RE: Maynard Hill TAM 5 Aspect Ratio  
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RE: Maynard Hill TAM 5 Aspect Ratio - 9/30/2004 7:13:31 PM   
destructiveTester


 

Posts: 109
Joined: 9/12/2003
From: london, UNITED KINGDOM
Status: offline 		JohnG,

I thought so, but I had an email conversation with the author of Profili who said that my approach would always be approximate because I wasn't modelling 3D flow.

I guess he has a point - I don't really know - but doing CFD is probably a bit beyond both my knowledge of physics and my ability as a Computer Scientist!!

The important thing is the relative performance of sections in the results, not the actual figures - the idea is that you can choose a good section for a planform against a bad one - you have to exercise common sense in interpreting data which can be influenced by lots of real world factors. Its still interesting and useful though I believe....

(in reply to Johng)
       Post #: 26

RE: Maynard Hill TAM 5 Aspect Ratio - 9/30/2004 7:47:41 PM   
Johng



Posts: 1821
Joined: 1/24/2002
From: Deland, FL, USA
Status: offline 		A few more thoughts -

My position is influenced by my experience with a 4m scale glider that just happens to weigh in at 11 lbs. That is with 4 lbs of battery and motor aboard. It's got a rediculously flat glide at it's happy speed somewhere around 45-50 mph. It can also take a hard pull-up at 90 mph that just looks horrible from a wing-bending perspective. I have a picture of this somewhere, but apparently, not scanned. So, I know that a good, efficient, survivable and controlable plane can be built like this. Aspect ratio around 18, and lands nicely, although I avoid yank-n-bank below treetop height. I do put it through it's paces up high, cuban-8s and other classic aerobatics are very nice. I'm not a "sailplane guy" though. I fly a lot of different stuff. I've often thought that the 4m sailplane would make a nice UAV testbed.

The Oswald factor is something that is an approximation in this case - because we didn't get it from testing. However, that's where engineering judgement comes in. Do you really think it makes a material difference that I used an estimate of .8 for e, when the real 3d flow might say .77 or .82? That's what I call bad engineering. Chasing down answers to the n'th degree when a ballpark figure is just as usefull. A 5% error in e becomes a 1% error in CDi which becomoes a .2% error in the overall L/D - range consideration. So, who cares? I don't mean any disrespect to Stefano and Profili at all though. I'm a customer and love his program.

As for the weight considerations of a Drela wing, the Aegea 3m wing can be built at 25oz for structure, or about 1.5 lbs. That's a pretty reasonable budget for an 11 lb plane like this. If you figure the bare structure must weigh about 3lbs, half that for the wing isn't bad. That's for a wing that will take a 200 lb load. Build this wing and you have a wing that will go to 18 g's or so. So, you could build it even lighter.

I do think that another improvement on Maynard's design may be in the airfoil area. Didn't feel much like sorting thru those today though. Sheesh, I been posting and figuring on this thread for a long time. I gotta get some stuff done around here.

_____________________________

John

(in reply to destructiveTester)
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RE: Maynard Hill TAM 5 Aspect Ratio - 9/30/2004 8:20:28 PM   
destructiveTester


 

Posts: 109
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From: london, UNITED KINGDOM
Status: offline 		JohnG,

Here is the best Lift Drag table against Selig wind tunnel data for your 4m soarer - all the suffixes after names are for things like trip wire turbulators. An "a" or a "b" refers to the test-model of the section used.

I agree with you re: Oswald, bad engineering, Stefano from Profili - but I'm a big fan of his program too!

1.5 pounds sounds OK to me for 2m wingspan - perhaps they (composite wings with kevlar, glass, foam and carbon) are just more work to make? Maynard had to make many airframes per season.

The structural performance of Mark Drela's wings is very impressive. He achieves an extremely good strength to weight ratio.

Re: sorting through data - that's why I bothered to code the program because it can get very time consuming trying to determine the optimum section.

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(in reply to Johng)
       Post #: 28

RE: Maynard Hill TAM 5 Aspect Ratio - 9/30/2004 9:06:37 PM   
Johng



Posts: 1821
Joined: 1/24/2002
From: Deland, FL, USA
Status: offline 		I'm having trouble figuring out what your L/D table is doing. What is Ci? Why does it seem that there are different values for Cl?

The Drela Aegea wing most definitely alot of work. The spar is vertical grain balsa with kevlar wrap and CF cap. Since balsa sheets are cut with the grain, the builder has to assemble the vertical grain core from stock. Of course, since it would be overbuilt for the mission, a lot of the material and time could be cut back.

I would guess you meant 3m wing at 1.5 lbs?

