eric_marsh

eric_marsh

Well Known Member
I'm starting to approach the point where I'll be building the fuel tanks for my RV-6. One of my objectives is to build this airplane to be aerobatics capable.

The drawings show an optional weighted fuel pickup for aerobatics but to use this pickup my understanding is that the fuel level sender should be moved to the next compartment, i.e. between the second and third ribs (from the inside). I'm not exactly wild about that idea. Unless I'm misreading this it means that the wiring will be immersed in fuel and if the sender fails it seems like it will be very difficult to replace it.

I'm wondering if there is a good alternative approach. A couple of ideas that have come to me are to move the fuel sender to the opposite end of the tank or to put the fuel pickup into the second compartment.

However I can see a downside to each of these approaches.

Moving the sender to the opposite end of the tank seems to me to be the best approach but the dihedral will prevent the last bit of fuel in the tank from being shown on the gauge.

Moving the pickup to the second compartment means that it won't be able to pick up that last bit of gasoline. I don't think that's too much of a concern as the difference in the lowest part of each compartment will only be a fraction of an inch. I'm thinking that the greater concern would be with the placement and direction of trap doors designed to keep the fuel in the compartment where the pickup is. The trapdoors would likely impede the flow of fuel when the airplane is being filled.

I'm just trying to think ahead on this.

Thoughts or suggestions anyone?
 
The wiring won't be immersed in fuel as the sender will be mounted on the rear baffle instead of the rib.
Yes, you will not be able to "see" the last bit of fuel on the gauge.
BTW, You really don't need the "flop tube" for most aerobatics. Only if you intend to do sustained inverted flight, in which case you will also need an inverted oil system.
 
Moving the sender to the outboard end of the tank will only let you see the top few gallons of fuel - it will read empty when you have more than half a tank - completely useless.

The real question is "what kind of aerobatics are you going to do"? I do them almost every day, and don't have flop tubes. I simply don't enjoy negative G aerobatics, so they aren't necessary. Our new airplane will have inverted oil and fuel injection - but still no flop tubes. We'll get enough time at zero or negative G to do vertical lines or momentary Humpty-bumps.

If you're going to do serious, competition-level Acro, the RV isn't the airplane for it anyways. For me, I have just not found the extra complication of flop tubes and altered level senders to be worth the effort.

Paul
 
one standard, one flop...

I ended up putting a standard pickup in the left and a flop in the right... the float in the standard pickup will range 0-16 gallons and in the flop configuration 0-17 (for reference the -7 tanks are 21 gallons each). One potential benefit about the flop tube and trap door; less sloshing away from the pickup in turbulence for a low fuel landing... I have had no issues with fuel delivery from either tank during any positive g aerobatics but if I ever add an inverted oil system of some kind (not likely) I have a flop tube.
 
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Eric...

Given the misunderstanding you had about "how" the Fuel Sensor is fitted with a Flop Tube, hopefully you can now see the decision is reallly "do I fit as per Vans" or not bother at all?

Re "need for a Flop Tube", testing an IO RV-8 with 1 x Flop Tube + inverted oil, I clearly tested the systems with sustained (1min) inverted flight. With the owners' permission, I then repeated the test with the non-Flop Tube tank. At 75% power, it took 23s for the engine to falter :eek:

As per Paul's post therefore, my advise would be, if you want to do aerobatics, requirements in order:
  1. IO engine
  2. Some sort of Oil/Air seperator
  3. FULL inverted oil in lieu of above
  4. Flop Tube - and only is you want to do extended negtive 'g' - say >15s
i.e. only even consider a Flop Tube if you have IO and a full Christen/Raven inverted Oil system.

I have done standard level (Sportsman?) Aeros in the RV-8, with IO, half an inverted oil and no flop tubes - again as above, it is not an ideal aeroplane even at that level. The RV-3 will have 1 Flop Tube and full Inverted Oil - buit for fun, not competions ;)

Andy
RV-8 G-HILZ
RV-8tors
 
Thanks for the feedback. I don't know what level of aerobatics I will do or even if I will do any at all. My sense is that it's something I'd like to explore though. For most of my life I've been into very high performance (ground based) vehicles and have done some competition. I think that plodding along in a Cessna 150 would get old pretty fast but high performance flying, abet with proper training and adequate experience, just sounds like fun.

As an aside, I was in the process of building a "Junior Fuel" front engine dragster but after taking the Frank Hawley Drag Racing School I abandoned that project to build my RV. Frankly, compared to riding my turbocharged drag bike driving one of his rails wasn't particularly challenging or exciting. But the stuff I saw guys (and gals) doing in airplanes - now that looks like a total rush.

At this point my feeling is that it's easier to provide for that option at build time than to retrofit and re-certify later.

I told my wife that I'd like a couple hours of aerobatic training for my birthday.
 
Andy Hill said:
Eric...

Given the misunderstanding you had about "how" the Fuel Sensor is fitted with a Flop Tube, hopefully you can now see the decision is reallly "do I fit as per Vans" or not bother at all?
I have done standard level (Sportsman?) Aeros in the RV-8, with IO, half an inverted oil and no flop tubes - again as above, it is not an ideal aeroplane even at that level. The RV-3 will have 1 Flop Tube and full Inverted Oil - buit for fun, not competions ;)

Andy
RV-8 G-HILZ
RV-8tors
Click to expand...

Yes, I see what everyone is saying. It might well not be worth my while. But now's the time for me to be asking those questions.
 
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