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|Subj: Fuel System on Your XL|
Based on photos on your Web site, I've made some guesses about how fuel is routed from the wing tanks to the engine on your Zenair 601XL. One photo shows a tank selector valve on the center console below the instrument panel. Is there any risk having the fuel flow up to the valve and then back down to the floor and on to the firewall?
Other photos show your fuel pumps on the firewall. What little reading I have done on fuel systems says that with regard to preventing vapor lock, sucking is "bad" and pushing is "good." Some books recommend putting the fuel pumps in the wings next to the wing tanks. But I've looked at a lot of XLs and all of them put the fuel pump(s) up by the engine. Why is this a safe practice?
|Craig Payne, Utah, Manual #6154|
|Reply from WW:|
|Theoretically, electric fuel pumps do a better job pushing fuel than they do pulling it. So, in a textbook,
you would have the electric pump right at the tank. Conversely, here's the real world take: If it can be avoided at
all costs, I avoid putting pressurized fuel lines through the cockpit. Just use your imagination to think about a
pinhole leak in the fuel line and ask yourself where you want the fuel pump. Gus and I did extensive tests of the
fuel pumps to ensure they wouldn't have a problem drawing the fuel, and would work in the series orientation in which
we have them. To back up this testing, we also have about 130 hours on the plane. This is split equally between the
first fuel system and the current one. The current fuel system reflects my own personal values on how the system
should be installed. You'll notice I don't pressurize the gascolator either, and the gascolator is placed above the
bottom of the firewall, where it wouldn't be smashed in an accident. While it's good to follow the textbook
recommendations, the actual fuel system you fly will have to be optimized for your specific installation. I personally
feel the one we have now is the best combination for a Corvair powered 601.
Ready to buy my carburetor for my soon to be turbocharged 164. I want to go with the Monnett Aero-carb. I had a brief discussion with them. They recommended the 35mm carb. Of course it will be upstream from the turbocharger so as not to lose atomization - correct? Do you agree with the size recommended? Sould I get the flange mount or spigot mount version? Thanks as always.
Terry Calderwood, Kitfox 7 Turbo, Missoula, MT
|Reply from WW:|
|In a turbo system, if the order of components is carburetor, turbo, engine, this is called a draw through. The system you're mentioning, where it's turbo, carburetor, engine, is called a blow through. Original Corvair automotive turbo setups were draw through. Draw through is analagous to virtually all of the U.S. planes in WWII, which were carburetor, supercharger, engine. It is far easier to set up an engine and get it to run reliably as a draw through, thus all of my work with aircraft turbos on Corvairs will be draw throughs. Modern car turbo books discount this, but they don't understand the need for reliability in aircraft, nor the fact that throttle response in an airplane need not be as fast as a car. A 35mm Aero-carb would be a good size for a draw through. It absolutely should be a flange mount. I only use flange mount carburetors on aircraft. Spigot mounts have a dubious history of coming loose in some applications. Just to illustrate the complications of blow through systems, consider that if your turbo made 5 pounds of boost, it would blow the fuel backward through the carburetor and back into the fuel tank. Even if you installed a fuel pump, it would have to have a regulator diaphragm to increase the fuel pressure exactly as the boost in the engine built. There are many other complicating issues, but by comparison, the draw through will always produce a simplistic installation that works well.|
|Subj: Zenair 701|
Hey, everybody. I know William has addressed the question of Corvair power for the Zenith 701 STOL. Zenith says the engine is too heavy. FWIW, I found this interesting comment on http://www.sportsplanes.com/zenith.aspx:
STOL CH 701 aircraft is being redesigned to meet the sLSA and eLSA standards. It was developed as an "off-airport" short take-off and landing kit aircraft to fulfill the demanding requirements of both sport pilots and first-time builders. First introduced in 1986. A new model is being developed for the sLSA and eLSA category. It will have higher gross weight.
Heard anything about this? Sounds pretty intriguing, especially if Zenith allows a little more weight in the engine compartment.
|Russell Groves, Louisville, CO|
|Reply from WW:|
|I just took a quick glance at the Web site you mention, and I'm not sure these people are affiliated with the factory. Your question may be better directed to Sebastien at the Mexico, MO, Zenith factory. Just looking at the Web site, I was surprised to see a company that wanted to take deposits on the sLSA version of the 601, which hasn't even been built yet.|
Haven't seen any updates lately regarding Dave's Wagabond project. Was wondering if any are forthcoming. Am also building a Wagabond and looking seriously at Corvair power.
|Thanks, Jim Wegner, Manual #5146|
|Reply from WW:|
|Dave spent the weekend covering the tail surfaces on the plane. The public debut of the plane will be at Sun 'N Fun 2005. We'll have more updates as it goes to final assembly. December was a real busy month in the shop, and the plane didn't get a lot of attention. But Dave's been back on it steady in January, and it should look like an airplane by the end of next month.|
I have a friend (Peter Johnstone) here in Australia who is presently trying a 40mm CV carb. While he plays with this, I have purchased a Stromberg carburetor NA-S3A1. I have the 35mm but you state that the 23 works well with the Corvair. Will the 35mm work with my Corvair? Your response would be greatly appreciated.
Also got my copy of The Corvair Flyer. Well done once again. You have stated that you are making Safety Shafts from 7075 alloy. Could 2024 be used in this application? I'm sure this will not be the first time this question is asked, so I'm trying to be the first. Hope to hear from you soon.
|Darren, Aussie Corvair VP-2|
|Reply from WW:|
|The CV carb should work and run the engine, but keep in mind that these carbs have the characteristic of
shutting down when exposed to ram air, and potentially stopping if they're subject to a diaphragm rupture. These types of
carbs have flown on a lot of experimental engines, but they're not what you'd call simple and trouble free.
I think you have a typo here and meant to say a 35 and 32mm. In the U.S., these are generally referred to as 1 3/8 and 1 1/4" venturis. We have extensive flight time on both of these. The 32mm runs very nicely and will not restrict the power output of the engine until it's running in the 3,300rpm range. The 35mm will support any power output on a direct drive Corvair, yet it will idle smoothly and not load up.
We selected 7075 because it is as strong as many alloys of steel. While 2024 is fairly strong, I don't think it's tough enough for this application. We tested the 7075 to make sure it matched the strength we're looking for. You might get away with less, but I feel better with the far higher level of safety. If you're looking to make a shaft from the drawing, make it out of steel, not 2024.
I know that you specify using Loctite 620 as the threadlocker of choice, but how about using Permatex High Strength High Temp Red? It's in all the automotive stores and readily available. Is this the same as 620? I thought Permatex was part of Loctite.
Thanks for answering these questions of mine.
|Ralph Young, Zenair 601, Emmet, ID|
|Reply from WW:|
|Permatex and Loctite I believe are one in the same these days. 620 is in its own category for strength and temperature
resistance, with gap filling characteristics. We've tested them all, and 620 has properties that blow away all other
more common thread-locking products. We had used High Strength Red on the Safety Shaft and Hybrid Studs, but since we
discovered 620, it allowed us to achieve excellent strength with head studs, and we've transitioned to using it
everywhere we use thread-locker in the engine. One small bottle will do a whole engine. It carries my highest