Aerobatics prohibited with wing tank fuel?

[GeeWhizz]

Ladies and gents

I've been doing my research on aerobatics and have read some POH, handling notes, and checklists for several aerobatic aeroplanes. Among them are the likes of the CAP 232, Pitts Spesh's, and Extra 300 series. Each of these stipulate that aerobatics are not permitted with fuel in the wing tanks. I cannot for the life of me find out why this is so.

Issues that are possible factors include W&B, internal tank damage (baffles?) under high G loading, AAIB/NTSB reports of fuel starvation when the fuel cock is on the wing tank feed, inverted fuel systems etc. But from my research none of these explicitly state why fuel cannot be in the wing tanks during aeros.

Any ideas?

GW

[stiknruda]

When spinning (for example) the tip tank fuel effects the spin characteristics as the extra momentum caused by the fuel mass can overpower the aileron inputs.

If this not a sufficient explanation, let me know and I'll go into it in far more detail, tomorrow!!


Hope this helps,

Stik

[GeeWhizz]

Thanks stik, glad you're answering this. I'll progress my question if I may.

Do the aeroplanes I mentioned all have tip tanks? I was under the impression they were root tanks that took up roughly half of the inboard wing area. Also the literature, albeit limited, seems to state that absolutely no fuel is to be in the wing tanks. I'm taking that literally meaning not even a drop; does it still have the same effect on spinning at minuscule amounts?

Thanks again

GW

[The500man]

With my Pitts, I can't fit two people and a full tank of fuel without being overweight. Usually the aerobatic envelope will have a restricted maximum weight lower than the normal envelope as well because of load factor and CG constraints (C of G is particularly important for inverted spinning for example). Any weight that is unnecessary is something you have to accelerate, or turn or slow down in manoeuvres, so I think that is the main reason.

[foxmoth]

Certainly in the Extra there is a problem when rolling, the roll rate is so high that you can damage the wing as the fuel is thrown out to the side!

[GeeWhizz]

Certainly in the Extra there is a problem when rolling, the roll rate is so high that you can damage the wing as the fuel is thrown out to the side!

This sounds most logical as (in my mind) it covers all quantities of fuel in the wing tanks. Are there any documented references for it?

Thanks all

GW

[boguing]

Quick rough calc, used on a tank centred one metre from the CL at 360 degrees a second says mass goes up by a factor of nine, so maybe just holding the tank in place is not easy.

[Zulu Alpha]

Two reasons,

1 The tanks are long and thin. When you roll or flick the fuel surges outward and can break the outside end wall of the tank when it hits it with force.

2 When you spin the fuel surges to the outside ends of the tanks and then you have a large mass a long way out. This gives you much more rotational inertia which means it can be more difficult to stop a spin.

[The500man]

Here's a good brief explanation of how spinning is effected by mass distribution.

[GeeWhizz]

Thank you Zulu Alpha, that all makes a lot of sense.

Sorry to ask again but are there any ADs/reports/document references for these?

Edit: 500man thanks also.

[NigelOnDraft]

There are aerobatics and there are "aerobatics"

Yes - the POH might forbid aerobatics with wing fuel, but these types are usually capable of quite extreme rates of roll / 'g' / gyrations etc. I think you might find some more modest aerobatics with wing fuel present will not result in the wing(s) falling apart. Else there would not be many Extra 300s with intact wings

Trouble is the "Aerobatic" Fuel Tank is often so small, you are limited to a competition / display type sequence for 5-10mins, then immediately land with little diversion fuel. There was a G- reg Extra 300 accident in Norway I think where (not relevant to the accident IIRC?) the investigators pointed out you could not legally aerobat an Extra in Normay since it could not retain 45mins holding fuel

NoD

[Zulu Alpha]

are there any ADs/reports/document references for these?

Not that I am aware of.

[GeeWhizz]

the investigators pointed out you could not legally aerobat an Extra in Normay since it could not retain 45mins holding fuel

I read this report, which was interesting and IIRC this pointed out the problem of fuel starvation occurring when the fuel selector was set to wings.

[The500man]

GeeWhizz I'm not sure exactly what you're after but the reason why aerobatic aircraft have wing/ aux tanks in the first place is because the main tank is restricted by available space, and because aerobatic sorties or air displays are generally short and close to an aerodrome anyway so not much fuel needs to be carried. The only purpose for a wing or auxiliary tank is to increase range to get the aeroplane where it needs to be.

It wouldn't make much sense to carry additional fuel in a wing tank, that couldn't be used during aero's anyway. I think most auxiliary tanks have to transfer to the main tank, which normally has to be done straight and level over a period of time. So by carrying that fuel you have the extra stresses associated with carrying that weight, loss of performance and the gyroscopic effects due to location.

If you want actual data you may have to ask a designer or a test pilot.

[GeeWhizz]

Of course. It's probably a really simple question that I'm looking too deeply into. It just seemed rather strange that there are many POHs that specifically state that no fuel is to be in the wings for aerobatic flight, without any explicit reasons in print as to why.

[The500man]

I think the reason for that is because it's an operating manual rather than an educational one. It doesn't for example tell you why to warm the engine at 1200 rpm or why you should avoid prolonged idling either.

[A and C]

A lot of good thinking above about the distribution on weight and the effect of inertia of the fuel in the wings on the aircraft when performing aerobatics.

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.

[Armchairflyer]

Maybe the risk of damaging the attachments of the fuel tanks might be a factor here. Some months ago I was told about a case where a tank attachment in a Decathlon or Citabria (don't remember exactly) had been ripped loose by aerobatics with too much fuel in the tanks (apparently without any damage to the structure of the wing or the fuel tank itself).

[boguing]

Quite happy to do the numbers.

Give me the dimensions of the tank in an Extra and I'll tell you what the actual load is in the worst case scenario. Which I think is flying at 90 degrees aob any straight into max roll rate.

Earlier I guessed at 40 kg becoming 360 kg statically. The free surface effect load will be even larger.

[foxmoth]

But which would be worst - nearly full tank with little movement or say quarter tank with it all moving from inboard to outboard as the roll starts?