_____________________________

John

(in reply to destructiveTester)
       Post #: 29

RE: Maynard Hill TAM 5 Aspect Ratio - 10/1/2004 9:33:18 AM   
destructiveTester


 

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From: london, UNITED KINGDOM
Status: offline 		JohnG,

Remember this is a summary of results for all possible sections on that particular wing. Ci should be Cdi (sorry) - coefficient of induced drag. There are different values of CL because the best LD ratio for a given wing/airfoil combination is at different velocities. Some sections work better faster than others - i.e. it depends where the minimum of the drag bucket is against the amount of lift required to hold the aircraft up I suppose.

If you can tell me what section your glider uses I can produce a table of LD/sink rate for all CL's for that section.

That Drela wing sounds hard to make accurately by hand. My father (who is an ex-engineer) takes the position that it doesn't really make much difference which direction the grain of the webs are pointing - but I have heard that grain should be vertical.

I suppose the point about the Drela wing example is that it has a very high strength to weight ratio. For the TAM 5 flight could a less strong composite wing been made - with say a 5G max load?

I meant a TAM 5 wing or a 3m Drela wing: roughly the same wing area.

(in reply to Johng)
       Post #: 30

RE: Maynard Hill TAM 5 Aspect Ratio - 10/1/2004 10:03:25 AM   
DipStick


 

Posts: 129
Joined: 9/4/2003
From: Holts Summit, MO, USA
Status: offline 		Wow a lot of information to look at. I was not suggesting that a higher AR plane might not be better, only that given the limitations, that the induced drag was not a concern at the speed the TAM flew at. The lower AR larger chord would produce the lowest induced drag when the plane was loaded with fuel if my analyzes is correct.

For my analyzes I used a rectangular form.
6 foot max span
Max weight 11 pound
Fuel weight 5 pound (2 oz per hour.....40 hour max....80 oz or about 5 pounds)
Speed range from about 41 to 48 MPH
equipment weight for all planes 1 pound
For every square foot of wing area I allowed 10.7 oz or about .667 pounds of weight

Steve

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(in reply to destructiveTester)
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RE: Maynard Hill TAM 5 Aspect Ratio - 10/1/2004 11:43:08 AM   
destructiveTester


 

Posts: 109
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From: london, UNITED KINGDOM
Status: offline 		Dip,

Nice analysis! Interesting to note that induced drag co-efficient doesn't change much: as the wing loading goes up (because of the span limitation) you get more induced drag.

Strength to weight would or should be better on the low AR wing though - thicker section at root for example.

(in reply to DipStick)
       Post #: 32

RE: Maynard Hill TAM 5 Aspect Ratio - 10/1/2004 2:38:17 PM   
Johng



Posts: 1821
Joined: 1/24/2002
From: Deland, FL, USA
Status: offline 		Yes, that is very interesting. More evidence that Maynard had the thing optimized given the rules restrictions.

One bone to pick though. At 10.7 oz/sq ft of wing area, the wing would make up 82 of the 96 oz of empty weight. Assume the wing weighs between 1 and 1.5 lbs and a more reasonable figure is 3oz/sq ft. Can you plug that in and see what happens?

_____________________________

John

(in reply to DipStick)
       Post #: 33

RE: Maynard Hill TAM 5 Aspect Ratio - 10/1/2004 2:56:45 PM   
dick Hanson



Posts: 10192
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From: slc, UT, USA
Status: offline 		The really slick part of doing the lower aspect ratio wing is that you can build it lighter (at required strength )- which allows for either more power (bigger fuel load) Or lower AOA for req'd lift --( less airframe weight)
It is a synergistic thing.
I don't have any charts to prove this but actual practice shows it works.

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       Post #: 34

RE: Maynard Hill TAM 5 Aspect Ratio - 10/1/2004 3:36:04 PM   
Johng



Posts: 1821
Joined: 1/24/2002
From: Deland, FL, USA
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quote:

ORIGINAL: destructiveTester


If you can tell me what section your glider uses I can produce a table of LD/sink rate for all CL's for that section.


I can't It's the DG-800 4.2m being sold by Icare right now. It was reviewed in MAN a few months ago.

quote:


That Drela wing sounds hard to make accurately by hand. My father (who is an ex-engineer) takes the position that it doesn't really make much difference which direction the grain of the webs are pointing - but I have heard that grain should be vertical.

I suppose the point about the Drela wing example is that it has a very high strength to weight ratio. For the TAM 5 flight could a less strong composite wing been made - with say a 5G max load?

I meant a TAM 5 wing or a 3m Drela wing: roughly the same wing area.


