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Author Topic: MA409 Airfoil - Penetration Performance Question  (Read 2071 times)
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Trebor
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« on: March 18, 2011, 01:40:41 PM »

Hi

I am new to this, or any other, aerodynamics forum and have an interest in multiple fields ranging from: low Re MAVs, ornithopters, RC gliders and electric flight. I blame this on watching Those Magnificent Men in their Flying Machines when I was a child, which made me want to be an aircraft designer. Unfortunately, this didn't happen and I would now like to take up model aircraft as a hobby. I am definitely not an expert but have studied and collected articles over the years and wish to increase my knowledge.

I have a question regarding undercambered airfoils as follows: It is often stated that undercambered airfoils have poor penetration characteristics including the following by Yak52 on January 11 2011: "Thin, cambered airfoils like MA409 are good for free flight but not found on (modern) RC gliders because they don't have the penetration ability - ie they tend to be draggy at high speed. " I am having difficulty reconciling this statement with the polars of the MA409 as measured in the low-turbulence UIUC wind tunnel by Michael Selig: Summary of Low Speed Airfoil Data - Volume 1. Looking at the graph of Cl v Cd for the MA409 and comparing this with other airfoils the following conclusion can be drawn:

1. The MA409 drag at lift coefficients between 0.0 and 0.6 appears to be equal or lower than that of SD7037 (F3J Thermal soarer) for Reynold numbers ranging from 45000 - 300000!

2. The MA409 drag is also equal or lower than that of SD6060 (a slope-racing section!) at lift-coefficients between 0.0 and 0.6 for Reynold numbers of 45000, 60000,100000 and 300000. The SD6060 has slightly lower drag only at a Reynolds number of 200000, at lift coefficients between 0.0 and 0.2.

If penetration is being defined as the ability to move fast at low lift coefficients, with low drag then the MA409 appears to meet this criteria. I appreciate that the other 2 airfoils mentioned are capable of working at higher lift coefficients than MA409 so perhaps this is not a completely fair comparison. Would I be right in assuming that the performance of the MA409 may suffer relatively to other airfoils, if flying inverted? If this is the case then maybe this could be used to advantage in a duration contest when you need to rapidly descend to land exactly after 10 minutes? Does the MA409 perform better than expected because it is only 6.69% thickness with a camber of 3.33%?

Apologies for the length of this first post and please point out if I am making any fundamental errors in my assumptions.

Rob
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Tmat
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« Reply #1 on: March 18, 2011, 02:40:03 PM »

I think you are looking at it the right way. At low lift coefficients, and higher Reynolds numbers (such as in a dive, or powered flight, or a fast, cruise glide case) you want to see the drag coefficient. The MA409 to me looks like a low camber(ish) fairly thin airfoil that would be suited for R/C gliders rather than free flight models. Free flight airfoils often have far more camber (and undercamber) and have large drag values at low lift and high Re conditions.
This is how the MA409 looks if anyone isn't familiar with it: http://www.worldofkrauss.com/foils/1053

L/D is probably a good comparison for penetration I would think.

Tony
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Yak52
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« Reply #2 on: March 18, 2011, 03:53:54 PM »

Apologies if my post caused any confusion: in fact MA409 wasn't the best example of what I was talking about in that thread. A true 'free flight airfoil' would have more camber as Tony says.

The polar for MA409 shows the drag bucket oriented towards higher lift coefficients. But it's still a good all rounder I would think.

The context we were talking about was good airfoils for (very) low reynolds numbers. I was really recommending it as an example of a low Re foil. Compared to SD7037 and SD6060 at Re 25k the MA409 does quite well at the higher CL's, probably because it's a thinner section, even though the Selig-Donovan foils have a wider drag bucket.

As far as I recall MA409 doesn't have a particularly high section power factor (for thermalling). I have seen it used on fast, small models so perhaps it would be ok from a penetration point of view.

