I have a Gremlin ARC (almost ready to cover) kit I bought a couple of years ago that
I'm finally putting together. I was wondering if anyone knew if Gorilla Glue would harm
the foam (I gather CYA will dissolve it). I was also thinking of epoxying a carbon fibre
strip to reinforce the leading edge (and maybe the wing spars). Anyone tried this?

thanks,
Stefan

Gorilla Glue will not harm the (EPS-expanded polystyrene) foam. It's safe to work with, and you can spray the parts with water to get a faster cure and cause the glue to foam up more to get into all the pores.

Back in the day, I did build a few gremlins with .007 CF laminate behind the balsa LE. It added a good measure of impact resistance. You'll still get dents, but they probably won't go all the way through the wing on LE impacts.

I presume that you sheeted the first couple of inches of the wing behind
the LE? Attaching with epoxy? (This is my first time so advice is very welcome).

I was just going to reinforce the spars with carbon fibre tape and then put the standard fibreglass tape over the wing and then cover with econocote/ultracote.

I'm out in the boonies so unfortunately I cannot get hold of CF laminate sheets
very easily (everything must be mail-ordered).

Sheeted a Gremlin? Nope. Just build it to the instructions. I assume the instructions are per Jim Reith's or Eric Henderson's original design.

The only reinforcement recommended was for the bottom center 6" or so to be glassed with lightweight cloth (2 or 4 oz, or 6 if that is all you have) and epoxy, but don't smear it into the foam. This is mostly for landing damage, but you can practically forget the glassing step for a lighter plane.

Eric's original design wings were not even covered -- just bare foam.

I hope I'm not leading you astray. The ARC kit may be different than the RA cores version. If you have two 6" or 8" plywood center spars glued to some pine bearers which take screws to your downspout fuselage, embedded into the top half of a symmetrical foam wing, than it's got the same basic design, and shouldn't need wing sheeting anywhere. Only fiberglass tape is recommended on the wing's thickest part.

The more weight you add to these, the worse they fly (the wider they turn)

I bought some 0.07 CF laminate (1") today to reinforce the LE and some CF tape (1") to reinforce the spars joining the wing halves. I'll brush on some 30-minute
epoxy first and then put Ultracote over everything (after applying the fibreglass and fibreglass tape as per instructions).

I hope that won't add much weight to the wing.. maybe a couple of ounces at most of epoxy.

Any advice on building this puppy (other than getting the C of G correct)? I'm talking to Jim Reith about it as well (the MAN). Unfortunately, I don't have a mixing radio (a Futaba 4-channel and a 6-channel non-computer) but I'll build a mechanical mixer.

thanks, /S

The CF across the middle won't do much good. There isn't really any flexing going on in the area, due to the fuselage mount and glassed center.

You'll find that the wings will flex a little in turns, and you get a lot of strength from the covering. In fact, I've built a couple of these without any strapping tape at all, and just covering for strength. The foam is stiff enough. However, for combat, the strapping tape helps with holding things together on impact.

You might find after a few flights that the downspout may split in the engine area. I had good luck with PVC cement and some glass cloth for repairs. You can also make splints of scrap PVC to bridge cracks with that glue. After a while, the ply mount always splits the PVC at the edges, and/or where it gets screwed to the plastic.

Be sure you get the CG at 1 7/8" from the LE like the instructions say. Also, limit the "elevator" travel to around 1/4" for first flights. make "Aeileron" throw as wild as you like. You'll want to test these by flying, and observe if the plane snaps out of loops or tight tturns, and limit the elevator throw to exclude this.

The lightest gremlins I've ever built were around 2 1/4 lbs, and they flew great. But they were highly modified, and covered with Mica Film. I still have a couple, but I consider them too pretty and too fragile to actually fly in combat anymore.

Someone in the composites forum suggested that I put in my CF laminate as additional spars (vertically through the wing) in very thin slots. I guess I'm not sure where the
stresses will come.

Good point on the engine mount. Probably gave out for a couple of reasons - overstressed plastic, bolts used for location not just clamping (loaded in shear), torsional stresses on plastic (from mounting only two bolts).

So do you think using CF tape on the LE edge would prove less than useful as well?

