What does the gear do?

[Gunfreak]

So it has to be low on start up, but i haven't really found a use for high gear. 

[razo+r]

You use high gear, or rather auto, for flights at higher altitude. 

[scoobie]

Don't quote me on that, but I think it works like this:
You start the plane in "MOD" (I don't know what "MOD" stands for, really) - this position of the switch keeps the supercharger at low gear/speed PLUS it prevents the supercharger from switching to the high speed automatically - just in case something went wrong, the supercharger automatic control went nuts, turned "high blow" at low alt and overboosted the engine or anything of this sort. You don't need trouble when taking off or low above the ground in general. That's my understanding of it, I may be wrong, though.
Then, when you are at somewhat safe altitude and planning to climb higher and higher, you flick the switch to "AUTO". Auto means that the plane will switch from low to high supercharger speed automatically at a "prescribed" altitude (which depends on a specific engine installed in the aircraft). You need high gear at high altitude to keep high manifold pressure, thin atmospheric air up there is not enough for that, nor is supercharger low speed (which is "tuned" for denser air).

 

Edited September 24, 2021 by scoobie

[grafspee]

37 minutes ago, Gunfreak said:

So it has to be low on start up, but i haven't really found a use for high gear. 

You fly too low then, climb high

[Bremspropeller]

[EGPWS voice]

 

Too low, Gear!

 

[/EGPWS voice]

[Reflected]

Use low gear (MOD) until you climb so high that even full throttles will only give you +7 boost. THEN flip it to auto. That's what the original pilots notes says anyway.

 

I've been wondering why, but @scoobie made a good point there. If the solenoid or whatever that is breaks, it still won't flip it to high gear at low alt and overboost the engine. If you follow the manual's recommendation, there is no way the engine would overboost.

[grafspee]

Just now, Reflected said:

Use low gear (MOD) until you climb so high that even full throttles will only give you +7 boost. THEN flip it to auto. That's what the original pilots notes says anyway.

 

I've been wondering why, but @scoobie made a good point there. If the solenoid or whatever that is breaks, it still won't flip it to high gear at low alt and overboost the engine. If you follow the manual's recommendation, there is no way the engine would overboost.

May be was not reliable enough.

[scoobie]

Oh, thanks for the tip! I really need to get down to reading the pilot's notes at last - so it is: ↘+7 boost @ full throttle => supercharger AUTO. Where are my tentative checklists... bloody mess...

 

Yeah, I wish I knew the exact reason why they put "MOD" position (I hope someone has more insight into this and can explain), but one simple thing is fairly sure - it seems to be a safety precaution, so someone who knew exactly how this automatic speed change mechanism worked (so an engineer, most likely) had to fear it may go wrong and kill the engine (and possibly a crew).

 

I can't remember clearly, but a simliar patent is in the Spit. However, AFAICR there you need to flick the switch to "M.S." (or something like that) EDIT: then back to AUTO and only confirm the supercharger remains in low speed. Then you leave switch in "AUTO" and go taxi, take-off etc. So it's a bit different in the Spitfire (or I have a bad checklist, which is also plausible ).

Edited September 24, 2021 by scoobie

[mosquito64]

From the Pilot manual:

System description:

23. Superchargers control When the superchargers gear change switch (51) is set to MOD, the superchargers will remain in low gear at all altitudes. When this switch is set to AUTO the electropneumatic rams are controlled by an aneroid, and will automatically engage high gear when climbing, at approximately the following heights : Merlin 23—8,750 ft. Merlin 25—7.000 ft. These heights are the correct supercharger gear change heights only when using maximum power (operational necessity). When using low power settings, the selection of high gear should be carried out by switching to AUTO as recommended in para. 41. When descending in AUTO, low gear will be engaged at slightly lower altitudes than those quoted above. Failure of the electrical or pneumatic system will cause the superchargers to remain in, or return to, low gear.

 

and Para 41 

41. Climbing (i) The speed for maximum rate of climb is 150 knots. (ii) Climb in low gear at 2,850 r.p.m. and +9 lb./sq. in. boost. When the maximum obtainable boost has fallen to + 7 lb./sq. in., change to AUTO. Above 18,000 ft. decrease the airspeed by 2 knots per 1,000 ft. (iii) When climbing for maximum range, climb in low gear at 2,650 r.p.m. and +7 lb./sq. in. boost, using the airspeeds recommended above. When the maximum obtainable boost has fallen to +4 lb./sq. in. set the supercharger gear change switch to AUTO and re-adjust the throttles. Above 18,000 ft. increase power to +9 lb./sq. in. and 2,850 r.p.m. and reduce airspeed as recommended. Although less fuel is used to reach a given altitude by climbing at high power the total fuel used and the time taken on the subsequent cruise is the same, whether the aircraft is climbed at 2,650 r.p.m. and +7 lb./sq. in. boost or 2,850 r.p.m. and +9 lb./sq. in. boost. (iv) When climbing with a boost setting of less than + 9 lb./sq. in. the automatic boost control cannot open the throttle valves fully and the boost will begin to fall off before full throttle height is reached ; the throttle levers should be progressively advanced to the gate to maintain the desired boost. (v) For operational necessity at any altitude, select AUTO and 3,000 r.p.m. and move the throttles fully forward (see para. 20).

