MJD

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Not! But it should be. What a riot. Pylon is cool, but straight line all out top speed is a different thing, although a good pylon model is always a reasonable contender..

That said, I've been putzing casually with this thing.. a Boomerang conversion to Profi Rambler .049. I really wanted the piped speed version but they haven't materialized yet, what with the challenges getting stuff from Ukraine at times. Anyhow this is the proto speed/open exhaust 1/2A speed version. I have flown its 1cc combat brother on a 25" span homebrew sport pylon model nowhere near as clean as this, and it is a little rocket.

So I have high hopes for this little guy. I'll be surprised if it does not crack 140, and I would not be surprised if it can reach the 150 mark.. but we will see, no point making any claims, let's just see how fast the little sucker goes.

I had this posted in the 1/2A forum, but I think they are all hibernating over there. Besides I bet this will be able to leave a significant percentage of the prop jets in the dust..

AUW will be under 10 ounces. 9.6-9.8 predicted. 108 sq in or 118 sq in, depending whose specs you read, max thickness about 3/8". 4.2x4.2 APC, Eliminator 4.125 x 4.5, 4x5 props to try out. I'll test fly it on a combat prop to keep the speed down.






The rough beginnings of the cowl plug. You can see the printed templates for the cylinder cowl top and bottom. Lower plate is NACA 0030, upper is slightly thicker. Blister is for the fuel line, I'll use a ball driver for the NV. I pushed a small pin into the wood blister, this pokes through the tape covering the fuel inlet to keep it lined up under the plasticine. Lots of work to go, still roughing out the shape, a week of evenings puttering with plasticine and little pieces of wood and it should be ready for PVA and splitter plate in prep for laying up the mold. I "wet sand" the plasticine with a piece of soft cotton cloth and mineral spirits.. it softens the surface a hair. Not much evidence of it here though, I just finished adding some material in front of the intake.

One question left in my mind is - if I don't care about pylon turns and fly the whole time at low lift coefficients.. I wonder if it would be faster if I zipped off the tips just outboard of the ailerons. The wing loading would still be light. Maybe I should see if I can score another wing.

CafeenMan

Am I missing something or are you planning on landing on the engine?

airraptor

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i wouldnt cut the wing tips just leave them. you will gain more by making a tunnel for the engine. less ing area means a higher AoA and more induced drag.

MJD

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The part not there in the pictures is the cylinder cowl, so it will end up looking like a typical CL speed cowl. The drawing is the top and bottom profile templates which I haven't made yet. The plasticine part is just the bottom end and it has some work to go - it will graft into the cylinder cowl. So yeah I'll land on the cowling but not on the engine per se. On grass and under 10oz, it is not going to hurt anything. There will be a small cooling inlet, air forced through the cylinder/head fins and either through the back or a top outlet. I'll probably flare the TE of the cowl airfoil and shape it to create negative pressure at the outlet.

iron eagel

Vent your cooling air out the back if possible, straightest run possible. Don't go nuts with the outlet it only need to be 140% of the inlet size, but a bit of flair to get to the point (at least internally on the cowl) will work out nice.

MJD

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That's the plan, 1.4:1 area. I will most likely use a rectangular rear exit slot on the TE of the cowl.

iron eagel

Rounded would be better, but admittedly that is splitting hairs. Try to think of it more as the exit nozzle of a rocket engine type of thing.

MJD

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Well, rocket nozzle design is something I do know about.. the goal is to expand and accelerate the sonic nozzle flow to ambient pressure, trading as much heat for kinetic energy as possible for highest efficiency. The edge at the exit plane remains sharp/square however, or you reduce efficiency. Here, we want minimal external and internal drag, and I'd like to create a bit of negative pressure at the outlet. So the airfoil profile of the cowl remains as is until near the outlet, where it flares slightly. The top and bottom of the exit slot would be rounded, not square.

iron eagel

Well you know how nuts I went on my cooling exits.
Too bad you didn't have the room to incorporate a bit of an expansion chamber as well.
I've been told you want to stay away from a high aspect ratio for the exit, you may want to keep that in mind. I think that with the rear exit your already going to be in an area of relative low pressure, I get the feeling the flare isn't really needed, and will add to the profile drag.

MJD

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You could be right about that, the flare thing was just me thinking how to improve the breed. As you say it should be in negative pressure anyhow at that point. Aspect ratio affects efficiency for sure, but then there are trade-offs on the profile of the exit. Let's see where the "TLAR" method takes things as I sculpt the pattern.

iron eagel

This is the part that DaRock refers to as "black arts" as far as model airplanes go.

MJD

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This is an example of what I was blathering about above with regards to profile plates. These are rejects, but they get the idea across. The lower profile is too long, it was supposed to be 3/8" from the edge of the cylinder to the nose of the airfoil, and I botched it. So once I like the look of the two profile plates tacked in place, I'll sheet the outside in 1/64 or 1/32 ply, build the outlet and inlet as needed, fill, putty, sand, massage, tweak until it looks like a speed cowl. Then make the parting plate, then lay up the mold, oh goodness what have I gotten myself into...

What am I complaining about, at least I didn't have to make the airframe.

iron eagel

I feel much the same way about my latest project.

