Challenging thermal plane

[Bernd Brunner]

I want to introduce and discuss my design for a scratch-build allround sailplane, which we want to build with our club youngsters. We will buy the fuse and build the rest. The wings will be made of foam with spar and a thin koto-venner.

AOW 750 gr ( 26.5 oz)
Section MG06 mod. with 2% camber and 9% thickness
span: 1900mm / 6.23 ft.
wingarea : 29.9 dm2 = 463.56 in2
(Thanks JonStone!)

The intention is to build a allround sailplane: good for thermaling, but also with a excellent L/D at higher speed. Full-house wing configuration for easier landing and more flexibilty. What do you think about it?
I know, it´s very different to the “normal” thermal planes like BOT or GL. It´s more like a mini F3B plane. Too challenging for youngsters with one year flying experience??

Looking forward to your comments!

Endless lift!
Bernd

[Peytr]

Hello Bernd,

Just bumped into it and a bit late now. But I will take a better look later on. I'm trying to downsize 'high performance' concepts and designing a homebrew 3.2 mtr F3J derivate myself. Looks good at first sight:cool:

Any dihedral?

Took a look at the foil you choose, and I'd prefer someting like MH32, thinned Clark Y (9 or 8% thickness, camber pro rata reduced), thinned SD 3037. They give me a bit more Cl max (for thermal flying) and more than enough speed when slightly reflexed. Your mod MG06 might be faster though, but not much. I would make the flaps a bit longer and the ailerons a bit shorter, especially when you intend to have dihedral breaks at these lines.

Will be back.

[AustinTatious]

good for thermaling, but also with a excellent L/D at higher speed.

Go with an RG15 ..... great airfoil for that!

you may want to skip flaps or at least have that as an option to keep the cost down ( 2 less servos)

15 oz sounds optimistic but go for it!

[nuevo]

750 gr = 26.5 oz
1900mm = 74.8 in = 6.23 ft.
29.9 dm2 = 463.56 in2

Looks like a great design to to me.

[Bernd Brunner]

Peytr:
After a discussion with a member of the famous F3B Logo-Team, the dihedral will be 4 deg. on each side. That seems to be pretty much, but look at Shark and Wobbegong from the logo-team or the beautyfull Mark Drela planes. We will try it!

Peytr, AustinTatious:
I have attached the calculated polar diagrams from the mG06-2-9 compared with the MH32 and the AG43. I think it´s much better than the mh32, especially at those low Re numbers.


Bernd

[Peytr]

OK, the dihedral is a good idea in my mind. Like it for thermal flying especially.

About the foils I don't fully agree with you. When flying at low Re, you're in the high lift end of the low drag bucket of the MH32 (or one of the others I mentioned). The high end lob of the polar is abt where you are at this point and this is all perfect for the foils I mentioned.

As soon as you go at a lower Cl, the plane speeds up and the Re rises. This makes the upper half Cl part of the low drag bucket of the other foils deeper which cancels the MG advantage. In fact the MH 32 does a better job at any Cl over 0.45. It only just shows if you look at the right part of the polars at the right Re numbers.

At very low Cl (whistling now) you're at high Re numbers. The low drag bucket around Cl0 of all profiles you plotted now deepens, so you have speed enough. The MGs (including your mod) is faster, that's for sure, but not dramatic. About the difference to win or loose a speed run in an F3B comp. :D but not much more. So it all boils down to the basic concept you have in mind: Putting the emphasis on thermal flying or speed. MG foils do better at the latter but lack a bit efficiency at Cl 0.5 - 1.1, at the representative speeds.

Please feel free to correct me if I'm making a mistake somewhere, the discussion is what this is all about. At the end of the day I'm sure your plane will fly fabulous, with any of these foils :)

The basic dimensions are fine with me, I like a higher TVC for thermal flying, but this will do well and is a bit more agile. I would however try to lessen the narrowing of the tips on the tailfeathers. Re nrs gets very low here when thermalling which reduces the effectivity of the stab. when you want it most.

