Tee Bee

I feel like I'm opening a can of worms with this one but here goes: I've built and set up reedie .049 powered planes with great success using Sullivan 1oz tanks. Care was taken to achieve a good flop tube setup and these planes have been flown without any fuel draw issues. I'd been looking forward to flying with a TD .049 so I set my latest 1/2A plane up with one(Das Splat Jr) using the same Sullivan tank that's worked well for me in the past. It's turned out to be a real headache so far. The single vent line exits the firewall and faces the prop blast. The fuel mixture changes with attitude and is causing it to be difficult to tune the engine to run for a full tank. If tuned to run on a downline it will die or run poorly on the upline, or vice versa. It also tends to starve during the handlaunch if thrown very hard. I've experimented with tank height(adjustment is limited) to no avail.

Question: Does the TD design have less fuel draw than a reedie? I was hoping to avoid tapping the crankcase to pressurize the tank as I'm not sure it would be very effective. I'm not very interested in switching to a balloon tank. FWIW, this setup works fine with my .09 engines but maybe the additional displacement draws fuel more effectively? I'd like to resolve this situation since the plane is so much fun to fly. Thoughts?

combatpigg

My Feedback: (3)

A backplate with a built in pressure tap should be available from somewhere, like Texas Timers or ZL or Reggie. I found that it helps quite a lot to run pressure.
You can JB Weld a hunk of tubing in the venturi to help fuel draw, but I've never bothered with that. I like to run them full strength on bladder.

icerinkdad

If the venturi has been opened up on the TD that can impact fuel draw. Also the tank needs to be as close to the engine as possible for a suction setup. Using an aftermarket backplate with a pressure tap will help with fuel draw issues even with the tank several inches from the engine but it makes it hard to throttle an engine as the extra pressure causes the engine to flood as you throttle down. Before you do anything drastic try adjusting the engine with the nose pointed straight up. On the stopping when thrown the fuel is probably getting interrupted by the little bubble of air that moves around quite a bit when you toss a plane.
I know you are not interested in a balloon tank but it really may solve all your problems....
Bob

EX Model Engines

If your going the pressure route, I would say go for the backplate pressure tap. Check out ebay listing number 260538548873. I know he has a bunch of other types as well, and his work is excellent.

Regards, Matt.

Andrew

It looks like he is using a standard muffler tap in a partially tapped hole, leaving it to the buyer to thru drill it. 6061 drills fairly well, depending on the temper, but drilling a hole the size needed for crankcase pressure can dicey, plus needing a small diameter chuck.

proptop

My Feedback: (8)

Actually, that's all one piece (I've got a few of them ) I know the guy...he's a very good machinist [8D]
Haven't decided yet though what size to drill the hole?

Tee Bee

Thanks for all the replies and PMs, fellas. Lots of food for thought. That's why I love this forum. I like the idea of pressure tapping the rear plate but that won't work on this particular mount since the engine is so close to the firewall. Might use it on the next one, though.

Toad

I drill mine with a .015 brill bit. I have used .020 , I prefer the .015
I love the TD on backplate pressure and the venturi opened to .156 (5/32)

Andrew

Hmmm...........

A variation that would work better for me, since I'm not set up for drilling accurate, small diameter holes, is a backplate that's drilled and tapped to accept a pre-drilled nipple. Someone could buy a backplate and one or more nipples with different hole sizes - testing performance differences becomes as simple as swapping nipples and you don't have to worry about ruining a complete backplate with an incorrectly drilled hole.

andrew

proptop

My Feedback: (8)

I was thinking it would probably be somewhere around .015 to .020" (don't think I have anything that small...will have to get some small drills )

I had been thinking though...if for example I accidently drilled it out too big, maybe a piece of .015" (give or take a couple thou ) dia. SS or music wire, coated w/ vaseline, and then put JB weld or epoxy and the wire in the hole...and then slip the wire out after?

combatpigg

My Feedback: (3)

The hole size isn't critical, at least as far as I know for running wide open only.

It could possibly matter for some amount of throttle ability....[more like mixture control to drop rpm].

Toad

Well, I looked at the Norvel pressure back plate for the AME's, it has a large pressure hole.
I tried a large hole on a TD, My TD didn't like it.
In the good ole days a TD pressure back plate was easily made using a Fox 4-40 pressure nipple available at Tower
Drill and tap the plate 4-40, place a piece of stainless .015 wire (safety wire from local FBO) in that huge hole and solder in the hole, the solder wont stick to the stainless, The wire is easily removed via the handy needle nose pliers and a .015
Pressure back plate is born, the nipple will be too long inside the engine, easily corrected to flush with the back plate
Some times I can find good one's on the bay
I hope to read your comments if you run one over .025.
This way you don't have to drill the .015+ hole, very convenient and not hard to do at all.
You probably have a few stock td back plates, the nipple was under $3 from tower a few months ago.
hope this helps if ya wanna do it on the cheap.
It does not need a larger hole, smaller the better I think.

small_rcer

I have always found that I need to mount my TDs sideways to get good fuel draw. It lowers the center of the venturi. With most fuselage shapes the tank is usually too low in relation to the venturi if the TD is mounted upright. I also found that using 3/32 rather than 1/8th brass tubing made a big difference.

Jim H

MJD

My Feedback: (1)

That's true for so many installations with many engines. In general the tank bottom needs to sit just about on the engine bearers to line up but that's not often possible.

I've become a big fan of side mounting all my suction draw engines, deltas notwithstanding. Unless the tank can be set up in line with the spraybar, but with common engine sticking out in the breeze installations that means a deep fuselage behind the cylinder area generally which is not always in keeping with looks or needs of the design.

MJD