Rather than building the mechanical mixer I'm just going to use a GWS V-Tail mixer which will give me electronic mixing with my 4-channel. I presume that I need only use two servos rather than three. One for elevator/aileron and the other for throttle?
I can understand how the mechanical mixing works (elevator servo moving aileron servo on tray) but I cannot see how I would need two servos for this using electronic mixing. I would just use a single servo that would respond to either elevator or aileron input, right?

cheers, /S.

Sorry but you'll need two aileron/elevator servos, unfortunately the servo can't do "angels in the snow" movements and so you'll have to have a servo in each wing for the aileron, which will then also work as half your elevator.

Duh.. now that you point it out, it's obvious to me. So one servo per aileron, but with dual inputs to each (elevator and aileron). So it can serve as both aileron or elevator depending on my inputs. Cool.

At least I don't have to build that darn mechanical mixing tray (and that saves some weight as well as preserving my C of G).

When I measure the C of G, should I have the tank half full of fuel or full or empty??

Thanks, /S.

With the tank empty. I aways cut holes in the elevons, and vertical fins to help with the CG.

There is nothing wrong with the mechanical mixer it works great. I have also mixed with the transmitter.

On mine I didnt use any type of spar, other than the short ones to attatch the fuse to the wing.
On low rate I set them up so that it wouldnt snap at wide open throttle when you used man up or down.

Hope this is some help

Not if you DO want to fly combat. It's probably the single lightest thing you can add that will keep the LE together. CF along the entire LE makes it hang tight and resist splitting big time, cause the CF will be in tension for front end impacts. IT's not perfect, but it helps a lot for the weight it takes.

BTW, for a nice electronic mixer, check out :

http://www.canterburysailplanes.co.n...%20mixers.html

prices are New Zealand Dollars.

Thanks very much. I'm using the CF tape to reinforce the leading edge (using Finishing resin thinned with alcohol to emplace it) and am fibreglassing the center section as well. Used Scotch Extreme Applications tape instead of the standard fibreglass tape to do the longtitudinal strapping.

Though I do appear to be ending up having to put in place two sets of servo rails as three abreast (Futaba S3004s) appear to be pretty damn crowded.

Has anyone reinforced the engine mount area before it cracks? The PVC really looks quite thin! I was thinking about fabricating a magnesium or titanium plate to sandwich the PVC with the plywood.

thanks, /S.

I once made a backing plate out of thin aluminum sheet which mounted on the outside of the PVC and received the ply bolts. The PVC eventually cracked in a line even with the ply and then around the perimeter of the aluminum plate. I was able to fly quite a while in the cracked configuration, as the crack didn't proceed down the rear side of the ply and aluminum plates.

The PVC is really difficult to make resilient to vibration and impact. It's soft enough that reinforcements always seem to cause weak spots at the edges. An ideal reinforcement would taper the structural area as it goes to the edges. This is one of the reasons we eventually abandoned the whole PVC fuselage altogether and just went with a lighter flying wing design once we figured out how to mount the engine and servos to the wing. The other reason it that it's heavy for the strength it does have compared to other materials. It's chief benefits as I saw them was it was cheap, very easy to work, and easily replaceable -- much like other SPAD designs which utilize PVC parts.

Yes.. I suppose one material that leaps to mind is CF. I have never made my own structures with it but I should imagine that making a square cross-section tube would be fairly easy. One could laminate a titanium/(other light metal) in the engine mount area pretty easily.

But from what I gather you have other designs that build even faster? Where do I go to get these? Spadworld? Do you have a particular type of design in mind that is derivative of the Gremlin (for when my PVC inevitably cracks )?

cheers, /S.

Quite a few Gremlin drivers use this type of skid to help stiffen the mount area and keep the engine out of the dirt better:



It is just a scrap of PVC with the mount plate bolts through it & the fuse into the mount. The plate distributes the stress of the bolts & mount plate over a wider area of the fuse pipe.

I also use four 1/4-20 nylon bolts to secure the engine mount plate. In an impact, the bolts break away saving the fuse pipe.

HTH,
D

Thanks very much Demon!

Great photos and very helpful..

cheers, /S.

The tufFlight predator is basically a gremlin made from EPP with the wing flipped backwards for easier balance. I wouldn't say it's "derived" directly from a gremlin, because there're 5 years of technology development with EPP, lexan, UHMW and other materials now widely used in modern combat designs.

