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2015 Design Contest - Airrow Two

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2015 Design Contest - Airrow Two

Old 05-31-2015, 05:02 PM
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H5606
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Default 2015 Design Contest - Airrow Two

1/2A Class - centerline twin - originally designed around the Ace constant chord foam wing but will now be built-up D-tube construction; so far, fuse and tail parts are all 3/32" balsa with some plywood for firewalls and formers. All thrust and incidence angles to be set at 0-0. Challenge is to get all radio gear forward of wing LE to achieve proper CG.
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Old 05-31-2015, 06:05 PM
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Interesting design - you're off to good start!
Old 05-31-2015, 06:05 PM
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This should be cool to watch fly, hope you can vid it. Do you have a phenolic or metal thrust washer for the pusher engine?



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Old 05-31-2015, 06:08 PM
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Looking good..!
AFAIK, this push / pull plane is a "first" at this forum. Others might have threatened to do one, but never did.
Old 06-01-2015, 08:42 AM
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H5606
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Originally Posted by Pond Skipper View Post
This should be cool to watch fly, hope you can vid it. Do you have a phenolic or metal thrust washer for the pusher engine?



I have a video camera, however it has no viewfinder and is useless to track airplanes using the flip-out screen; wondering if taping a small dia tube to the top will act as a viewfinder or if anyone has tried something like this. No real mods to the pusher engine other than hi-compression head, removal of spring starter, and a need to de-gunk it. Does running these engines in pusher config cause performance loss w/o thrust washer? Pretty cool trick - how did you "negativize" that image?

Last edited by H5606; 06-01-2015 at 11:10 AM.
Old 06-01-2015, 11:19 AM
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I put it in paint and made it more skinny to reduce the thick airfoil and fuselage then reversed the colors to give it that top secrete blue print look = )

Yes without the thrust washer you will rub aluminum to aluminum this will create a larger gap in short time thus the flywheel and rod pin will move back further
until it rubs on the reed valve assembly which will toast the engine Cox international sells thrust washers, their phenolic version would be best.
Why is they don't heat up like the steel ones and they do not wear down the aluminum crankcase and prop plate as fast.

I use a hat clip on my baseball cap stand in a mirror and set it up to face forward level with your eyes so where ever you look it looks.
I use a action cam they are smaller it has its own rear screen view but I use a wi fi app to view with my cell start and stop it set prefs etc...

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Old 06-01-2015, 05:20 PM
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Thanks for the advice; I took a quick look at Cox International and found their steel thrust washer (looks like the stock item included on all Tee Dees) but can't find the phenolic one... thinking of fabricating one from thin G-10. Cox had a few pusher design products (Backdraft and Shrike come to mind) but never owned or took note of engine details...

I'm becoming tech illiterate even worse than the way my parents were but you've given me some ideas concerning video. I've got a long way to go before I'm ready to think about recording a flight so we'll see if I can even build this airplane first.
Old 06-07-2015, 11:22 AM
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H5606
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Progress this W/E: I added tri-stock, stiffeners, wing saddle doublers, and lightening holes. Refurbished old Sullivan tanks with new bungs; 1 oz (really > 1) will go to the rear, and 2 oz will go up front, fixed pickups - under water tests are probably better but I did my usual lazy pressure test - cover vent, blow into supply hose & pinch, count to sixty, and release near my ear - if I hear a decent rush of air, I okay it.

Would like to know this: does vent line pointed into or away from the airstream help, hinder, or cause no affect to engine runs?
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Old 06-07-2015, 01:45 PM
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Having the vent pointed towards the oncoming air helps a little.
Control line flyers cut the vent tips at an angle to help trap more air.
If your engines are sensitive to changes in head pressure [tank position], then the rear engine will go rich during a climb while the front engine gets leaner. The opposite would happen during a dive.
I would definitely set the needles with the plane pointed so that the tanks are below each engine being adjusted.
Old 06-08-2015, 01:19 AM
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Hi H5606,

Sweet Project !
I have yet to , not quite there, burned out a Cox.049 in pusher mode.
Eventually, you will want/need the washers, after a few gallons.
An electric starter will cause way more damage to the engine, than running 'it' backwards.
One or two clicks rich, is your friend.
Castor oil dissipates heat,
Unfortunatly, pushers lack oncoming cooling air...

But, They are Frigging Cool!

Cheers,
Dave'crosscheck'Fallowfield
Maac 6437
Unabashed Combat Team
Old 06-08-2015, 03:47 AM
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CP, what you say explains observations I witnessed in a previous endeavor; I had difficulty telling what was going on during climbs/dives. Replies in another thread suggest turning tank around in pusher config but was wondering if engine can draw fuel through long hose w/o crankcase or some kind of pressure and would this change get rid of sagging during climbs? CC, I can't remember seeing dark, residual oil around drive plate/crankcase union on pusher config in distant past*, but what Pond Skipper said above has me a little concerned, so I'm undecided at this point on shim/washer. Will probably hand prop rear. My start procedure plan is to start rear first, tune and shut down, start fwd, tune, then start rear again and launch. Hopefully fwd engine cools rear some prior to launch. Thanks for all the advice though; I'm still learning stuff and that's cool.