As I said - mea culpa. It wasn't the best example to give in that context.
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« Reply #3 on: March 18, 2011, 04:25:23 PM »

Yes,
Something like a CH407 (Hank Cole's F1A airfoil) or similar would be a good example of a low Reynolds number airfoil with very poor penetration, yet a good glide.

Tony
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Trebor
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« Reply #4 on: March 18, 2011, 04:36:32 PM »

Tony, Yak52

Thanks for the replies. The data I was looking at was in a 15 year old UK magazine called Quiet Flight International, Issue 18 December/January 1996. The airfoil is in the F1C category and had the lowest drag at zero lift of all the contemporary sections then tested. At a Reynolds number of 40000 it appears to have a L/D at near the stall of approximately 33.33 CL=1.0, L/D = 34.09 CL=0.75, L/D of 22.22 CL=0.4 and L/D of 11.76 at CL=0.02. At CL=0 and RE=40000 the drag figure worsens to approximately 0.022. I am reading these direct from a relatively small graph but have tried to be as accurate as possible. These are the worst case figures for the airfoil, at CL of 0.73 at both Re=60000 and 100000 the L/D is >45.625. I am not sure how these figures compare to more modern free-flight sections but it seems to be a useful airfoil covering a wide range of Re 40k-300k. If you are interested in exact figures then I can supply this information.

What is the Re range in which you operate?

Rob
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« Reply #5 on: March 18, 2011, 09:14:16 PM »

That airfoil is very much like the one I used in 1988 on a Alum. skin wing FIC. It worked well for the times, there are much better ones now.
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« Reply #6 on: March 19, 2011, 09:25:12 AM »

...What is the Re range in which you operate?...

Pretty small stuff for me: FF scale and sport, around Re 15-35k.

Jon
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« Reply #7 on: March 19, 2011, 11:33:08 AM »

That’s an interesting pickup Trebor. I must admit I'm a bit of an airfoil enthusiast and particularly with RC Gliders. Thus I would like to add some comments on the airfoils comparison approach applicable to RC gliders

You were correct as Yak has admitted in finding that the MA409 did not compare all that badly to the SD7037 up to Re100,000. This is largely due to the lower thickness of the MA409 at 6.7% compared to the SD7037 at around 9.2% even though the MA409 has more camber at 3.3% than the 3% approx of the SD7037.

However as the Re’s climb to 200,000 and above the drag advantage of the MA409 occurs over a lower range of CL – approx 0.7-0.9. Depending on the wing loading and wing chords it’s also quite likely that the CL’s at 200,000 will be closer to 0.15-0.2(for RC gliders), where the SD7037 is actually better.

Thus unless the airfoils are compared either by nominating definite operating points of Cl and Re or by using the Reduced Re No for a particular wing loading it can be difficult to pick one airfoil in preference to another using 2D polar information. I actually have used the first approach on a few occasions now but XFLR5 makes the reduced Re No approach a lot easier than it was.

Modern Rc Gliders – particularly F3B are now using a lot less camber than the 3+% SD7037 and MA409 at less than 2% - from 1.8% down to 1.6%, while generally a bit thicker at between 7-8.5% both F3J and F3B.

The range of Re’s – below 100,000 where the MA409 outshines the SD7037, is really closer to larger free flight applications than RC gliders -thus Yak was basically correct in reference to modern RC gliders.

I hope this helps Trebor – you certainly have a good grasp of basics and I don’t regard myself as an expert either.
As a matter of interest I have included XFOIL plots of CL/CD for 45,000 60,000 100,000 200,000 and 300,000 for the SD7037, MA409 and something close to a modern F3B section – the DP187.83 – 1.8% camber, 7.83% thick. Note the low CL’s where the drag bucket is better than the other 2 foils.

Flaps are used to reduce the disadvantage during launch and to a lesser extent while thermalling.

