Chipmunk negative g

[elmorejames]

Could anybody please tell me what the correct max negative g is? We think we know... but someone thinks differently and I cannot find this info from any poh's or manuals surprisingly, so need a definitive source. Also, any views on the effects of negative g on engine life, or anything else for that matter? I'm talking strictly Chippie/Gipsy here
Thanks very much

[Lowtimer]

I can't document this, but what I've always believed is that the RAF release to service listed it at +6 and -3.

+6-3 were the limits marked on the accelerometer on the T.10 I had some experience of in the '80s (WZ868 / G-BCIW). I also believe, but can't prove, that the CAA required at least some Chipmunks to be restricted to +5-2.

The Chipmunk had a fairly difficult transition from military to civilian flying, with the CAA requiring a number of modifications which cost a lot of money. The absence of such mods never seeemd to make military Chipmunks fall out of the sky in unacceptable numbers, but that's government for you. The military Vne was a somewhat alarming 173 KIAS: in the civilian Mk.22 pilot's notes I have, Vne is listed as 155 KIAS.

However, in the real world +5-2 and 155 knots is more than enough to do anything with a Chipmunk that the reasonable pilot might wish, especially with no inverted fuel and oil and a distinctly "right way up" optimised wing section. The aeroplane does not really like negative G at all and anything more ambitious than a transient -1g slow roll or roll off the top of a loop might be regarded as rather unsympathetic handling of an aeroplane which has reached its half centenary.

Even transient negative G is not ideal for the engine. When I used to slow roll a Chipie using -1G in the middle, I was strongly encouraged to follow RAF practice in closing the throttle as the G went down past zero, and opening it smoothly only once positive G, and oil pressure, had been restored. The RAF in the Chipmunk's main service life, of course, had lots of money for engines. With a top-notch (meaning Vintech) Gipsy Major rebuild now costing as much as a small house in the North of England, and the engines only having a 1000 hour life on a transport category C of A anyway, I would be very much inclined these days not to go negative at all with any Gipsy Major I owned or might have to pay for. After all, that's what we have Yaks for (+7-5) nowadays!

Were you thinking of doing anything that requires a lot of negative G? I'm curious.

The worst thing I've ever seen done to a Chipmunk is in "the Great Waldo Pepper" where a modified Chippie is dolled up as a pre-war US stunt aircraft and subjected to a number of attempts at an outside loop from normal flight until it crashes, killing the pilot. :ugh:

[elmorejames]

Thanks very much Lowtimer; just the sort of info I need. I'm certainly not going to do anything that requires a lot of negative. Quite the opposite. I'm trying to prevent it happening by somebody in the group and I could use all the evidence against it that I can gather.
I agree entirely with your operating suggestions and did the same twenty five years ago when I wasn't paying for it.
If anyone else has relevant info about absolute limits I'd be obliged.

Damn right I got the blues

[Wide-Body]

Hi

A top response from Lowtimer. The Chipmunk is an old aircraft. For definitive answers to limits try

de Havilland Support Ltd
Building 213
Duxford Airfield
Cambridgeshire
CB2 4QR
England
Tel: +44 (0) 1223 830090
FAX: +44 (0) 1223 830085
e-mail: [email protected]

When aerobatting the Chippy well +4, -0 is not normally exceeded. A good range of manoeuvres can be flown within these limits.The engine does not like negative talk to Deltair for more info. Stress to your errant group member the age of the aircraft. I assume he wears a chute, if he persists if flying a 50 year old aircraft to limits he may need it. For the rest of your groups safety ensure you have a locking system on your accelerometer.

Good luck safe flying

Wide

[Sleeve Wing]

Pretty sure we'd elicit a response from your good self on this one, Wide.
.....and you're right, a well-thought-out techy. answer from Lowtimer.

Chipmunks weren't originally fitted with G meters and my bet is that only a few civvy ones have them ('896?),even now.
It's true that "in the olden days" +6 -3 was the ballpark figure for a number of military training aircraft but the first aircraft I remember to be fitted with one as standard was the JP. A great deal therefore depended on your instructor,to show you the correct way to carry out a manoeuvre and also to experience what acceptable "G" felt like.

