Of Flop Tubes and Fuel Senders

ptrotter · #1 01-19-2005, 08:33 AM

I am about to build my fuel tanks and it is time to decide on whether to use flop tubes or fixed fuel pickups and whether to use float or capacitive fuel senders. I am interested in anyone's comments these topics.

I expect it is unlikely that I will do any aerobatics beyond simple positive G maneuvers so it probably doesn't make sense to put in flop tubes. Also, for any maneuvers that really require a flop tube, I would need an inverted oil system as well, which I am not planning on putting in. On the other hand, I can't see any real downside to flop tubes as they may give me flexibility in the future.

With respect to fuel senders, I am leaning toward capacitive, simply because there are no moving parts. I have heard, though, that some people have had trouble calibrating them. They do allow me to put in flop tubes without relocating the float to a different location. Also, if problems arise with capacitive senders, it is possible to install float senders after the fact if necessary. Of course float senders are much easier to find guages that they work with.

I suppose I should just keep it simple and use the tried and true fixed fule pickup with float senders, but I hate to do something that might limit me i the future.

Any comments would be appreciated.

kellyp · #2 01-19-2005, 09:59 AM

I also did not want to limit my future choices - so I put in a flop tube. I will be doing positive G's unless I do a lot of engine mod's. But a future builder, or myself, can go negative if they want.

As for the senders - the Van's are simple and cheap - and that is what I did. The downside is they are not accurate at full tanks or in the flop tube tank (it goes in the second bay). However - you know when it's full because you filled it and checked it right? And when it's empty you had better be on the ground and parked. The rest is good flight planning or using a fuel flow meter.

Davepar · #3 01-19-2005, 10:10 AM

This is what I've read on the mailing lists. Hopefully somebody will jump in if any of this is wrong:

The flop tubes will need maintenance eventually. The flexible tubing needs to be replaced after some number of year. If you don't desire flop tubes, I wouldn't bother with them (I didn't).

If you install a fuel flow sensor and fuel computer (such as the ACS 2002), that will give you a much more accurate reading of fuel remaining than either gauge. You push a button when you fill up and the computer keeps track of how much fuel you've used. At that point, the senders will mainly tell you whether you have a leak and fuel drained out of a tank. Given that, I'd use the cheaper and easier float senders next time. Installing the capacitive senders was a bit of a hassle and they require an expensive adapter to use with most fuel gauges.

Dave

RudiGreyling · #4 09-27-2005, 06:12 AM

Got a fellow RV4 pilot here in SA, that says next time he will fit 2 flop tubes. His biggest gripe is that the normal pickup gives bogus fuel reports since it does not pic up the fuel all the way to the bottom. This means less usefull fuel.

Is he right?

Thanks
Rudi

Radomir · #5 09-27-2005, 06:48 AM

Rudi, that doesn't sound right.. "normal pickup" doesn't report anything.. it's just a tube.. I'm guessing he meant putting in capacitive senders instead of floats for better precision. but you can do that whether you use flop tubes or not.

rv8ch · #6 09-27-2005, 07:19 AM

Quote:

His biggest gripe is that the normal pickup gives bogus fuel reports since it does not pic up the fuel all the way to the bottom.

When the standard pickup is installed, you have to take some time to ensure that it is as low in the tank as possible.

With the design of the tanks, there will always be some unusable fuel, but putting the pickup as low as you can will help. I don't know why he thinks the flop tube will be better. It's of much larger diameter, so I believe you would have more unusable fuel with a flop tube than with a regular pickup.

RudiGreyling · #7 09-27-2005, 08:02 AM

His reasoning is that the flop tube lays flat on the bottom, get to all the fuel, the normal pickup not. He is not complaining about the measurement device only about the usable fuel.

Any further comments?

Thanks

rv8ch · #8 09-27-2005, 08:23 AM

Quote:

His reasoning is that the flop tube lays flat on the bottom, get to all the fuel, the normal pickup not. He is not complaining about the measurement device only about the usable fuel.

I think if you look at both, you will see that a properly installed standard fuel pickup will allow you to get more fuel than a flop tube. Here are two pictures. The diameter of the brass pickup on the flop tube is about double the diameter of the standard pickup. Sorry I don't have a side-by-side photo.

szicree · #9 09-27-2005, 08:41 AM

Even if that little screen is against the lower tank skin, that puts the actual pickup half an inch above the lowest point. The flop tube lies right at tht botttom

Steve Zicree
RV4

rv8ch · #10 09-27-2005, 09:48 AM

The flop tube has holes all around it, and at the end. It's about twice the diameter of the screen pickup. With the screen pickup installed as close as possible to the bottom, the flop tube will be sucking air before the screen pickup.

The screen pickup sits along the bottom of the tank, it's not perpendicular to the bottom skin.