The Drela wing is a fine lesson in sweat equity, and more performance requires more effort. The accuracy is in the foam cutting. Then you need to fit the spar into the foam wing. THe spar is end grain balsa a half inch wide. FIrst, making the raw spar from normally available sheet is work intensive, and then tapering it to fit into the wing right is work and precision. So, this probably isn't what your Dad is thinking when he claims grain direction doesn't matter. End grain balsa is rediculously strong in compression, and that matters here.

As far as weight, using less CF impacts cost more than weight. Less balsa would be less work, maybe. I think you could save a good bit of weight by leaving flaps & ailerons off and using lighter kevlar skin, without doublers. TAM only had one aileron on one wing. Also, the wing could be made one-piece, or the wing joiners could be lighter, weaker material.

At 1062 sq in, the wing is very close in size to the TAM wing. No one including Mr Drela would claim that the soaring airfoil would be the best for this mission without analyzing it first.

I've included a section of the Drela Aegea wing. THe grey is the balsa vertical grain core and the black is the CF cap.

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John

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RE: Maynard Hill TAM 5 Aspect Ratio - 10/1/2004 6:47:14 PM   
DipStick


 

Posts: 129
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From: Holts Summit, MO, USA
Status: offline 		Dick we may actually agree on this one...who would of thunk it? But I'm still a diehard numbers guy when it comes to knowing the world...but I can respect that you know things from a hands on perspective. I think I have a better idea what you have been on about and yes I would agree that a lower AR wing can be a good choose under the right conditions or limitations.

quote:

One bone to pick though. At 10.7 oz/sq ft of wing area, the wing would make up 82 of the 96 oz of empty weight. Assume the wing weighs between 1 and 1.5 lbs and a more reasonable figure is 3oz/sq ft. Can you plug that in and see what happens?


John...the reason I used 10.7 oz was that I was assuming the dry weight of the plane minus the equipment would change relative to the wing area. Bigger wing, Bigger tail, Bigger body more weight.... or smaller=lighter but relative to the wing area. If anything the weight is probably a little low for the 8 AR wing because the body would have needed to remain pretty close to the same size, strength and weight, further the spar or strength of the wing/ weight would need to remain about the same for all three planes because they are lifting the same weight, and finally the fixed weight…engine, servos, landing gear, telemetry equipment etc would probably account for more than the 1 pound.

In my example, the TAM total weight minus the equipment was 6-1=5 pounds.....5/7.5=.6667 pounds per sqft of wing area. So 1sqfoot of wing area equates to a plane weighing .6667 pounds per sq foot of wing. For the 8AR wing plane I’m saying the whole plane would weigh 3 pounds plus a pound of equipment. So maybe the wing would be around 1 pound and the body 2 pounds. If the equipment made up 2 pounds of weight then 4/7.5=.5333 then the TAM would still weight 11 pounds (5pound fuel, 2 pounds equipment, plus 4 pound of plane). So, the 8 AR plane would weigh (5pound fuel, 2 pounds equipment, 2.4 pounds for the plane, total 9.4 pounds). See new analyzes: John if you want to tell me what the max and min weights should be for each plane I would be glad to plug them in.

Steve

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< Message edited by DipStick -- 10/1/2004 6:49:36 PM >

(in reply to Johng)
       Post #: 36

RE: Maynard Hill TAM 5 Aspect Ratio - 10/1/2004 7:54:37 PM   
dick Hanson



Posts: 10192
Joined: 12/12/2001
From: slc, UT, USA
Status: offline 		Well --- the proof is in the puddin------------
I do know a bit a bout calculus-I think it is hard to remove from your teeth

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(in reply to DipStick)
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RE: Maynard Hill TAM 5 Aspect Ratio - 10/1/2004 9:27:38 PM   
Johng



Posts: 1821
Joined: 1/24/2002
From: Deland, FL, USA
Status: offline 		OK, now I understand what you were doing. That weight/sq ft was for the entire aircraft. The way I read it, it was for the weight of the wing structure. That's probably how I would look at it. Fix the fuselage, etc and look at how the weight of the wing varies. Thus my 3 oz/sq ft weight parameter.

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John

(in reply to DipStick)
       Post #: 38

RE: Maynard Hill TAM 5 Aspect Ratio - 10/2/2004 7:55:53 AM   
destructiveTester


 

Posts: 109
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From: london, UNITED KINGDOM
Status: offline 		John,

you are one lucky guy - that's a great looking glider!

http://www.icare-rc.com/dg800s.htm

The section is an HQ (HQ 2.5/13.5-11) (http://www.nasg.com/afdb/show-airfoil-e.phtml?id=784)

I don't have the polars for it. I could use profili/xfoil, but this tends to give optimistic results - far higher lift co-efficients at low Re, for example than real-world data from my limited experience with it.

I am having trouble understanding the structura