Cheers
John
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Trebor
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« Reply #8 on: March 19, 2011, 11:54:41 AM »

Tony

I have spent some time over at the worldofkrauss site www.worldofkrauss.com and notice that there are some significant discrepancies between the polars calculated using Javafoil and the actual UIUC wind tunnel measurements by Michael Selig. As the coordinates used to calculate these polars are also extracted from the UIUC then this is a cause of concern! The worldofkrauss site allows comparisons to be performed between different airfoils at a fixed Re of 100k and I would have expected that, although computed and actual values may not exactly correspond, the relative performances of the airfoils could be evaluated. This does not appear to be the case because the airfoil that one would judge to be superior (lower drag at the lift coefficients of interest) as shown by worldofkrauss is actually inferior when looking at the wind tunnel polars.

For a specific example compare the MA409(smoothed) with the A18(smoothed): http://www.worldofkrauss.com/foils/show_compare/?id[]=702&id[]=1053&chord=6.5 . The actual wind tunnel results have lower drag for the MA409 at lift coefficients between 0.0 and 0.8. The lowest drag for the MA409 at Re=100k is at a Cl of approximately 0.175 where the A18 airfoil drag is significantly higher. How trustworthy is Javafoil in the results computed when compared to actual measurements from a well respected low-turbulence wind tunnel? Would you expect the codes used to be less or more accurate at lower reynold numbers?

Rob
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« Reply #9 on: March 19, 2011, 12:19:58 PM »

Trebor,

Java foil does not model separation bubbles so at low Re, and especially with thicker and higher camber foils it is probably a little optimistic... Don't forget that empirical testing has it's accuracy issues too! Different institutions can produce some quite different results, especially at low Re.

Have a look at Martin Hepperles site: http://www.mh-aerotools.de/airfoils/javafoil.htm There is a section on the limitations of Javafoil.

Add to that the fact that it's unlikely anyone can replicate an exact airfoil on a real model - especially if tissue covered. I personally (though I'm definitely not an expert either Smiley) tend to use the UIUC tests where I can... but the World of Kraus site is handy for general comparisons. The shape of the drag bucket tells you a lot - above critical Re of course...

Jon
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« Reply #10 on: March 19, 2011, 12:30:21 PM »

I have reattached the plots as png from my previous message they didn't appear to open as jpegs.

Also i agree with YAK's response and it is the reason I use XFOIl which I drive from Profili. Mark Drela's XFOIL is a much better low RE airfoil programme.

John
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Trebor
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« Reply #11 on: March 20, 2011, 07:54:11 AM »

FF Bruce

Please can you supply details of which you consider to be the current state of the art F1C airfoils. I ask this because even the might of Google is failing to make an impact with this question! I am not sure what it implies to be in a field that is so niche that this information doesn't appear to be easily available to non-insiders. Other FF categories are hardly mainstream but they appear to be shouting from the rooftops by comparison Smiley

Rob
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« Reply #12 on: March 20, 2011, 10:14:31 AM »

The current state of the art of F1C foils is not an easy question to answer. There are two reasons for this. The first is that the current best models are likely those of either Verbitsky and Babenko (Ukraine). Since they sell "factory" models, much of their aero work is a "black box" of information. That is, there isn't any! You buy the model and hopefully are satisfied with it. (most are it seems). The other issue is that the current F1C state of the art uses either variable geometry (folding wing) or variable camber (flaps) or both! (folder with passive flaps). The folder airfoils are tough to analyze as they are so dependent on the joint when the wings close, and you'd really need 3D analysis to determine the drag of the smaller, folded wing compared to the unfolded wings of others. Also, both of those great flyers rarely publish their work, and when they do they seem to eschew co-ordinates for their airfoils. I think they actually use a more pragmatic cut and try approach!

Brian Eggleston has designed several new F1C profiles that according to x-foil should out perform a flapper (not sure about the folder) but you would have to contact him directly to obtain his airfoil details.
Contact me at: tmathews180@gmail.com and I'll forward your enquiries to Brian.

