Moni Motorglider

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lr27

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According to the Incomplete Guide, to Airfoil Usage it's a Wortmam FX 67-K-170. Is there any reason to change it? I suppose if you're not going to wipe off the bugs or build really precisely, a more ordinary airfoil might be ok.
 

Victor Bravo

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If it is the old FX 67K airfoil, that's probably not a real good candidate for a riveted wing on the Moni. Remember that this airplane was originally designed to be glued, leaving a perfectly smooth skin, and so they probably thought this would allow them to use the super whiz-bang airfoil that was on most of the current racing gliders at that time.

But I can tell you from experience that the 67K airfoil does not live up to the wind tunnel performance in the real world. Anyone else who owned a racing glider with that airfoil will tell you the same thing. Mine was a Mini-Nimbus... but ask any other old washed up racer who had a PIK-20, LS-3, Glasflugel Mosquito, Kestrel, etc. Unless you were spending hundreds of hours sanding and filling and polishing and SANDING (maybe every racing season!) you were not getting the full brochure performance out of the wing.

Then add dirt, bugs, raindrops, or anything else outside of a laboratory clean room, and your wing performance takes a noticeable hit anyway. But as the great Ron Popeil would say... but wait, there's more!

That Wortmann airfoil was 100% designed for camber changing, in the 15 Meter and Open Class sailplanes. The low drag "bucket" on this wing was in a narrow range of AoA. In the real world this meant that you were using the flap (camber) control as much or more than you use the elevator (pitch) control.

Now the Moni has full span ailerons that could be converted to flaperons easily enough. But overall, based on quite a bit of field experience (and no formal aero degree) my gut feel is that a Moni built today should not use that particular Wortmann airfoil. It's too fussy and too demanding and too much work for what you get.

Now if Pete Buck or John Roncz or Barnaby or anyone else with significant formal education in low speed aero and laminar wings wants to disagree, I'll certainly listen and yield to their higher level of knowledge. My education on this is solely from the point of view of the "end user", not the research scientist.
 

Bille Floyd

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...
My education on this is solely from the point of view of the "end user", not the research scientist.
Seriously --- for me, that is the only opinion that really counts ; the
opinion of someone that actually used the thing. It's no different in
any other sport i do, (Hang glide, paraglide , kite-boarding , or snow
skiing ; every year , companies advertise their new stuff, with all the
"invalid" reasons that you should buy from them.

Bille
 

BoKu

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If it is the old FX 67K airfoil, that's probably not a real good candidate for a riveted wing...
Full agreement, except that you misspelled "piss poor." The old woodsplitter was very much an academic product reflecting a relatively weak understanding of soaring practicalities. By the time Felix got to FX 81 he had given up on the vast tracts of laminar run, and was designing for less linear distance but more robustness in the laminar flow against bugs and woopsies. It paid off in spades with the LS6, where the right and left wings are so different it's hard to know which one, if either, has the correct airfoil. Fortunately, they both work great.

I forget which, but one of the big-name high-altitude drones uses the FX 67. I'm sure it seemed like a good idea at the drawing board.
 

wwz7777

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Hello VB,

Yes, it may be a Workman section but modified. I think because of bankruptcy, JM et al won’t answer any questions about the Moni. Period. Maybe JM will bring the Moni home like the Sonerai. We’ll see. FWIW, I want an airplane, not necessarily a slow efficient wing for gliding. I have figured out how to change the full span ailerons into ailerons and flaps. I’ve had suggestions to use a Riblett, a NACA 230xx. The kit ribs only had a 1/2” or 5/8” flange so riveting would have been challenging anyway. I totally understand using the Moni spar, but that spar can not be purchased anywhere anymore. JM kept the dies that formed the extrusion and supposedly uses them to produce the caps for the Sonex. If a Moni user dings the spar there’s no fix. That’s another reason I wanted a built-up spar was to spare the Moni extrusion. But, nothing is set in stone. I’m open to suggestions.
 

Victor Bravo

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If you do not have the kit wing ribs or spar, I'd look into using one of the more modern NLF airfoils... I'll bet that BoKu can rattle off which one is a likely candidate right off the top of his head.