*Doesn't mean it didn't happen - just don't remember...

Last edited by H5606; 06-08-2015 at 08:10 AM.
Old 06-08-2015, 08:17 AM
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You'll be fine if you needle the rear engine first as suggested. Then go front back to start. The rear engine won't melt down in the short period of time while you get yourself organized and launch, but your thoughts towards minimizing static running are good.

The shortest path between the fuel and the engine is a balloon tank facing the rear. Suck out the air, replace with fuel. Regards the pusher/clunk.. I've run clunk tanks on pushers fine - with the tank outlet facing forward as suggested - but I did indeed have muffler pressure. So on suction I don't have a worthy opinion.
Old 06-08-2015, 09:07 AM
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Weigh both of those plastic tanks, then weigh a pair of party balloons. Like MJD says, you can arrange the party balloons for short lines up to the engine. I would like to try crankcase pressure taps if it was my project, but there is always a chance of botching something while performing surgery. With recessed backplates I've attached 3/32" brass tubing inside a puddle of JB Weld with good results. Pilot drill the backplate [with a 1/32" drill or so] and leave the drill bit in the hole, then slide the short hunk of brass tubing over the drill bit, then pour in the JB Weld. Let cure for a week in a car parked in the sun. Everything needs to be immaculate and sanded rough before gluing. Aluminum oxidizes so fast that you need to glue right afterwards. This might not be feasible with your engines, but I like to outline how this is done for those looking in who don't have access to a factory made pressure nipple backplate.

Last edited by combatpigg; 06-08-2015 at 09:10 AM.
Old 06-08-2015, 05:45 PM
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H5606
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Some more learning questions: Re using the balloons, could anyone please walk me through or reference a link about making them? Syringe or fuel bulb for fueling?

I'm assuming reedies can't use crankcase pressure; I remember way back talking to another 1/2A racing newbie about the Kustom Kraftsmanship pressure backplate and was thinking of drilling out what looked like absence of hole at fitting - he said "don't do it, you'll ruin it" - as he said he did - actual hole is pinprik tiny (almost invisible and <1/32") so just wondering if size of hole is not as critical as told originally.

Last edited by H5606; 06-08-2015 at 05:56 PM. Reason: pin***** filter
Old 06-08-2015, 06:23 PM
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This looks interesting. Looks like you have a nice start on it. Could you tell us how long it's going to be?
Do you have any special way you are going to launch it?
Old 06-08-2015, 06:23 PM
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As to the hole size, I can only say I've rigged 3/32" ID nipples on .049s on up to .50 size engines. I THINK it would work with .020s. 3/32" is a handy size for Sullivan Brand SMALL fuel tubing to slide over. Regardless of orifice size, if the fuel system is air tight it doesn't take long for a small engine to lay in a pressured atmosphere.
Since your engines are reedies, you would need to drill an undersized hole into the crankcase, heat the case up and press in the brass nipple.
It's a gamble and you would want to practice your technique on similar thickness scrap aluminum to be sure.
Otherwise, if you keep the plane's maneuvers big and graceful using a suction feed should work just fine.
To use a party balloon, you can use a standard fuel tank "stopper" [some call it a bung] and stretch the balloon opening over the stopper. With a balloon tank you do not need [or want] a vent. The balloon can not be filled to the point where any pressure is developed on the fuel. Before filling, you suck out all of the air with a syringe, then clamp the balloon, then load in some fuel. Keep excess air to a minimum will allow nice, consistent fuel delivery. For an .020 you probably only want 1/2 to a 3/4 oz of fuel, so a standard [small] balloon ought to be just right. The fuel line / fuel pickup should be arranged inside the balloon to allow fuel to be dispensed. Prolonged climbs and dives will starve one engine or the other, so keep that in mind while flying. An .020 can run for at least several seconds on residual fuel in the feed line and the feed line only needs random dips into the fuel to restock the feed line before the engine quits. I've done a lot of 1/2 A RC flying with fixed fuel pickup lines and they allow all the maneuvers except for prolong inverted flight and of course prolonged dives.
With the size of plane you show, I think the weight of those 2 plastic tanks is quite a load. You might be able to use the built in tank of each engine as a "header tank"...?
I've heard of this being done. The auxillary tank has it's feed line plumbed into the backplate tank's vent. I've never tried it, but it might help smooth out head pressure changes a little bit while the plane is climbing and diving..?

Last edited by combatpigg; 06-08-2015 at 06:29 PM.
Old 06-09-2015, 03:43 PM
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Originally Posted by A. J. Clark View Post
This looks interesting. Looks like you have a nice start on it. Could you tell us how long it's going to be?
Do you have any special way you are going to launch it?
Thanks but nothing really new here: Dornier did it with their 335 back in the '40s, Cessna did it later with their - IMO - rather ugly 336 & 337 sans driveshaft but with booms - I hate booms but it may have been a better way to go here for wt distribution... Real inspiration came from a one-off design I saw at Sun-n-Fun back in the late '80s - IIRC was the DG-1 - Mazda rotary PPs designed for unlimited class at Reno.

