MJD

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Well, here's a question..

For a while now my standard 1/2A servo has been the HS65HB, since ditching HS-55's for glow engine stuff.

But... is there a slightly smaller frame servo that is known to hold up under the abuse of high revving 1/2A engines? I have squeezed an HS65 into the wing for aileron control, but it is a snug fit for the elevator, and the elevator itself and linkage have way less drag and breakaway force than the ailerons.

MJD

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You probably posted it elsewhere, but what are we looking at in your picture? I see a piped small engine..

Mr Cox

That's a "Little Tony", with an old MVVS .15RC.
It does 22000rpm on an APC 6.5x5 on the ground, I need to try a little more prop load I think...

MJD

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If it's like my MVVS .15, I think peak hp is at 20k. If your goal is lots of speed but also some traction in vertical and out of corners, I think a 7-5 would be about right. On this example this I think you would gain pull but no top end speed. Have you clocked this model, I.e. Radar or Doppler?

Mr Cox

I haven't flown that one yet...
Still working on the set-up (but got side-tracked by other things) the engine goes rather lean when it is on the pipe. Might try with pressure to the tank from the middle of the pipe. Perhaps a lower compression ratio is better too.

aspeed

My MVVS .15 is a real dog, It won't go over 14,000 rpm with a 7-5. Slowest .15 motor I have, tied with the TT for hind ***. The .21 is stronger.

MJD

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Right now I am picking away at the engine area so visual progress is slow. The PVC tape is two layers, I'll screw the motor down with some epoxy filler at the front end, then remove the engine and sand flush, this will give a minimum clearance opening. Later I think I may put PVA release on the motor case, and fill the area under the front of the crankcase fluh to the case. Then when I mount the engine I can put a thin cost of RTV on the front case and then screw it in place. Won't hurt anything, and might damp the case a bit, who knows, another 25rpm?

Last night I prepped the aileron servo (cut off mounting tabs and scuffed the case, made a short double end output arm), carved out the servo opening and bonded it in using methyl methacrylate adhesive and standing off on a short piece of hard TE stock to angle the output arm up a bit for clearance. Now figuring out if I can mount an HS65 for elevator and have room for the Rx battery and Rx in the back. Probably ok, just some perseverance needed, I think if I stand it off 1/8" it will clear the lower fuselage radius and work out fine.

I'll need some advice as to how to route the stubby antennas.. it is a Kevlar/glass fuselage with CF in the corners and a couple of circumferential bands. So the fuselage is reasonably RF transparent. Don't want wire whiskers perpendicular to the fuselage on the outside. Parallel with trim tape over them is cool. Inside better. I might use one of my bare board Rx and solder the elevator leads directly, and solder in a short extension with receptacle for the aileron servo. That way I can DS tape the Rx against one side of the fuselage with the antennae running fore and aft on a CF free panel and taped to the fuse side.. sound like a good/bad idea to any radio savvy folk out there?

MJD

My Feedback: (1)

But that is static with the full flight prop. So if anything it should be struggling to get on the pipe and this leanness would be symptomatic. Are you running a 90% diameter cut down prop on the ground for pipe setup? You're right, they do have a lot of compression, a shim or two to start would not hurt.

MJD

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I have been wondering if I painted myself into a corner with the way I did this front end - meaning I had trouble visualizing how to make the removable top cowl mate nicely with what I had carved out below. But I think I know how to do it now - after shaping the plug, I will build a frame from heavy tape, offset about 3/16" outboard of the cowl from the parting line. This will create a step that will telescope together with the half-shell in the engine bay, and a narrow lip to blend in rearward on the belly of the fuse for the exhaust and cooling outlet. Gotta print that last picture above so I can sketch cowls on it.

MJD

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Nose is done, aileron linkages done and servo mounted/centered. Time for a weigh-in - I stuffed the Rx battery in the back with the loose elevator servo, screwed in the engine and fitted a prop.

So at this point missing:

- cowl (10 grams)
- elevator pushrod and linkage (3 grams)
- regulator (3 grams)
- fuel bladder (8 grams)
- prime and paint nose (3 grams)

I think I am being generous here.

total: 27 grams

Current weight: 245 grams / 8.64 ounces

Plus the above total => 254g + 27g = 281g / 9.59 ounces

11.7 oz/sq ft

I can live with that.

Mr Cox

I have been trying a few servos and the high quality ones seems to hold up fine, just avoid the chepo indoor stuff like HXT, Tower etc...
The HS-45 is slightly smaller and still has the carbonite gears. There is also an HS-35, but I haven't tried it myself.

In my sheet wing racer I'm planning to use the Dymond 47. These are very precise in their movement and small, but don't have carbonite gears.

MJD

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Hitec says the HS35 has metal/Karbonite gears as well:

"The HS-35HD Ultra Nano servo offers unprecedented durability thanks to our exclusive Metal-Karbonite™ gear train."

So I may have to check these out on a micro "something".

SO... I completed the elevator linkage and servo installation on the Boomerang, and dropped in the receiver, Rx battery etc. and taped on the wing so I could play "vroom vroom" and wiggle the servos. Nothing left to do but the cowl - which, mind you, is at least as much if not more work than what I have accomplished so far.. but I count it as "just one more thing".

Big news is.. minus cowl but with everything else = 253g = 8.92oz. I'm liking this, should end up under 9.5oz. That ought to work.