[Bernd Brunner]

Hi Peytr,

I have made another comparison between "my" MG06 and the MH32 at Re=150000. The polar curves at higher Cl are quiet similar, but it seem to me, that the Mg has advantages at lower re. I know, that we have to fly with flaps while thermaling. It will be interesting to see how they will work.

ps. one reason for me to try the MG is . i don´t want to build plane no.1001 with the mh32 :rolleyes:

Bernd

[Peytr]

We're a kind of chatting in here :)

I see what you mean in the second polar diagram and the MG will definately scoot well :cool:.

I edited my last post with a few remarks concerning the general layout. Didn't expect you here this fast :).


ps. one reason for me to try the MG is . i don´t want to build plane no.1001 with the mh32


I can perfectly understand what you're trying and often feel the same. As I wrote earlier on I build extreme small gliders. They don't outperform the large ones many times but when they do I smile from ear to ear. Experiment is at least half the fun.

[Peytr]

One more remark.

Take a look at the foils at Re 40.000 which you will encounter in thermal flying, at about 100 mm from the tip. The polars at Re >60.000 of flapped foils are hardly relevant imo. When thermalling you want minimum sink and min. speed enabling you to centre in the core.

Like Eppler already realised, gliders are at minimum, or maximum Cl and (likewise) at max. or min. Re numbers. As I look at your design you'll be at about Re 40.000 at 100mm inwards of the tips just before stalling, or you'll be at 120.000 - 300.000 when speeding.

I'm not propagating MH32 (I actually never used it myself in the light of the same arguments you stated) but I'd prefer a bit more of Clmax, and the higher drag at Cl min., having the youngsters you mentioned in mind.

Good luck and please report back how the plane flies. I'll make some room for other input in this thread now :D

[Bernd Brunner]

Thanks Peytr, i agee we you, that i should look for a section with a higher cl. I found the ag35 or Ag44looks very promising - also at the low Re

@all:
Does anybody knows, whether it´s a good idea to use the ag35 or ag44 for foam-veneer wings? Where should we place the hinge-line? At 70% from LE?

Bernd

[m_acree]

Kudos to you and your club for making the effort to introduce the youngsters to building and then flying their handiwork. Where were you 25+ years ago when I was a kid? I don't know how set you are on actually designing, cutting your wing cores, tails, etc. but there is a very nice and proven full house TD ship "Supra" that is just now being offered as a kit. It sounds pretty much like what you're after aside from the design it yourself aspect.
http://www.rcgroups.com/forums/showthread.php?t=292797

If you're not interested in a kit all the specs., plans, etc. to do a full scratch-build are online. This plane has a 3+m span but you could go with the same planform and just scale it down.
http://groups.yahoo.com/group/Allegro-Lite/files/Supra/=292797

Good luck and have fun!

[Peytr]

@M-acree

Just scaling down wouldn't be the best thing to do, again because Mr Reynolds is looking at our work. The Drela designs are excellent, but at the cutting edge of what's possible with low Re foils. The low weight of the Supra, combined with the narrow tips would bring us in trouble if we just scaled it down. It could be scaled down, if the chords are roughly left the same as they are in the original. Please keep in mind the Supra is very light, which makes it slow (when wanted). Mark Drela uses different foils for every chord, to match the respective Re numbers. Scaling it down would do harm to the characteristics of the design, possibly with ill results.

@Bernd

Since it's a bit quiet in here I'll post again. (never to good at keeping promises like these :o )

The AGs you mentioned are a perfect choice imho. I don't think Mark Drela ever used the 35 flapped, but it sure looks promising in Xfoil. Don't know about foam wings with these profiles, but I can't see any reason why it should not work.

[m_acree]

Peytr:

I can't take credit for the idea of scaling down the Supra. That question was posted in the Allegro-Lite group awhile back by someone interested in making a 120" version (originally 134" I believe). Of course that isn't quite as drastic as scaling it down to the 6.23' that Mr. Brunner is after which would probably have a more drastic effect on the airfoils. Now that I am rereading the original message I see that he isn't saying to scale the foils only the spans, again he is only shrinking it down by 14" ~10% so not as critical. The question and reply from Dr. Drela are in message 5902 and also copied below.

From: "markdrela" <drela@m...>
Date: Sat Jul 17, 2004 2:11 pm
Subject: Re: 120-inch span Supra?

--- In Allegro-Lite@yahoogroups.com, "monkeytumble" <spamhole@m...> wrote:
> Mark,
>
> If one wanted to make a 120-inch variant of the Supra wing, could
> you offer any suggestions regarding where and how you'd decrease the
> panel span lengths?