One must now treat the Chippie as the ageing lady she is, watching particularly throttle handling when inverted. Reducing the RPM when neg. G forces come into play will allow the engine to produce thrust a little longer, prevent the cylinder bores being washed with neat fuel when the oil pressure drops and will also prevent the engine from inadvertantly speeding up before oil pressure is reinstated.
Of course,the only real problem with neg. in a Chippie is the way it copiously chucks oil out of the breather, all over the under-fuselage of your beloved aeroplane !! Half an hour getting filthy wiping it all off for a few moments of indulgence......????
Why not keep your aeroplane in much better shape and get your negative G kicks in a Pitts ?
Rgds, Sleeve.
:ok:

[Milt]

Maximum g

Be aware that max g limitations on all aircraft manoeuvres relate to the design/build specifications and the margin applied to provide a safety buffer before it breaks.

Don't know what the margin was for a Chipmunk but probably two thirds ultimate.

Stick force per g for a Chipmunk or similar should not be less than five pounds.

Also be aware of the additional limitations you should apply in respect of Rolling Pullouts where you load up the upgoing wing an increased amount over what you will see on an accelerometer.

Perhaps there should be another thread on Rolling Pullouts for the further benefit of the uninitiated.

[jabberwok]

Odd how the old memory plays tricks because as soon as I read your post I dived for the handbook convinced the +/- G values were in there. And indeed you are right because Publication CHK. P.N.-1 A.L.9 simply states:

"The maximum positive acceleration which the structure has been designed to withstand without permanent deformation at a weight of 2100lbs is 6.0g. Intentional manoueuvres must be confined to load factors well below this value."

The list of "approved" manoeuvres in the manual are all positive G - no negative G envelope is mentioned at all.

Again it may be fading memory but I also recall some documentation (CAA?) about civil Chipmunks suggesting they did not qualify for full aerobatic status i.e. +6/-3g and were only to be considered "semi aerobatic" which I think was to +3/-1.5. I'd prefer if someone could verify the "semi aerobatic" limits as I don't trust my grey cells these days.

There is a line in the handbook saying the aircraft is not designed for sustained inverted flight but that was not a G question - more to do with the engine (no inverted fuel capability and loss of oil flow).

[Lowtimer]

Hiya, Wide,
Enjoyed meeting you briefly at soggy Sandown the other weekend, pity about the weather. Yes, absolutely, +4 and zero are all I would wish to use in a Chipmunk nowadays, I am older (as are the aeroplanes) and I hope a bit more mechanically sensitive than I was in my early 20s. Even in the Yak I've never exceeded +5 -2, but then I'm not competing in it and I'm only really a Sunday afternoon looper and roller.

Sleeve,
I don't think I'd really want to do aerobatics in anything without a G meter, or indeed fly any aeroplane that was being aerobatted by lots of other people if it didn't have a G meter. And I'd forgotten about the oil dripping all off the lower fuselage, it was even worse than cleaning up the Yak!

Milt,
You make a very good point about the way the stresses build up when G is applied in conjunction with aileron, and one which is not very often considered in operating handbooks in my experience. The troubles of the T-34 fleet in the USA are a case in point. Use of significant aileron must not only be done with consideraton of speed, but also G, and in the absence of any formal G / aileron envelope it is wise to unload, roll and pull in best fighter pilot fashion. For those occasions when one must pull and roll at the same time, I work on the basis of half and half, i.e. maxiumum of half aileron with a maximum of half the G limit. That's plenty to accomodate barrel roll entries and similar manoeuvres.

Jabber,
On everything non-aerobatic I've seen, Normal category is +3.8G, and Utility category is +4.4G, so +3G sounds a bit low for a Semi-aerobatic category. I think anything allowed to do aerobatics at all would have to be placarded at no less than +4.4g.

Gliders might be an exception, though, most of them in my gliding days were placarded at +3.5G, and that seemed to be enough for the occasional loop. Mind you, those were the days when we used to do spin training straight off launch at 1200 feet in the K-13 at North Weald, two turns and into the downwind leg at 600 feet, fair gives me the quivers to think about spinning that low down these days.