If you don't mind one more intrusion.
The lower profile should be a bit longer to the rear behind the cylinder. You really want it to blend into the fuselage as much as possible (not a bump that sticks out). The top one on the other hand should be a bit shorter to the rear, not much, about 80% or so of the current length, angle the rear down just a bit as well (avoid any flat surfaces as much as possible type of thing). A better way of putting it is the top of the cowling should be airfoil shaped as well, but you may already have that in mind as far as the top.

MJD

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Yeah.. you really can't see what I have in mind from this picture. But have faith that I have a vision of slippery aerodynamic buttery goodness in my mind, with nice drag body curvatures and blending. No bumps, all I was doing with the plasticine was beginning to get the bottom shape worked out.. it will all become one.

The lower profile is a precise NACA 0030. It will melt like chocolate into the airframe, or at least I hope it ends up looking that way. Of course it could be ugly as heck.. hope not. Purposeful looking will be fine with me.

iron eagel

Cool and I forgot you still have the rear exhaust thing to deal with.
One thing that may be a consideration given the original profile of your plane is that your adding a lot of side area ahead of the COG, you may have to adjust your vertical stabilizer area to compensate.

MJD

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A big brother project to this Boomerang! Why not? Check out this Ariane ( http://www.soaringusa.com/Ariane-P5.html ) with a .21 for sure, Novarossi I think. Did 169mph, this is a 1.4 times big brother in a way at 157 sq in, and with the engine and pipe hanging out in the breeze which according to the builder Bernd held it back and he was planning to cowl it for the next event. This was at a Speedcup event 2005 or so.

I'm ordering a Profi F2A .15 to do a similar conversion.. fully cowled however. The Ariane was one of the candidates on my list, Sunracer IV another. Not sure smaller is always better, these lekkie pylon models are slippery devils. There are several F5B models in the 205-220 sq in range as well, and I can't help wondering if they wouldn't be just as capable or more on a manic .15 like that.

MJD

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Yes, I have to have enough area for the exhaust and cooling air. The exhaust duct inset into the fuselage is a an old piece of paper-phenolic rocket body tube I had lying around from competition days (rocket guys remember CMR, 1970's? This is their phenolic mini engine size tube, now it is used as a slime duct.). You can sort of see it under the bottom plate.

iron eagel

I thought that material looked familiar behind the motor now I know why.
Any one of those are a good candidate for the type of thing your doing.

MJD

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So now I made two more profile plates, these were keepers, and tacked them in place lining up a witness bar on top level with the stab. Sanded some soft 1/2" sheet to fit between the plates for rhe rear half of the cowl top, and used plasticine for the front half. This is the dress rehearsal stage here, but I like how the shape is coming together. The tricky thing for me is figuring out how to allow for layup thickness, but I think the tape idea for the fuselage mating joint should work fine.

iron eagel

That's pretty cool looking. It's going to be an interesting airplane.

MJD

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Interesting is the perfect word - no matter what happens it ought to be something to see.

Ought to be pretty slimy in the back end of that cowl after each flight.

iron eagel

Treat the exhaust port as a rear facing nozzle and at least the residue will not be for nothing.

MJD

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Yeah, I wondered about the outlet shape but I decided that a slightly flared TE with a vertical oval outlet was probably the way to go. Problem is, this isn't just a cooling air outlet which I would size at the old rule of thumb 1.4 times the inlet area. It also has to vent the exhaust, which at this point has expanded and cooled a bit.. how on earth do I figure out how much exhaust area to use.. I don't even have a SWAG at this point. I guess I could start with the exhaust outlet area and add 1.4 times the cooling inlet area, but I doubt that takes enough relevant factors into account.

??

I've been puttying and shaping and licking and smoothing and adding more and bumping into it and fixing it several times and bla bla bla.. but I like where this is going. Need more outlet area yet I believe, just by eyeballing it against the inlet which is my best freehand marker on plasticine work. Just visualizing..

If the Profi doesn't cut it, I have a secret weapon on standby.. you can see it here. I believe it may even be capable of sustaining level flight on this model, but not sure on that just yet.



The squirt bottle in photo #1 is a cat deflector ray gun, very effective. I find the old Tornado yellow 6-3 to be a very nostalgic looking 1/2A prop.

I had to put a prop on the engine and tape on the wing so I could play with it a bit while I was taking photos. Nyeeeeeeeeaaaaaaarrrrrrrrrrrrr.........

It at least looks like it ought to boogaloo. Should be a heap of fun regardless.

iron eagel

A thought about venting. Use a pair of NACA scopes configured for outlets on the sides as close to the rear of the cylinder as you can get them, leave the exhaust outlet on the rear exclusively for that. You need a curved baffle just behind the cylinder (Internal on the cowl) to feed the NACA outlets.
I am pretty sure the pressure of the exhaust gas would pretty much stop (or severely limit) any airflow around the cylinder from the intake.

MJD

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That's the sort of thing I worry about. There is also the top vent like a lot of the RC speed guys us, a kind of a flush outlet. See here:



It wouldn't be hard to baffle cooling out out there and let the exhaust dominate the rear outlet.