 

[scoobie]

[deleted] (I thought I got it, but not quite.)

Thanks! Now I know why flick the switch in flight., so why you need a switch at all. What I still don't get is why be so scrupulous to set "MOD" before take-off. If MOD = AUTO (at low alt), who cares if it's MOD or AUTO? They built it "the safe way", i.e. malfunction = low gear (safe).

Edited September 24, 2021 by scoobie

[Reflected]

I’d also be interested why this safety precaution is not mentioned in the Spitfire’s pilots notes. There you leave it on auto and that’s it

[Bremspropeller]

Probably because if there's only one engine, it'll switch gear whenever it does.

With two engines, there might be an altitude band where one engine goes into the high blower-gear, while the other one won't.

 

It's probably some pressure-diaphragm with an associated hysteresis which deems whether to switch gear to high, or not.

Have two different units with two slightly differing hysteresis curves and you might have one engine switch early, leading to yaw and other non-niceties.

 

Hence you only switch the gear to auto at an altitude, where the switch-over can be assured.

 

/educated guess.

[Reflected]

@Bremspropeller good thinking, makes sense!

[Diesel_Thunder]

Another aspect of this is that driving the supercharger in high gear also uses more power from the engine. Up at altitude the power gain vs the power needed to drive the compressor is worth it.
 

However it hurts at low altitude as you are dealing with denser air. You end up with a double whammy where the compressor is using more of the engines power instead of going into the prop, and you have to back off the throttle to avoid overboosting the engine meaning that your engine is already making less power to start with. Not a good combination. 
 

Having one control for both engines also allows for both engines to do the supercharger gear change at the same time, which keeps the power level similar between both engines. If each engine had separate controls, one engine may switch gears at a different time than the other leading to a power difference between the engines and asymetrical thrust. 

[Sarge55]

6 hours ago, mosquito64 said:

(v) For operational necessity at any altitude, select AUTO and 3,000 r.p.m. and move the throttles fully forward (see para. 20).

This subsection of Para 40 is interesting.  I may be too literal here but to me it says that you can switch to AUTO when in a dogfight or final run in on a target (operational necessity)... or do they mean if the engines are not operating correctly?

 

Wonder what Para 20 says?

[scoobie]

(I don't know the answer to what Sarge55 asks.)

@Bremspropeller, @Diesel_Thunder - thanks a lot!

 

[grafspee]

With automatic boost regulator it is impossible to over boost engines. Operating engine at second supercharger gear at low alt will kill engine power significantly due to higher power draw by supercharger and due to increased charger heat up, i made this graf for P-51's engine it will look similar for mossies engine too.

 

[grafspee]

14 hours ago, Sarge55 said:

This subsection of Para 40 is interesting.  I may be too literal here but to me it says that you can switch to AUTO when in a dogfight or final run in on a target (operational necessity)... or do they mean if the engines are not operating correctly?

 

Wonder what Para 20 says?

They mean if pilot don't have time to baby sit engines, example plane is in combat situation, switch should be in Auto position. 3000rpm ensure that over boost won't happen because max boost possible is safe at 3000rpm.

[mosquito64]

On 9/24/2021 at 5:56 PM, Sarge55 said:

Wonder what Para 20 says?

ENGIN E CONTROL S 120. Throttle controls The black friction nut is for the throttles: the (larger) white one is for the propeller levers. Normally the throttles can be pushed forward to the stops only. When the small catches on the levers are squeezed the throttles can be pushed fully forward. Merlin 25 engines give 4-12 lb./sq. in. boost at the stops and +18 lb./sq. in. when fully forward ; there is no cut-out. Merlin 23 engines may still be fitted. They give +9 lb./sq. in. at the stops and +12 lb./sq. in. at the fully forward position. If the boost control cut-out is pulled, +14 lb./sq. in. will be obtained in low gear. De-rated engines of either mark give +9 lb/sq. in.boost at the stops and +12 lb./sq. in. at the fully forward position, and do not have a cut-out.

 

This be para 20.