[elmorejames]

Excuse the belated response, but I just want to say thank you to those superb replies. Excellent info; I now have all the ammo I require!

[Fg Off Max Stout]

Disregarding the structural considerations, I would suggest that any aeroplane without an inverted fuel/oil system is not designed for intentional, sustained negative G. Now taking into account the structural considerations, the pilot would be wise to fly to reduced G limits rather than those published when the Chippy was a shiny new aeroplane. After all, fifty years of aeros is going to cause cracks and fatigue.

A chippy can do very nice aeros without exceeding 0/+4G, so why push it further? After all you would do handbrake turns and doughnuts in Bentley Blower would you?

[ShyTorque]

Good topic and interesting replies.

Sleeve Wing,

You said:

" Reducing the RPM when neg. G forces come into play will allow the engine to produce thrust a little longer, prevent the cylinder bores being washed with neat fuel when the oil pressure drops and will also prevent the engine from inadvertantly speeding up before oil pressure is reinstated".

This is an interesting statement, could you explain the first two parts of this please? :confused:

[G-KEST]

What a good thread and excellent advice all round on an aeroplane which is likely to be over 50 years of age (and, unlike me, looks ever more elegant....!!!). I have been doing aeros in Chipmunks for longer than I care to think and plus 3.5 and minus 1.25 is perfectly adequate. Any negative G should be minimised and the throttle closed when the engine cuts until you are back into positive G.
Certain examples on the US register and operated in the ezperimental category were highly modified by guys like Art Scholl to be much stronger and fitted with larger flat six engines up to 260hp with C/S props. They were used for the type of manoevers flown at unlimited level in the late 1960's and early 1970's and mainly used for airshow work. I well remember Art flying in the UK in 1970 when the US team flew at Hullavington in that years WAC. His lomcavaks were awesome and flown at very low heights.
Nowadays stick to the standard level manoevers and you should not trouble the aeroplane. Treat it like your grandmother though she does like the occasional bit of blue beneath brown and green.
Better a short life in the saddle, Lord: than a long one by the fire..
Cheers,
Trapper 69
:ok:

[Dan Winterland]

In military service, it was +5 to -3.

From AP 101B-5510-15 (Chipmunk T Mk10 Pilot's notes) Part II )limitations Section 3 (c):

'Normal Acceleration. Accelerometer readings are permitted within the range +5 to minus 3 g.'

Although, I (or my students!) managed to overstress on more than one occasion. The engineers didn't start their checks until +6g. I defy anyone to push more than -3g in a chippy!

There was always a lot of 'fokelore' regarding the stopping of the engine under negative g. The common misconception was that the engine was starved of fuel while the aircraft was inverted. However, the design of the Hobson carburettor meant that the float floated to the bottom of the float chamber (upside down!)and the metering valve was fully open, therefore the cut while inverted was a rich cut and not fuel starvation. The advice to close the throttle was to reduce the ammount of fuel entering the cylinders allowing a quicker pick up when positive g was restored.

I never subjected the aircraft to anything more than hesitation slow rolls in the neagtive sense. The key to successful Chippy aeros was maintaining your energy. I saw too many people trying to impress by pulling hard - the net result was that they ended up at base height sooner!

[ShyTorque]

Dan, your explanation of the "rich cut" is a good one.

Sleeve Wing,

I don't see how an engine at normal operating temperature can have its "bores washed with neat fuel" on inversion of the aircraft, because the fuel will be vaporised as it enters the manifold / inlet ports. Neat fuel is a liquid.

Closing the throttle reduces the pressure drop across the venturi, which would help prevent the engine pulling in excess fuel flooding from the float chamber to a greater extent.

[Ripline]

Nowt to do with g loading on Chippies, but:

After all you would do handbrake turns and doughnuts in Bentley Blower would you?

Leeds Castle, late 70's, Balloons & Bentleys, competition (very quietly sponsored by one R Maxwell, MP), self as part of retrieve crew in rear of "Wicked William's" dark green monster doing a 4-wheel drift to a halt in the gravel at the entrance to the Castle for Southern TV cameras as event winner.

They weren't ready, so they asked him to do it again.

Bliss....... :cool:

Don't even ask about the Gorilla suit :D

Ripline