Merlin negative G cutout too quick?

[klem]

I hope to get into the library this week but please don't hold out too much hope on that as if there were a publication I'm sure someone would have popped up with it by now. The chances of bumping into a Hurricane pilot are extremely thin!

However, on this general subject, a small point for the devs:

To confirm the reversed Rich/Lean problem, if you open an info Window you can see that the Hurricane lever in the rear 'Rich' position is giving 0% Rich and in the forward 'Lean' position is giving 100% Rich. You get the same results whether you are using an Axis or Keys for your mixture control. I assume that is what is feeding into the simulation and its not just showing my Axis position.

On the Rotol prop Hurri FT-N, when you close the throttle it pushes the mixture lever to the rearward opsition, i.e. Lean not Rich (remember its backwards) which is not what you want when starting the engine, you want Rich, and I have to move the Throttle forward to around halfway up the Rich-Lean gate so that I can move the mixture lever out of fully Lean to start the engine. My point for the devs: Strangely, when I then close the throttle the info window shows the mixture doesn't change until I give my axis a slight tweak when it goes to 0%. Just another tweak needed when they can.

Rich for the Spit should be forward and Lean backwards. It just reads the wrong way in the cockpit model. Physically it is correctly implemented.

[klem]

Originally Posted by 41Sqn_Stormcrow

Rich for the Spit should be forward and Lean backwards. It just reads the wrong way in the cockpit model. Physically it is correctly implemented.

I'm sorry but that's wrong:

http://spitfiresite.com/2010/07/anat....html/03it_001

also some 'gathered' notes checked over by a Spitfire pilot
http://forum.keypublishing.co.uk/arc...p?t-72714.html

Also this is from the A2A Simulations "Accusim" Spitfire which seems to be a well repsected simulation of the Spitfire MKI amd MKII which particularly models engine behaviour, wear etc.:

Mixture Controls - The Mk I and II have an automatic mixture control which will weaken (lean) the mixture as height is increased, regardless of whether the mixture control handle is set rearward to RICH, or set forward to WEAK (lean). If the mixture control handle is set to WEAK an extra weak mixture will be provided with a 3% drop in R.P.M. DO NOT use the extra-weak mixture at more than +2 ¼ lbs./sq. in. Boost.

As you'll know the Spitfire has two position mixture control, unlike the Hurricane which is adjusted by hand and again the Rear position is Rich.

The actual effect on mixture is reversed.

Mh. I thought to have read in the Pilot's note of the Spit that mixture should be forward for rich. I try to find this passage.

EDIT: You're right. I must have mixed some stuff up. The pilot notes say richt is backwards and lean is forwards. So actually the lever positions in CoD are wrong.

PS: However I know how mixture ratio works for the early Merlin nonethelss

[41Sqn_Banks]

Has anyone tried to climb a Spitfire or Hurricane to 30 000 ft? Above 20 000 ft I suffer constantly from misfires and negative g cut outs.

[fruitbat]

Originally Posted by 41Sqn_Banks

Has anyone tried to climb a Spitfire or Hurricane to 30 000 ft? Above 20 000 ft I suffer constantly from misfires and negative g cut outs.

yep me to.

[klem]

Originally Posted by 41Sqn_Stormcrow

Mh. I thought to have read in the Pilot's note of the Spit that mixture should be forward for rich. I try to find this passage.

EDIT: You're right. I must have mixed some stuff up. The pilot notes say richt is backwards and lean is forwards. So actually the lever positions in CoD are wrong.

PS: However I know how mixture ratio works for the early Merlin nonethelss

Yes and if you get the Engine Info in an info window you'll see that: 0% is at Rear position (should be 100%), 100% is at forward position (should be 0%). And of course its a real problem on one Hurricane because the Throttle moves the mixture rearward when it closes, forcing it to Lean when it should be Rich.