At Lost Hills in Feb. Babenko had a new folder design and had switched to a direct drive motor instead of the geared engine with the larger prop. Apparently his still air tests indicated that the larger propellor used on the geared motor produced a significant glide penalty even though the geared motor produced a 10 to 15 meter climb benefit.

I hope this helps.

Tony
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« Reply #13 on: March 20, 2011, 11:53:12 PM »

So, Tony, does the current F1C World Champion fly a folder or a flapper?

John
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« Reply #14 on: March 21, 2011, 01:02:15 AM »

Neither, as you know.
But, according to Mike Woodhouse, Pete isn't flying anymore and is not going to the upcoming W/C in Argentina. He does have a Eggleston airfoil model that apparently goes brilliantly. But alas, we probably will have to wait for someone else to fly one...

Pete Watson showed that a conventional model can win in the right hands. No conventional models were able to beat the flappers or folders this past Feb in Lost Hills.

Tony
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« Reply #15 on: March 21, 2011, 11:10:08 AM »

Sorry Tony, I was just trying tease you!

It's a pity that Pete W. seems to have lost interest in F1C (actually, I think he has completely lost interest in long distance travel - or the case of Argentina, very long distance travel)!

One of the conundrums (conundra?) in a these discussions is that talking about aerofoils doesn't take into account the concerted development that goes into these 'specials' (folder or flapper), which isn't really the case on the normal models where development is much more steady. Years ago Thomas Koster worked supremely hard on flappers, gaining 2nd at the '71 champs - but won the '77 champs with an unflapped model.

I make no claim to know which direction is best, but I did say in an email to you a few years ago that I thought FAI f/f aerofoils carried too much camber. It will be interesting to see what comes up in the coming months.

I've just got XFoil onto my PC, and am slowly learning to use it!

John
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« Reply #16 on: March 21, 2011, 11:54:04 AM »

Camber is going down now in F1A models and probably soon in F1B. But it's not so simple as you know. The current trend is towards airfoils with a symmetrical "chin" on the bottom to reduce drag at or near zero lift angles. The realization is that none of the FAI classes are pure "gliders" anymore and can benefit from multi purpose wing sections (or variable camber/layout). Super thin and highly cambered airfoils will soon be passe' imo.

Tony
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« Reply #17 on: March 21, 2011, 12:23:01 PM »

Off topic, I know, but has anyone tried a flapped F1B, Tony?

John
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« Reply #18 on: March 21, 2011, 12:42:42 PM »

Yes,
Several over the years (one at least in the Zaic yearbook) and 2 or 3 in the last year. No reports of great benefits that I've heard of and recent calculations show only a small potential benefit. Speeds at the start just aren't high enough it seems. See here for a recent development in F1B flappers: http://www.facebook.com/?ref=home#!/album.php?id=100001665493789&aid=5505

We have some new BE low drag airfoils for F1B that have the same burst drag as the flapper and as good or better glide.

Tony
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« Reply #19 on: March 22, 2011, 12:35:56 AM »

Tony I am curious as to whether there may be an advantage in combining the Eggleston Vortex Nose(Free flight Quartely #37) approach on a basically low cambered thin foil- say around 2-2.5% camber and around 7% thick with flaps.

In this case as I see it - the flaps would be dropped 5-10deg only for duration and the clean low drag foil used to maximise the climb.

My gut feeling is that this would be a better approach than using a reflexed flap on a higher camber foil for the climb. I had a quick look at this on XFOIL, flapping a low camber foil - DP1.8 7.83(1.8% camber and 7.8% thick) and although my early attempts could not match the high Cl of the BE50 (Verbitsky) - it was left well behind on minimum drag at a Re of 100,000 Ncr 9. This foil BTW is a F3B foil designed by Dirk Phlug, an Aero Engineer and modeller who is making a name for himself.

I also had a look at a higher camber foil - 4.5% camber (based on the same DP foil) which had impressive high CL - around 1.4, but did not respond all that well to reflex.

