biber
I am doubting my results because EVERYONE is telling me that a glider should have an angle of incidence between 1-2 degrees. I've measured some of my plans at home and they are around 1-2 degrees too. The gliders seem to fly fine at those trims, but i dont think it is the most efficient flight. But nonetheless, that is why i doubt the results so much. I was expecting to come up with an incidence around 1-2, and instead I get 6-7 for each airfoil i look at.
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