You can billet machine a Moni style spar, surely wasting a little pile of aluminum, but eliminating all of the riveting and precision assembly work. You can also machine in the steps and joints that allow the finished skins to be perfectly smooth with no chordwise seams, or at least not in the laminar areas of the airfoil.

While you're machining the spar you can put in the right/left overlap joint you are looking for (leaving the bottom longerons alone) and all the tapered transitions, thicker areas of the spar web, etc. Yes it would be a big machining job, but look what you'd get for your money.
 

karmarepair

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That narrow rib flange was not much good, IMHO, for bonding either.

John saw what Richard Schreder (they were friends) and Jim Bede were doing with bonding, but didn't seem to get a full understanding of what Dick in particular was up to. The relatively wide, closely spaced FOAM ribs of the later HP kits meant a LOT of bonding area, and the somewhat resilient foam could deform and accommodate some inaccuracy in forming the aluminum skins and still get very minimal (if any) gaps between the ribs and the skins. I think the cellular nature of the foam helped the bond as well. Even at that, there were incidents of disbonding in Schreder's designs, as-built by homebuilders, something BoKu almost certainly knows much more about than I do; there is a lot online about it, by Steve DuPont, Stan Hall, and Dick Schreder himself.

At least one MONI literally disintegrated in mid-air in moderate turbulence, luckily the pilot was wearing a chute.

Even before dis-bonding reared its head, the MONI had a lot of growing pains. I'm not sure what engine John had in mind originally; they settled on the KFM 107, but it wasn't All That. High fuel burn (it was a two stroke), broken crankshafts, poor support from the Italian OEM. And to get the CG right, they had to bolt steel plates to the engine mount.

The INAV debacle (AFAIK, he didn't go bankrupt, his buyer did) was a bad business. He sold out at the height of the ultralight craze to a British investor everything he'd done up to that point. Design rights to the MONI, MONERAI, SONERAI, all the tooling, AEROVEE conversion inventory, etc. Said investor also spent heavily on a Lotus-designed engine for ultralights (a high RPM flat 2, that used the camshaft as a gear reduction, same strategy of the Continental Tiara engine series), a dry hole (there is NOTHING on the web about that engine, you'd have to search old SONERAI newsletters for a picture or description) that brought it all crashing down. John and his wife (Betty? Nothing but good words about her from anyone who ever dealt with her...) never got paid what they were owed. Bankruptcy court dispersed the assets, which is how Steve Bennet of GPASC ended up selling SONERAI plans for many years - he needed an airframe for people to hang his engines on.

Put ALL this turmoil together and it's easy to understand why JM (who I found, in my EXTREMELY SLIGHT acquaintance, a little cranky ANYWAY) might not want to talk about the MONI.

If wwz7777 is right about the fate of the MONI spar cap extrusion dies, one MIGHT be able to buy Sonex spar caps and roll on. http://www.sonexaircraft.com/eshop/cart.php?target=product&product_id=16453&substring=caps
 

BoKu

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...I'll bet that BoKu can rattle off which one is a likely candidate right off the top of his head...
I'd scale the FX81 root profile to the chord and depth required, and use that. Its maximum depth is a bit further forward (38% vs 41%), but that's probably not a huge issue.
 

lr27

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I wonder if the FX-81, in metal, with a few bugs smashed on, would have much less drag than some old, but suitable NACA section with no special pretensions about laminar flow.

I'm also imagining aluminum sheet with a bondable coating or VHB transfer tape* vacuum bagged onto a solid foam core. Maybe Highload 60 foam or something like it.

*Transfer tape grabs right away, so you only have one chance to get it right. I wonder if they have one that's only sticky when it's warm?
 

BoKu

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I wonder if the FX-81, in metal, with a few bugs smashed on, would have much less drag than some old, but suitable NACA section with no special pretensions about laminar flow.
That was our thinking for the HP-18B wings. Unfortunately the one builder who started one passed away before finishing, so we never really got to see how well it would work.