While not completely state of the art - the Verbitsky BE50 may still be a good starting point for F1C.

John
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« Reply #20 on: March 22, 2011, 10:20:48 AM »

The vortex nose (aka "Stanfoil") is useful for low Re only, so F1B is about the largest category that can make use of it. Brian looked at it for F1C and found it was not beneficial.

The approach of using a low cambered airfoil with flaps for the climb is interesting. My suspicion is that the climb benefits might be offset by the glide penalty that the kink from the deployed flap would present. Remember that the profile drag is only a portion of the total model drag in the climb. A 20% reduction in profile drag does not produce a similar reduction in total model drag (as I'm sure you know).

I know that Brian looked at a hybrid foil recently that was interesting. He had created an F1A airfoil with a bit more camber than his more recent F1A, low drag profiles. This was designed as a "gentleman's" F1A airfoil, that is, for those older sportsmen that cannot run as fast anymore. It was biased more towards the glide than the ballistic bunt phase. But it still has a generous "chin" on the lower surface so is still a low drag airfoil. He added virtual flaps to this airfoil and got some very interesting drag values at high Re and low CL. It's a tricky thing to design an efficient flapper airfoil. The new airfoil that Verbitsky is using appears to be cleverly shaped. It has a nice shape in both the reflex and flap down mode.

Both Verbitsky and Babenko seem to be looking for ways to improve the glide at the moment rather than the climb.

I agree that the BE50 is an excellent place to start.

Tony
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« Reply #21 on: March 22, 2011, 11:20:08 AM »

These descriptives make me think of Thomas Kosters all moulded 'laminar flow' flappers of the 80's (?) - the wings were quite thick, with a limited flap movement, but the result was;
a) A lack of kinks - flaps up.
b) A lack of kinks - flaps down.

I have a dim (= unreliable) memory that he was using Eppler foils.

I looked through the F1B flapper pics on Facebook, Tony, very interesting and well done, but that wing looks very kinked in the flaps up mode. Intuitively it looks wrong!

John
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Trebor
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« Reply #22 on: March 22, 2011, 11:32:14 AM »

Tony, John

The Verbitsky BE50 was also featured in the UIUC wind tunnel results (featured in QFI Issue 18 Dec/Jan 1996) that inspired my first post. Do XFOIL comparisons betwwen the MA409 and BE50 confirm the wind tunnel results ie:

1. Penetration of the MA409 is superior to that of the BE50 at low Cl 0.0 - 0.3 at all Re (down to 40k) and much better at Cl 0.0
2. Penetration of the MA409 is superior at Cl 0.6 - 0.8 at low Re 40k - 60k
3. The sweet spot for the BE50 (lowest drag) is at Cl 0.4 -0.5 between Re 60k - 100k
4. The wider drag bucket of the BE50 gives it an advantage in L/D at Re > 200k and Cl > 0.8
5. Both the MA409 and BE50 exhibit hysteresis (gradient of lift curve reverses) at Re 60k at Cl of approximately 0.6 and  0.75 respectively

Tony did you receive my email?

Rob
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« Reply #23 on: March 22, 2011, 11:34:07 AM »

Rob,
Yes I saw your E-mail. I've forwarded it to Brian Eggleston so that he can respond to you directly.

I've not tried to run either airfoil using x-foil so I can't answer your other questions.

tony
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« Reply #24 on: March 22, 2011, 11:40:20 AM »

John,
I remember Thomas's laminar flappers. They were low aspect ratio to keep the Re high as I recall. I believe that he was using a Wortmann airfoil (IIRC).

The funny kink in the flapped foil is very common with the F1A flappers as well. While it looks "wrong", the drag reduction compared with flaps down is huge.

That you could achieve equal drag reductions just by clever shaping of the foil and still achieve an excellent glide with no flaps is remarkable and non intuitive imo. At least on x-foil it looks achievable. See the attached foil (F1B 3rd generation)

Tony
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