They don't build faster, and the materials employed dictate they are certaily not as cheap, but durabilty is vastly improved. Flying will be about the same if the weights are the same.

Keep your Gremlin light, and you'll smile every time it's in the air.

cheers,
Joe

these planes are for .25's right not .15's

Dude..

you gotta expand these acronyms for us novices (EPP???). UHMW??

Vis a vis exhaust. It looks like it is worth building a mousse can muffler for the extra RPM. Everyone seems obessed by it
but I'm not sure that the extra RPM on the top end (if that is alone what it gains) is worth it. Most of the horsepower will
come at somewhat less than max RPM. But if it improves the throttle response from say, 1/3 to full power, it would definitely
be a plus.

Having never flown in a combat I can only conjecture that you're not flying at max RPM the whole time. It would certainly
hurt your turning radius, right? Or are the aerodynamics of these planes so ridiculous that no .25 can possibly produce
enough horsepower to expand the turning radius?

A 9x4 seems to be indicated. Any particular brand (APC, Master Airscrew, wood?).

Any input would be appreciated. Is there a primer anywhere on the basics and more advanced topics? Perhaps someone
has written down the basic tactics/engine mods/tuning recommendations in a single document??

thanks, /S.

EPP is expanded polypropelene. It was first used in R/C airplanes for slope soaring, and tufflight introduced it for fuel power planes 4 years ago. It makes planes bounce when they hit something.

UHMW is ultra high molecular weight polyethelene. It's practically the same plastic used for fuel tanks. Think CUTTING BOARDS -- super tough, and hard to break-- they bend. Tufflight introduced this to the combat world 4 years ago as well.

You're right- RPM and speed is not everything. All planes slow down in turns, and you need to manage energy. A dream engine pulls really hard all the time no matter what throttle setting you have. You are limited in speed by the need to stay below "streamer shredding" speed.

Norvels are the best power to weight around (see picture above) and a stock .25 will run really good with an APC 8x5 prop. 18.5K RPm is usual with this combo. If I use tuned mufflers, I like the Master airscrew 9x4 prop, and a good Norvel (they aren't all created equal) will turn a MA 9x4 prop at 19.2K RPM, and a great one will go over 20. The MA props stay together much better if you hit someone else -- the APC's tend to shatter, but they do give more thrust.

For more info on combat, and state of the art stuff, you really should check out the RCCA website:

www.rccombat.com

Didn't mean to turn this into a combat commercial, but, hey, you asked...

Hey Demon,

I just noticed that you didn't use the rear bolts of the engine to attach it to the fuse. Is this a mod you did just for the Norvel or is it so that the fuse doesn't crack from the strain of the motor torque?

Or are you trying to increase the effective length of the fuse by mounting the motor further forward?

thanks, /S.

Acutally, we do spend most of the time at full throttle during combat. While you do turn a little tighter at a lower throttle, that only works for a short turn, maybe 180 degrees or so. You need all the pull you can get to keep turning at high AOA, if you get in to a turning duel with someone, you can go around in circles for quite a bit, so you need a plane that can maintain speed at some reasonable turning radius.

Btw, besides UHMW, there is also HDPE, High Density PolyEthelene. It's the same thing, just not quite as dense as UHMW. It's a little stiffer than UHMW, and almost as resistant to breaking.

The Norvel engine do have the best power-to-weight, but not necessarily the most power. And the Norvels can be a bit hard to get running reliable until you are used to them.

The stock Gremlin fuse pipe comes cut so the engine is recessed into a notch. The fuse pipe in the pics is one I cut without the notch for simplicity, and to avoid weakening the area. However, with modifications, the same type of skid arrangement can be used on the stock Gremlin mount configuration.

As for the UHMW and HDPE, I've made mount plates from both of those materials with great results. Both are MUCH more durable than the birch plywood stock piece.

One thing more:
The bottom corners of the mount plate should be rounded so it fits snugly into the corners of the pipe (look closely at the pics above). If the edges are left square, they create stress risers where they contct the PVC causing the fuse pipe to eventually break.

HTH,
D

PS
Throttle? What's that? I only use the left stick as an on/off switch.