...I'm also imagining aluminum sheet with a bondable coating or VHB transfer tape* vacuum bagged onto a solid foam core. Maybe Highload 60 foam or something like it.
That kind of gives me the heebie jeebies. I've used VHB on PVC and styrene foams, and was not impressed with the results. I'd go with epoxy or polysulfide to bond the skins to closely-spaced PVC ribs and interstitials just like the HP-18.
 

karmarepair

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I've used VHB on PVC and styrene foams, and was not impressed with the results. I'd go with epoxy or polysulfide to bond the skins to closely-spaced PVC ribs and interstitials just like the HP-18.
Have your tried moisture cured urethanes for bonding? They have better mechanicals and heat resistance than polysulfide and are available at any Home Depot. I've been tempted to make up some test coupons and come up your way and break them with the Break-A-Tron.

They use a lot of Acrylics in the automotive world for bonding things to other things. Any experience there either?

Last time I had to spec a "Field Repair" bonding aluminum we ended up just scuffing up the aluminum and using a 3-M prepackaged construction epoxy sold with a static mixing tube, the best stuff we could get in Hawaii in time to make the next patrol. This was on 5000 series aluminum 100 feet in the air, we scabbed a big doubler over a crack in the mast of a Government Vessel. I WANTED to use 3-M Sol-Gel or silane treatment over scuffed surfaces, but we couldn't get the materials for either from LA to Honolulu in time.

I'm contributing to the usual HBA Thread Drift, Mea Culpa, Mea Culpa, Mea Culpa Maxima. I'll give it up for Lent.
 

lr27

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That kind of gives me the heebie jeebies. I've used VHB on PVC and styrene foams, and was not impressed with the results. I'd go with epoxy or polysulfide to bond the skins to closely-spaced PVC ribs and interstitials just like the HP-18.
The solid core would provide 5 or 10 times as much bonding surface, and the Highload foam ought to be stronger than the usual stuff. Of course you'd still use rivets at the edges. Did the VHB let go, or the foam? In any case, an adhesive that sets up slowly is probably the better option unless it's all jogged so that it can't go on wrong. Some years ago, there was a popular rd glider that had foam cores and obechi sheeted wings. I don't recall any delams except from crashes. Not that my memory is so great...
 

wwz7777

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I wonder if the FX-81, in metal, with a few bugs smashed on, would have much less drag than some old, but suitable NACA section with no special pretensions about laminar flow.
This is what I'm thinking of doing. Not worried about laminar flow, just an honest airfoil that works well with riveted aluminum. Any suggestions?
 

Victor Bravo

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Look at the old NACA derivatives they used on the last generation of wooden sailplanes, they worked fairly well IIRC and most of them were not as horribly critical in terms of perfect contour.

Others here may be able to confirm or deny this far better than me, but if you borrowed the airfoil from the Standard Austria, SHK, Ka-6E you may get pretty close. Bob am I on the right track here?

If you can get a Moni wing to perform 3/4 as well as any of those gliders you've won the lottery IMHO.
 

Victor Bravo

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The Moni probably deserves a little more of a "sailplane airfoil" than the Cessna 100 series, IMHO. It has the potential to be a pretty efficient little critter, even with a 22-24 foot clipped span.

One important thing you may consider is that if you are going to be flying it in MN, there may be a remote chance that you may encounter one or two insects and/or raindrops on the wing over the course of your flying :)

That reality really must outweigh several other factors in which wing you decide to build. The highest performing whiz-bang laminar wing performance will likely go to s**t pretty fast with bugs and rain and dust.

BBerson's suggestion above regarding a Grob wing is probably good advice. But the very best advice is that you still need to hear from one or more people who are trained to understand this stuff. You can guesstimate it with your thumb out in front of your eye, but you have a pretty good chance of having it not work as well as you want.
 

wwz7777

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The Eppler 580 appears to have a cusp at the trailing edge, an I seeing that correctly? I’m guessing that since it’s a gentle curve it would work in aluminum?
 

danmoser

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According to the Incomplete Guide, to Airfoil Usage it's a Wortmam FX 67-K-170. Is there any reason to change it? I suppose if you're not going to wipe off the bugs or build really precisely, a more ordinary airfoil might be ok.
I measured the Moni wing profile at the tip to fabricate wing extension molds and discovered that it is definitely NOT the FX 67-K-170 section as reported. It seems to be a close fit to another Wortmann section, the FX 61-147 , but not perfect. Here's some points plotted.
 

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