[IvanK]

Here is a response to the question on this issue passed by a warbird engineer friend to a current early model Spitfire and Hurricane pilot in the UK:

Pilot quote:
Basically, the Spit/ Hurri with the standard float carb (SU) has the cut-out issue with a definite unload. The problem does not occur in normal flight or bumpy conditions but, does happen with low +ve G.

Engineers opinion:
He has not explored the limits of the problem as, you
avoid it by rolling and pulling (no suprise!). The limit is "between
zero and +0.5G".

My Opinion:
So in general there is agreement in these comments and the info written in the AP2095 that has been quoted in this thread. To me this confirms that the current setup is too sensitive.A good solution imo would be < +0.25G for more than 0.5seconds results in the cut.

[klem]

Well, I spent a few hours in the Tangmere library today but did not find any factual data or reports, just some observations from former Spit/Hurri Mk1 pilots, Alex Henshaw (Spit test pilot) and the pilots notes. It paints a picture which seems to be "negative G is quick to affect the engine but not instantaneous and it recovers in a couple of seconds" but here's what I found so you can form your own opinions:

From the cockpit: Spitfire by Wing Cdr T.F. Neill DFC AFC
"it caused the carburettor to flood after the briefest period of negative G"
"engine ceased to pull for a second or two"

Spitfire - The Biography by Jonathan Glancey
"When the Spitfire is thrust into a sudden dive the carburettors would flood causing the engine to cut out."
"the Merlin always came back on song in a matter of moments"
"All this took precious seconds"

Pilots Notes Spitfire IIa and IIb
Merlin XII
"Inverted flying. This is Normal"
"A moderately slow roll is best as the engine can be kept running normally...best if slight barrel roll... if engine shows signs of beginning to fade the stick should be brought back a little, almost imperceptibly"

"True Slow Roll* This can be done if high speed is used at the start but the engine will cut out when inverted. If the engine is throttled back** as the roll is started it will be possible to get the engine started again earlier in the final stages of the roll."

*In opening sequence of the film "Battle of Britain" (is that a "True Slow Roll"?) if you look carefully the roll is begun with an upward pitch and a slight barrel element iaw the Pilots Notes. From the moment the lift vector ceases (inverted) there is about one second before engine response and about two seconds after rolling out before it picks up again.

** Presumable reduces flooding

Sigh for a Merlin by Alex Henshaw
"I would open the engine flat out in a vertical climb and at approximately 1200 feet push the nose over forward and with the engine closed complete the half of an outside loop... usually round off to a few feet above the ground *** ... push the machine into an almost vertical climb.... then pull the control gently over to form a half loop, hoping as I did that the engine would burst into life"

***(klem:inverted)

There are frequent references to diving in pilots notes, Jeffrey Quill's and Alex Henshaw's books etc with no mention of engine problems. I expect the severity of the dive would have had some influence, perhaps a low -G or reduced G pushover to a sustained dive would allow the Carburettors to keep up? Some of these dives achieved very high speed and were quite steep.

There is a time element to onset but it's hard to quantify and almost certainly related to the severity of the pushover as the floats and fuel rose at various rates vs reducing G value. A sudden severe pushover would presumably have had the floats and fuel wanging up in the float chambers.

The recovery or catchup appears to be a matter of only a couple of seconds once more normal G values are recovered.

I understand what you want to say and I agree to it when it boils down to simulate ingame the thing.

I would like to know now if the cut out was an either on or off event with nothing in between.

Here is the drawing:


Is this pre- or post-modification?

As far as I understand the floater shall restrain fuel from flowing into to chamber when negative g's occur. That's above all in terms of function I can see.

Depending on the damping due to hinge friction my guess is that initially the floater will go up when one pushes the stick forward due to inertia that forces all objects to remain in the state they are until the forces get the better of them. So when it shortly gets up it will reduce the pressure loss at the reservoir entry by opening it. Hence more fuel will flow into the reservoir for a short time.