Thorp Air Command -

Supporting Owners, Builders and Pilots of the Thorp T-18 and its variants.
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PostPosted: Mon Apr 25, 2011 5:03 pm 
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Joined: Mon Oct 26, 2009 1:33 pm
Posts: 178
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Location: USA

First of all, thanks again for responses on primers, etc. I think I'll forego installation on that item, at least until I get it flying first, if I ever get to that point. Secondly, I'd like to provide response of my own to a couple of messages I read this week.

Material to use for the ring on a conical engine mount: I had the same issue trying to find it and kept looking until I got the right stuff. But often I wish I knew why John picked certain materials for certain parts. Perhaps at the time it was readily available. On the other hand, there may be unseen aspects never explained. The 478 ring must support a cantilevered load with propeller vibration and landing shocks. Material strength inside the cowling for either kind of aluminum starts to dropping at 250F. It may be about 50% at 350F. Something you might not think about. I like 2024 for its strength and always wondered why 6061 was so popular. It must be cheaper and easier for mass production and forming, but doesn't have the same strength. You'll find it's also kind of 'gummy' and plugs up the drill and milling tools, compared to the former. A/S lists yield strength of 2024-T3 as 42ksi, and that of 6061-T6 is 35ksi. I'll leave it to you which one to use. There is no material labeled T351. For those who don't know, numbers following the "T" imply heat treatment, and are just as important pertaining to strength as the alloy type. In my humble opinion, the important consideration on 478 is the fasteners, if you make the splice for the impulse-coupling type mag.

Another topic I might make comment is fuel surging to "plug" the vent in the tank: forgive me, I'm not being smart or critical, but don't think this would happen, and have this thought to offer: Just think of any container with a drain and a hole anywhere in the opposite end and hoses attached. Even if it were filled with liquid up through the hoses, it would find its way out through the one at the lowest point (hopefully we also fly so that would be the drain). Like siphoning from your car tank; although the tube on top is full of liquid it flows nicely down into the gas can. The issue is not so much "plugging" of the top tube. But there could be a serious problem if such "siphoning" starts to occur backwards from what you wanted.

With the vent hose routed to the belly like most of us do, it is quite possible that fuel being accelerated up into the vent can start to siphon out. It would have to overcome ram air pressure, but that may be easy at starting speed. Doing so could possibly create starvation to the engine.

What I plan to do is make sure I always operate between a minimum and maximum amount in the tank. A minimum so when I accelerate or climb on takeoff the fuel doesn't slosh back and "uncover" the drain. Not really; the minimum is to ensure head pressure since I don't have a pump. A maximum, so an overfull tank doesn't slosh up into the vent and start to siphon out. I put my vent in the tank as plans show.

Thanks for the forum, Les.

Last edited by Anonymous on Mon Apr 25, 2011 5:03 pm, edited 1 time in total.

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Bill Williams
PostPosted: Tue Apr 26, 2011 2:24 pm 
Bob and I repaired a Thorp a few years ago and put in about 3 gallons of fuel. Plane setting in a three point attitude would not throttle up above 1500 rpm before it would quit. Added another 3 gallons and it ran fine at high rpm's. Most low wing aircraft have two fuel pumps, I'll copy them.

Last edited by Bill Williams on Tue Apr 26, 2011 2:24 pm, edited 1 time in total.

Rich Brazell
PostPosted: Tue Apr 26, 2011 8:33 pm 
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Joined: Wed Oct 21, 2009 12:33 pm
Posts: 2908
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Location: Jamul, CA (San Diego area)
Ditto on the fuel pumps ! Yes I know gravity is simpler, but I have a complex mind.


Last edited by admin on Tue Apr 26, 2011 8:33 pm, edited 1 time in total.

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Richard H Woodcock
PostPosted: Thu Jun 23, 2011 6:00 pm 

Joined: Mon Nov 23, 2009 11:09 am
Posts: 41
Location: USA
Tony Bingalis has a good writeup for fuel flow testing in one of his books. Basically block the plane up to climb attitude, disconnect the fuel line from the carb, and measure the flow rate. Should be 150% of maximum for the engine, as I recall.

I'm with both ljkrume and Bill, though. Either measure the flow, and set aside an amount as "unusable", or put in two fuel pumps.

Rich Woodcock
N114RW - T18CW

Last edited by admin on Thu Jun 23, 2011 6:00 pm, edited 1 time in total.

PostPosted: Mon Jun 27, 2011 8:59 pm 

Joined: Fri Oct 23, 2009 12:28 pm
Posts: 36
Location: USA
The discussion about the fuel being forced aft because of the planes forward motion should not be a factor in "steady state" flight; i.e., without acceleration. I've been down to the last three gallons (when I didn't realize the inaccuracies of my fuel gage when near empty). You can bet your life that I handled the controls with kid gloves -- the gage was reading zero. When you are that low on fuel, the last 3 gal., it just isn't prudent to be doing accelerated flight maneuvers. But I feel safe flying it w/o an extra fuel pump.

Last edited by admin on Mon Jun 27, 2011 8:59 pm, edited 1 time in total.

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