Topic: Aeros and Avgas Fumes

[Stewart]

Have any of you, who regularly aerobat the Chipmunk, experienced increased levels of avgas fumes in the cockpit after deliberate/inadvertent zero or negative G manoeuvres?

Is this normal or otherwise to be expected?

Without wanting to condition responses, I have been wondering whether we may have a leak in a vent tube within the rear fuselage, just upstream of the vent duct connection. When the rear seat had been removed previously for access to the rear fuselage, I noted that these light aluminium tubes had tended to become crushed - presumably over the years, by boots carelessly planted either side of the control box tunnel.

We also have the classic unbalanced tank feed (max difference can be 2 galls), and, since the vent duct is shaped to generate a slight overpressure in the vent systems and tanks, this would be consistent with a small air leak on one side.

Any thoughts?

Stewart

[yakxx]

I also get a lot petrol smells and always have....but cant find any obvious leak.......As for the classic unbalanced feed....gosh only 2 gallons...lucky you...mine is nearer 5 gallons...Ive changed the non return valves and checked the vents etc etc.......Ive also run one side to Zero (Almost)   and then the other side is at about 4 gallons and then that side happily goes down all the way until about one gallon and then .well I landed...I did that over my local field just to see what happens.....

[Rod Blievers]

I concur with Yakxx.

Cheers,

Rod.

[Stewart]

yakxx and Rod,

Thanks for your contributions.
Maybe I should just focus on avoiding the neg or zero G!

Stewart

[Bob]

I've only noticed it in MT if the tanks are full, but I try not to get negative

[kenkelso]

I haven't noticed excessive fumes during aeros Stewart.

I have been reading "DHC-1 Chipmunk The Poor Mans Spitfire" (congrats on a great book Rod) and it mentions successful tests being flown to see if the fuel would feed with one tank empty. It also mentions this as part of the original specification for RAF acceptance.

Ours used to feed fairly evenly until the wing was removed for recovering and then it started assymetic feeding with a difference of 3 to 4 gals. We have done every concievable check including filling the breather pipe with fuel to see if there is an air leak.

Strangely I flew last Saturday and checking after flight the difference now seems to be minimal.

Ken Kelso

[john henderson]

When we first rebuilt MC, it would draw only from the starboard tank, we swopped the one way valves, then removed checked and refitted the ball valves above the tank and finally checked (blew through) the bleed pipes all the way back to the underside vent. None of this had any effect, but we did find that the port tank seal around the " filler/fuel sender unit" was leaking - there were traces of fuel stains after flight.

Since then, MC takes about 2 galllons from the starboard tank and then draws evenly from both. as we have 12 gallon tanks i can live with this, since the fuel endurance is way beyond that of my bladder or my backside! 

De Havilland recently set out an explanation in their news letter (I think) saying it is probably due to diffrerential air pressure and to check the joints in the vent pipes (flexible plastic with hose clips in the wings), but especially the fixed unions and their connections in the fusilage.  Check also the tank filler seals - ours are cork.  If one of them is not sealing, this will allow a pressure differential in flight.


if you get significant fuel smells during aeros, check the ball valves on top of the tanks.

[Dick Gower]

Quite a lot of fuel spils out of the carburettor when inverted so I think the fumes are part of the Chipmunk experience - certainly on the ones I have flown.

[Stewart]

Our problem had recently worsened, with a differential increased from 2gal to 4-5 gal, more avgas fumes, one instance of fuel venting post flight and, last but not least fuel level increasing in the stbd tank during flight.

Stbd anti-syphon hole looked clear, but was barely letting air through (bend over wing l/e, insert well into mouth and blow! Ignore sniggers from airfield 'friends'). A broddle with fine wire cleared it.
Vent ball bearing check valves appeared OK (hearty thump on wing adjacent to vent standpipe - listen for rattle of the ball).
Wing fairings and P2 seat and control box removed.
Stub pipe on top of NACA vent found to be broken and vent pipes within fuselage both bent/crushed. Removed, repaired duct and replaced both pipe sections.
Stbd N/R valve removed and inspected. Found 1" piece of fine dry grass stem stuck in the cage which retains the sealing disc, preventing disc from seating. Cleaned/replaced. (BTW ours was pt no. SPE.1957. Held together with 4 6BA nuts/bolts and easily serviceable)

Result. Fuel levels now less than 1/4 gal differential and only slightly increased avgas fumes after severe provocation with -ve G.

During the prior fault finding thought process, I did the sums assuming the NACA duct is 100% efficient at converting dynamic head to static head.
90 knots cruise  airspeed would be equivalent to 200mm head of AVGAS! Airflow over the vent face is higher than this, being in the prop wash. It's entirely consistent, therefore, that even a small leak in the vent system will cause large differential in fuel levels, and a catastrophic break such as ours could cause 1/2 tank depth differential.
Aeros up to 130 knots will double this theoretical head of avgas.

Stewart

[Bob]

You'll be telling us you can do algebra next . (I only scraped a maths "o" level!)
Very useful info,I'm on the way to the hanger to continue with the recover/paint/ C of A so will double check all your points.

Thanks
Bob

[Dick Gower]

Interesting discussion and outcome thanks Stewart.

Dick Gower