Aerobatics prohibited with wing tank fuel?

[Zulu Alpha]

Having been involved in the structural repair of a number of Extra 300's I don't think that the movement of the fuel within the tanks is likely to result in any structural damage, what is likely to be an issue is the lack of a reliable inverted fuel flow from the wing tanks.

I have heard that wing tanks have leaked after rolls with fuel in them. I can't find any official reports.

I think most auxiliary tanks have to transfer to the main tank,

Not on the Extras and Edges. You choose the feed to the engine from either the central aerobatic tank or the two wing tanks together. Its difficult to use all the fuel in the wing tanks as you have to wait until the engine coughs. Any turbulence or banked turns will expose the fuel outlet in one tank and interrupt the feed so the last few litres are only available in still air. This means that you can't easily transition from a ferry flight to an aerobatic one as you could still have fuel in the wings.


My Edge wing tanks are the curved section of the wing in front of the main spar. They are basically the empty space within the wing structure so are not added later. They extend from the wing root to 3/4 of the wingspan. So with a wingspan of around 27 ft they stretch from approx 2ft from the centreline to 10 ft from the centreline and hold about 60 litres of fuel. I think the Extras are similar. The fuel pickup is in the wing root so any bank with low wing fuel means you expose one of the pickups and suck in air. This is why the POH requires the fuel to be on the main acro tank for takeoff and landing.

Roll rate can be 420 degrees per second so the pressure on the outer end from fuel surge during a roll would be considerable I think.

[boguing]

foxmoth.

Definitely the latter is worst. A full tank is a totally predictable load

A partly empty tank will behave as a free surface, which rolled the Townsend Thoresen ferry and didn't help the Concorde fatal.

It's that wrong speed that sitting up in a bath can cause a bathroom flood.

Liquids are not compressible, so you need to imagine a tank of ball bearings.

[BackPacker]

This means that you can't easily transition from a ferry flight to an aerobatic one as you could still have fuel in the wings.

Out of curiosity, how do you handle this then? Obviously planning ahead helps massively, but do you really plan to arrive at your destinations (aeros contest or display) at the precise moment the wing tanks are dry, leaving only the acro tank as reserve/hold fuel/diversion fuel?

Or do you drain the wing tanks when reaching the destination?

[Genghis the Engineer]

I don't know and sadly have never flown a Pitts either. I did try to look it up on the Society of Experimental Test Pilots papers database, but surprisingly there are no papers there on any of the Pitts designs.


I'll offer a best guess however, just from having run a lot of certification and flight test programmes. Probably, the flight test team didn't see enough need to fly aeros with anything in the wing tanks, so couldn't justify the time and resources to test that. Since it wasn't tested, it's prohibited. As I said, guesswork, but it would be consistent with a lot of other restrictions I've known of in other aeroplanes.

G

[The500man]

Dunno about the edge or extra, but the Pitts aux tank is fully transferred during flight. Usually however you would takeoff with aero fuel and maybe a bit extra for transit which would all fit in the main tank. You can't fly aerobatics with a full main tank in the pitts.

[Zulu Alpha]

I think the Pitts wing tank is central in the upper wing so relatively easy to gravity feed to the main acro tank.

Low and mid wing monoplanes can't do this as the wing tanks are lower than the top of the mid/acro tank.

I agree that half empty tanks are the biggest problem with surging fuel.

Out of curiosity, how do you handle this then? Obviously planning ahead helps massively, but do you really plan to arrive at your destinations (aeros contest or display) at the precise moment the wing tanks are dry, leaving only the acro tank as reserve/hold fuel/diversion fuel?

Or do you drain the wing tanks when reaching the destination?

I am fortunate that I have almost 2 hrs in the central tank so rarely have to use the wings. If its quite calm I will use the wings until the engine coughs in level flight and then change to the acro tank.
Extras have much less in the acro tank so people often use the wings to ferry to a display. Some people do their display with a few litres left in the wings on the basis that this won't create much fuel surge. Otherwise you need to stop and drain the fuel.

With most aerobatic aircraft you display with low fuel so there is also the problem of getting home so normally you would need to land shortly after the display and refuel.

[A and C]

Fuel leaks

I can't recall having a problem with fuel leaks on Extra wing tanks ( but I have a nagging feeling that we did have a fuel leak on a CAP235 wing tank).

With the Extra the centre tanks both fuel and smoke oil are the usual suspects for leaking.