HQ with fuel slosh
 

I’m constructing a flight test plan to look at the effect on HQ of a large low baffled fuel tank mounted within the fuselage of an aircraft. The tank has a fineness ratio of around 5:1 so fuel movement is significant. Static analysis of the fuel movement within the tank at pitch attitudes of +/-10 degrees indicates a CG movement of +/-2.7 degrees or a range of 5.4 degrees. Dynamic does not appear to be that bad.

Tests I’m considering are:

1. Pitch capture tasks (looking for changes in Fs/g) over range –10 to +10 deg
2. Thrust transients when pitch attitude = 0
3. Combined 1 and 2 from above in manoeuvres such as a go-around
4. SPPO about 0 pitch attitude at high and low IAS
5. Emergency descent man. (60 deg AoB to –15 pitch)
6. Takeoff and landing HQ
7. Large aileron throughs at low speed (ie. with alpha on-roll coupling potential)

Testing at the upper end of the airspeed range seems not so important as the low, and am considering down to 1.2Vs for config. The host aircraft is generally well damped longitudinally with strong LSS and LMS. Am planning around a fuel tank level of 50-66% as being most critical for fuel slosh.

A complex thread I know, but many minds produce better solutions than one. Thoughts appreciated on test techniques, what to expect HQ’wise of large CG shift within manoeuvre, and any other. Also on safety considerations. Please don’t all suggest heaps of tank baffling as this is a last option.

Thanks.

What effect will sloshing of the fuel will have upon the CG of the whole aircraft? It should be fairly straightforward to calculate since you already know the CG movement of the fuel.

Had you considered the effect on handling of a rapid stall entry, followed by pitch down on recovery? (FAR/JAR 23&25 would normally use a 3 - 5 kn deceration rate - para 203 I think, which in some types can produce a marked pitch down.) I suspect that LSS will vary through the manoeuvre as the CG moves back during the stall entry (causing a low stall speed, high elevator power, and thus steep nose-up stalling attitude), followed by a more FWD CG during the pull up, causing reduced elevator power when you actually need it.

I suspect that you'll probably find that the SPO is more damped than usual - there's a device called a bifilar damper which uses mercury in a tube that tends to oscillate in antiphase with the motion; I suspect that your tank might well do the same.

Interesting question, I look forward to seeing what wiser heads have to offer.

G

Thanks Genghis,

In answer to your first question, +/-2.7% is an approximation for the movement of the aircraft CG following fuel slosh from a +/- 10 deg pitch change. Good point about stall entry and recovery, will look into those.

Some more info. Have done a basic analysis which seems to indicate that their is no critical slosh level (33% full, 50% or 66%). Seem that whilst the fuel CG movement is greater at lower fuel levels, the mass is also reduced. Thus the effect is the same as for a greater fuel volume moving a lesser distance.

Look forward to more test ideas.