Fuselage length is ~28"; original wingspan of Ace foam wing is 35.5" w/ constant chord of 6". Going to a built-up - thinking tapered wing planform right now - for aesthetics but unsure if I'll regret the loss of wing area... Being shoulder wing - launching is overhand.

CP, thanks for the detailed response; I'm on the fence yet as to what I'm going to do especially since I don't want hatches aft but there's certainly something to be said for consistent engine runs and getting weight out of the rear should be of paramount importance. Dumb questions: Do you need to be able to see balloons while fueling? Can you run pressure from reedies?

Last edited by H5606; 06-09-2015 at 04:04 PM.
Old 06-09-2015, 04:49 PM
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Do you need to be able to see balloons while fueling?

Not if you measure in the fuel - syringe, etc. Same as pressure bladders.

Can you run pressure from reedies?

I haven't figured out why it would be different than a front rotary intake engine in terms of case pressure, but there may be a reason. I'm curious.
Old 06-09-2015, 04:55 PM
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I remembered some twit posted a how-to using a Visine bottle spout as a balloon tank fitting - here it is:

http://www.rcuniverse.com/forum/1-2-...ml#post9215076
Old 06-09-2015, 05:18 PM
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That's cool - thanks for the link too.
Old 06-09-2015, 05:32 PM
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Like MJD says to do it.
Rehearse your procedure on a test stand so you'll know exactly how to operate when the balloon is hidden from view.
I forgot to mention in a earlier post that the balloon should be wrapped with a few turns of thread, [then tied off] so that it stays put on the rubber stopper. A small tie wrap or very fine safety wire also works.
When I use a bladder type tank, I rarely have an access hatch on the model. The bladder is inserted through a 1/2" diameter hole that leads into a fuel proof compartment behind the firewall. If the bladder / balloon breaks I simply let the fuel drain out the access hole and fetch a new bladder.
The access hole usually isn't too unsightly once the model is 200 feet or so down range at flying speed.

Last edited by combatpigg; 06-09-2015 at 05:34 PM.
Old 06-21-2015, 02:02 PM
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I had my wife pick up some balloons while grocery shopping at Target - looks like they're either too big or too small from this batch; reset piston con rods and fabricated thrust washer per Pond Skipper's recommendation for pusher from .010" G-10 and sanded down crankcase and thrust plate a skosh for clearance. I cleaned up both engines from quarter-century congealed castor as well - heat gun worked well to unlock engines. Last pic shows two white, 6x3 Grish Toronado props - one tractor - one pusher. Nice but perhaps negligible thing about this setup is both engines turn same direction but when oriented in push/pull config with tractor and pusher props - they'll turn opposite each other and thus cancel torque. A trusted source tells me to go to higher pitch in rear since pusher is moving already "thrusted" air, so may try using tractors on both engines with higher pitch in rear at the expense of thrust cancellation - showing cut down Cox competition 5x3 and MA ?5.25x4?. Since I'm using reedies, direction of rotation should be no issue.

I'd like to post video however, so far I've failed to upload to site; I'd prefer not to link elsewhere....
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Last edited by H5606; 06-21-2015 at 02:42 PM.
Old 06-21-2015, 05:24 PM
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I'd initially try the same prop load on both engines and listen for signs of the rear engine revving up [unloading]. I'd be surprised to hear this happens with such tiny engines / props.
As long as both engines are keeping their cool, sucking their tanks dry and giving adequate power I'd call that a win.
I'm not sure how much cooler the rear engine will run thanks to having the forward engine blasting it. Even with the additional air, the rear engine sits hidden from the airstream. Be alert for any signs of running hot, [that burned cookie smell] if so the size of the prop and the fuel delivery is in question.
Old 06-23-2015, 09:04 AM
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I just read combatpigg's response... I am now hungry for cookies (unburnt)... preferably chocolate chip cookies.

Av8rsodt
Old 06-25-2015, 07:12 PM
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Hi H6505,

I'm using a bladder tank and a hatch on my Go-Fer design.
I've found the thumb area cut from surgical gloves give ~3 minute run time.
I use a Sig syringe for filling. 1/4 filled, plunger up/nozzle down. Suck air out to collapse bladder, fill, then 'burp' any bubbles out by sucking/filling fuel in and out. Takes 10 seconds to do.
If you have a larger bladder, once the air bubbles are out, just put in the fuel needed. Works great.

I'm cleaning/running engines next. I mount them on a 2X2 clamped in my WorkMate stand. When running, I can release the stick, and do nose up/downs, inverted and do 'simulated' handlaunches. Bladders are better than hard tanks IMHO.

I seem to remember a TD pusher design, the Shockwave from RCM perhaps.
The article covered pusher props and tank setups. It had sweptback wings for CG reasons. Looked cool.

Project is looking great !

Cheers, Dave'crosscheck'Fallowfield

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