Poor fuel economy

Yes, the TD-s are very thursty and very powerful, all the same. I was alwas wondering why a TD051 slurps around 2 oz of fuel in a 10 minutes run, while a 049 reedy gulps only 8cc for a 2 minutes+ or so run, i.e . 7,5 times more fuel for a 4 times longer run, i.e. 1,88 times more consumption per minute for the TD.

But if you consider the power needed to turn the same prop is proportional with the prop speed on the 3rd power, the 1,88 times more fuel consumption means the TD should turn the same prop by 23% higher rpm than a reedy, (the cubic root of 1,88=1,23), and my TD 051 happily turns a 5x4 prop of Bernie at 22k, the same prop thus should be driven by a reedy at 22/1,23 = 17,9k or so.

In fact I measured exactly this speed ratio over last weekend when I started up a 40+ years old 049 postage stamp product engine from an old Stuka (details in the next issue of Bernie's COX Engines Quarterly) and measured 18k on it with the same prop that my TD051 runs at 22k.

Please check now if the 2 props on your 2 Bees with the stunt tanks have the same dia and pitch. With the same tach the TD cylinder engine should have a larger dia and/or pitch prop than the other.

But you never know...I built a reedy with a new set of TD051 stock cylinder and piston, and it was never able to reach the same speed as a stock TD, possibly because the constraints imposed by the reed valve, limiting the top speed of a reedy.

RknRusty wrote: I always wondered if the short skirts developed as much torque with less inertia of the lighter piston.
Rusty

The thing I was wondering about is, how does the lighter piston affect the balance of the crankshaft. Is it more than the effect of a slightly unbalanced prop?

The Unbalanced Mark

Because the piston does not have rotational speed I guess this lighter piston will not affect the shaft balance.

How big is the venturi hole in the rear of the tank back? There was many differences in production which could also explain to a certain level as to why one engine is using more fuel. The other option is that it may also be quite possible that fuel is possibly seeping out of the screws of the tank back itself. I've seen this many of times. Generally, I use Bernie's stainless screws new out of the package to try and remedy a screw leak. Pressurize the tank and place a few drops of oil on the screw heads and this will quickly reveal if the screws are even seeping. If new screws don't stop the leaks, I've wrapped cotton from a Q-tip around the screws and this has not only sealed the leak but also allowed for a service removeable joint. In addition to leaks, check the tank itself during pressurization. Even minor leaks as I've suggested can contribute to poor economy not to mention they're almost undetectable initially. Ken

Ken Cook wrote:             How big is the venturi  hole in the rear of the tank back? There was many differences in production which could also explain to a certain level as to why one engine is using more fuel. The other option is that it may also be quite possible that fuel is possibly seeping out of the screws of the tank back itself. I've seen this many of times. Generally, I use Bernie's stainless screws new out of the package to try and remedy a screw  leak. Pressurize the tank and place a few drops of oil on the screw heads and this will quickly reveal if the screws are even seeping. If new screws don't stop the leaks, I've wrapped cotton from a Q-tip around the screws and this has not only sealed the leak but also allowed for a service removeable joint. In addition to leaks, check the tank itself during pressurization. Even minor leaks as I've suggested can contribute to poor economy not to mention they're almost undetectable initially. Ken

I'm not sure about the hole size in the tank backs. When I pull the engine apart to swap out the piston/cylinder set I'll re-check the gaskets and check for tank back leaks. I've never hear of the cotton thread sealing trick, but I'll try it on re-assembly.

Someone here mentioned anaerobic sealant. If whoever posted that could give me a name brand or a picture of the tube, I'd appreciate it. Just going to the auto parts store looking for anaerobic sealant want's very satisfying.

The Unsatisfied Mark

Mark, I was the one who mentioned the use of the anaerobic gasket maker. http://www.permatex.com/products-2/product-categories/gasketing/anaerobic-gasket-makers-flange-sealants/permatex-anaerobic-gasket-maker-detail

I use this product exclusively for backplates. The way this product cures is through the absence of air. While it would probably work as a tank to tank back sealant, I haven't had to good of luck on the screws with this product. My initial experiment did work for a short period of time. Many I know have used the RTV gasket maker on the screws. This essentially makes the part a non serviceable item. I don't care for that so I've tried everything from the above to small o-rings. What I found to be the most effective is tearing off a bit of cotton and rolling it up and wrapping the thread and screwing it in. It doesn't impede the slot of the screw like RTV does and it works great.