I'd multiply all spanwise positions by 120/134, and
round off to nearest inch or 1/2 inch for convenience.

[Bernd Brunner]

hi,

Merlin (that´s the name now) has been maiden this weekend! It´s a little bit heavier than expected, cause it´s the prototype and i´ve tried out a lot of things.

Anyway it´s a dream to fly. Good highstarts, high zooms, extremely easy to circle in a thermal (you can put the thumbs off the sticks!). High L/D and last but not least it´s very fast. Good acceleration. One of the spectators visiting our flying fieled today asked me, what kind of drive i´ve build in... :) Handling is much more like a 3m f3j moldi. The best plane i´ve ever designed and build.

Bernd

[talonxracer]

When you run the numbers for the MG06, are you taking into consideration that the MG06 was designed for 30% chord flaps, and the ability to actually use reflex and camber to really change the airfoil? Just a thought.

Doug

[Peytr]

Hello Bernd!

This plane looks excellent! You're progressing a lot faster than I am, I only have the outer wing halves finished at this time. Did you end up with one of the Drela foils or did you use the MG?

[Bernd Brunner]

@Peytr

cause we have made too less foam kernels i have decided to continue building my mg06 wing. All the other planes will get the AG44 and will be with the weight below 800gr - mine has 900gr :rolleyes:
The AG44 is modified too, because we are going to build some polyhydral wings without ailerons and flaps.

@Doug.
yes i do. The reason for more camber was, that i want to build a thermal ship not a slope racer, and due to this i want to have a little bit more cl (lift) without using flaps. After the flights of this weekend, i must say, that the modified section is working like predicted. The flaps are working exellent, a litte bit more camber is increasing the cl significantly, and the LD with a little negative flap is unbelievable. I can recommend this section.

Bernd

[Wing-span]

Brunner,

Was there any further development with this design?

[glidagida]

Brunner,

Was there any further development with this design?

Hi Bernd

Any progress/ reports of flying? Ease of teaching youngsters?

How is the design standing up to launches?

Is the moment arm of the No2 fuselage too short?

Cheers
Dave

[Bernd Brunner]

Hi Wing-span (do you have a name?),

further development? Currently not. Last year i´ve flown Merlin many hours.
Summary: It should be a bit lighter: 30g/dm² is too much, 25g/dm² would be better in light air. A Merlin-V2 would have a AR of 10 not 12 For hard launches the spar should be stronger - but this will be much more effort, and then it wouldn´t be possible to build it with youngsters.
I have learned a lot about the importance of stiffness. Every part and every joint has to be stiff! At Fiss (Austria) i have flown merlin 8bft wind. There where some improvements necessary: e.g. the fixing of the tail wasn´t stiff enough.
The dihedral of 4deg on each side is perfect. Thermaling is very easy.
Merlin is very sensitive on the cg location. It took many flights to find out what´s the "best" cg. But it depends on where you fly. The cg for flat-land thermaling (100mNN)is different from the CG i´ve use in Fiss (2000mNN) - different cl´s ...
A V2 version would use ht14 airfoil on the v-tail instead of the flat-plate currently used. The possibility of adding ballast would be nice.
But with a deeper wing i would need a longer fuse.

Hi Dave! How are you?
Launches: the wing is still in one part! I´ve made a lot of launches with a strong rubber used for F3B planes. Tension 12 to 15kg (30lbs).The wings are flexing a bit. I wouldn´t try full-pedal winch launches or two-man F3J tow.
But for anything else it´s wonderful. As written above: A strong spar would be nice but a lot of effort. And it´s not easy to build a foam wing light enough. May be with 0.6mm Balsa (instead of Veneer), Carbon D-box and a separate build "Drela"- spar. Or a bagged wing...
Flying: i like it very much. Thermaling is very easy (hands-off the tx!), landing with butterfly (Crow) is similar to the big f3x ships. And the launches.. steep, fast and with high zooms :-))

The no2 fuse is light and strong -and long enough for the current configuration. But with more wing-area i would extend it by 10cm.

Oh, the youngsters.... Building planes isn´t cool enough. Most of them haven´t finished their planes. wouldn´t try it in this way one more time.
But those who have: they like it very much.

Bernd