MJD

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This is not a computer plotted drag body, this is a crude French curve thing I made out of card stock - I just wanted to start visualizing how this cowl would look, my 3D imagination needs some work. I'll plot a NACA drag body profile with the aspect ratio that just covers the cylinder OD and has a LE apex flush with the rear of the venturi. (unless anyone knows a better profile, I don't). Then I'll blend in a section that is flush to the venturi inlet, bla bla.

combatpigg

My Feedback: (3)

That will look really sharp..!
What would improve my "cowl game" would be to attend a C/L regional contest as a guest and just camp out at the Speed area. In all the years I attended those things I never found the time to pay any attention to details like the slickest ways to lock a cowl in place. The best I've done is to make a complete tongue and groove joint at the parting line and hold the entire cowl with just 1 screw right behind the cylinder. This idea requires a "roll bar" to be positioned under the cowl to give the single screw an anchor point that's tied to the plane. This idea / method involves weight...it's a "heavy" idea, but similar to how the bigger RC Speed planes are done.
#2 button head screws to hold the cowl look OK but they are a pain. Vibration seems to always turn screws counter clockwise, so the obvious answer is to use reverse thread wood screws...[ Vibration always moves items on the work bench towards the floor instead of against the wall, even if you turn your work bench 180 and move to Australia the items on the work bench will always know which direction is the floor. This is the sort of evil that us lovers of little screaming things that vibrate must endure.

DeviousDave

With servos, you want to pay less attention to weight and more to centering ability.... A lot of these smaller servos were designed with park flyers in mind that will fly straight with 1/8" of rudder deflection. Hitec is a good bet, but some of their servos are better than others and will leave you with an airplane that can't be trimmed and the trim will change based on your last control input direction. Centering is everything when your total throw is less than 3/32". ..

MJD

My Feedback: (1)

I wondered about this. The HS65's - so far - seem to bring the aileron TE back in line with the wing TE pretty nicely. But where they sit in the status quo is something I don't really know. I wish I had drilled the elevator output arm even closer to the center, but the elevator moves freely and the linkage is low drag and slop free, so dialling down the travel for low rate doesn't worry me much as these have lots of torque for the application. Live and learn, it's all "Gooped" in now. I'm happy with the tightness of the aileron linkage too.

Yeah, about weight.. I agree, for example using two 11g servos instead of 8g is all of a 6 gram penalty.. no biggie if it's a quality upgrade.

aspeed

I remember using 1/16" ply to wrap around the cyl. and gluing it on the top and bottom of the cowl. This is a bit smaller, maybe 1/32" would work. We never put cooling holes in, just left 1/8" of the head sticking out. They only ran about a minute. It almost looks like you don't need a full cowl really. Just tuck something behind the motor? even just make an aluminum clip to fair in the back of the motor. Another method is to cover the motor with saran wrap and plasticine and fiberglass over it. Whatever you do won't likely be measurable in speed IMHO.

BMatthews

Yeah, lots of the CL speed models in the later days and in smaller sizes relied on the head only for cooling.

But as you say they only ran for a short time. Just long enough to whip up onto the pipe then do the timed laps. But this is a little different in that it'll be running for quite a lot longer. So some cylinder cooling seems to be called for.

MJD

My Feedback: (1)

These will be 3 minute flights or thereabouts.. so I need to ensure that cooling can keep up. I am committed to making a full cowl, as much for the sake of doing it as functionality. I believe a small improvement in top speed is possible, but I do not expect miracles. The fit will be close to the cylinder and head to ensure airflow through the fins.

The game plan is to wrap the engine and build around it, using a couple of laser cut profile plates like the paper doodad in the last photo, plus whatever other bits, pieces, plasticine, Bondo are needed to define the plug shape. It would be blasphemy not to pay due attention to the looks and function of the cowling, after butchering this otherwise fine model.

combatpigg

My Feedback: (3)

One idea I've never had the nerve to try for forming a cowl, but might someday......
Tape off the engine and plane with a good quality electrical tape.
Dab on grease or wax for a release agent.
Make a simple box out of card stock to conform to the plane, surround the engine and to provide support for aerosol foam.
Once the foam cures, it ought to be pretty easy to shape it to form a mold.
Some of the "skill" is knowing how much foam to squirt into the form. A millisecond's worth of squirting is probably enough to do two planes.

MJD

My Feedback: (1)

CP - take a look at this! Works for full scale. I'd use two part pour in place expanding foam versus aerosol, but aerosol might work fine too.

http://www.instructables.com/id/New-...ntal-aircraft/

For this little guy I'll use plasticine, wood formers, and whatnot.

combatpigg

My Feedback: (3)

What a beautifull job on a gorgeous little plane..!
Bear in mind how much easier fiberglass work is supposed to be compared to hammerformed sheetmetal. The sheetmetal guru would need to develope wood patterns that he would have to HOPE hugged the contours of the engine just so. It would be very interesting to give the same job to a sheetmetal guy and keep track of the time it takes to produce a cowl that's a nice as this FG job.
Either way, I wouldn't be able to afford that sort of craftsmanship.

MJD

My Feedback: (1)

Yeah, three months for two guys evenings and weekends. I worked evenings and weekends for two years on a certain old biplane, but that doesn't narrow it down much either.. it could mean a casual 12 - 15 hours a week for some and 40+ for others.

What a neat little sport scale 1/2A or .10-.15 model that could make. Probably already been done. Almost any size would be cool really. I don't care if this is a limited racing class, it sure takes balls and skill to do what these guys do.

http://en.wikipedia.org/wiki/Cassutt_Special

Well there you go.. http://www.ebay.com/itm/RC-Modeler-M...-/231034434260

Here's the .15 conversion: http://www.rcsuperstore.com/SeaGull-...-p/seax164.htm

Mr Cox

The planes from seagull looks great and their miniseries looks just right for .15 engines...
Quality is seems better than other ARF's that I have seen.

I have one of their mini Nemesis, and it's getting an MDS .18 which will fit inside the cowl (I've turned the cylinder to get a rear exhaust);