Low/High Supercharger transition?

[Buznee]

What's the best method for transitioning between low to high with the supercharger as you climb with altitude?

 

It seems under standard prop and power settings as you get up towards 9,000 to 10,000 ft you end up full throttle to keep the manifold pressure up and the prop is set to the cruise or max continuous power rpm. At this point the supercharger does not seem to switch over when in automatic mode unless I push the prop lever forward some and then since the throttle is all the way forward as well itll switch to the high supercharger setting and the manifold pressure will sky rocket past the safe levels.

 

My thought is either you manually control the supercharger with the low/high switch or, you lower the throttle down, go to full forward on the pitch, and add throttle to get it to switch over. I don't know though, it doesn't seem right.

 

Maybe it's right under my nose but I can't find any checklist in the manual for normal procedure regarding the climb and transition from low/high on the supercharger.

 

I would have thought though that this feature would have been more seamless, you wouldn't have to go through a procedure to safely transition to the high setting.

[Buznee]

Here's what the manual says..

 

"High Altitude Flight Characteristics

The P-51D’s 2-stage, 2-speed supercharger provides plentiful power up to well above 35,000 feet. As a general rule, the greater the altitude, the greater the control movement required to achieve the same response.

The supercharger blower will automatically shift into high speed at between 14,500 and 19,500 feet. This change will be accompanied by a momentary surge in power and increase in manifold pressure, until the manifold pressure regulator catches up. There is no noticeable effect when the supercharger shifts back into low blower on the descent. As a precaution, attention should be paid to the Supercharger control switch. If the amber light isn’t out below 12,000 feet, the supercharger should be set manually to LOW. When running in high blower, care should be taken to handle the throttle smoothly, as any rough handling will cause the engine to surge, greatly decreasing the efficiency of the airframe at high altitude and increasing control efforts."

 

So apparently its okay to let it go through a momentary surge in power? Do real mustang pilots have a better method of transitioning smoothly through this condition?

[Nybble]

As far as I know a surge like that is fine and is not detrimental to the aircraft. In a combat situation the aggressive throttle use or WEP activation would put the aircraft in the same or worse state, and for longer.

 

A smooth transition would be nice, but not necessary at all. I will however load up the mustang now and have a shot at it.

[Nybble]

I did a bit of research in the sim if your still interested?

 

I found you can get a much smoother (You can't notice it at all) transition by setting the supercharger switch to HIGH while at a lower altitude. Note I had to manually bind the switch as the cover was always in the way so I could not click it.

 

The trade off however was that you take a performance hit changing over so early, using the second stage of the supercharger at this altitude kills the power a bit, and it takes longer to climb up to high altitudes.

 

From what I'm aware of unless something was to fail pilots would simple leave it in the AUTO position for the climb.

 

I could be wrong across the board though, it was Christmas today after all so was pretty distracted :D

[Buznee]

Thanks Nibble I'm really curious if the real mustang pilots checked this portion of Dcs p-51d for accuracy and how they transitioned.

 

Here is an Interesting thing I found regarding the charger. See if you can solve this riddle. Watch this video and tell me why the manifold pressure increase during the switch from low to high on the supercharger but the horsepower remains the same.

[Nybble]

It's simply a factor of the superchargers design limits, I'll try to explain.

 

As the aircraft is climbing at full throttle it is letting as much air as possible though the throttle and into the inlet manifold. As altitude increases the pressure begins to drop as the air is thinner, and the first stage of the supercharge cannot utilize the intake as efficiently. As a result the horsepower begins to drop, as the lack of air starves the engine.

 

At the critical altitude, 17,000 ft in this case, the second stage kicks in, which further compresses the air, in turn creating higher pressure at the inlet manifold, despite the thinner air. With the second stage in use the maximum horsepower that the engine can produce is 1400. Think of the second stage as an overhead that only benefits the aircraft at higher altitudes. Even though you have excessive MP the engine cannot utilize it at this stage, and you could even drop the throttle back while maintaining 1400 HP.

 

Imagine an inverse of the system, where the second stage is enable by default. If this was the case on takeoff at low altitude you would be limited to the 1400 HP, despite having a full range of MP available, due to the overheads that enabling the 2 stage supercharger brings. So naturally you would kill the second stage, however once at altitude you would lose pressure and have to turn it back on eventually, or suffer a huge drop in power and speed.

 

I hope my explanation is not too messy ;)

[Yo-Yo]

It's simply a factor of the superchargers design limits, I'll try to explain.

 

As the aircraft is climbing at full throttle it is letting as much air as possible though the throttle and into the inlet manifold. As altitude increases the pressure begins to drop as the air is thinner, and the first stage of the supercharge cannot utilize the intake as efficiently. As a result the horsepower begins to drop, as the lack of air starves the engine.

 

At the critical altitude, 17,000 ft in this case, the second stage kicks in, which further compresses the air, in turn creating higher pressure at the inlet manifold, despite the thinner air. With the second stage in use the maximum horsepower that the engine can produce is 1400. Think of the second stage as an overhead that only benefits the aircraft at higher altitudes. Even though you have excessive MP the engine cannot utilize it at this stage, and you could even drop the throttle back while maintaining 1400 HP.

 

Imagine an inverse of the system, where the second stage is enable by default. If this was the case on takeoff at low altitude you would be limited to the 1400 HP, despite having a full range of MP available, due to the overheads that enabling the 2 stage supercharger brings. So naturally you would kill the second stage, however once at altitude you would lose pressure and have to turn it back on eventually, or suffer a huge drop in power and speed.

 

I hope my explanation is not too messy ;)

 

I think it's a common mistake both in Russian and English community naming SPEED shifting as STAGE shifting. The point is that blower rotor is switched to higher or lower speed using a gearbox and a clutch almost as in a car.

THen, the term "critical altitude" that is the same as "full throttle altitude " is a term for the altide where the blower begins to be unable to produce necessary MP.

Critical altitude depends on the MP that is used as a reference and on the engine rpm. The higher is rpm for the same MP the higher is the crytical altitude. The higher is MP for the same rpm the lower it will be.

 

 

And answering the question - how could it be that after switching to the second speed the power remains the same - the higher speed compresses the air more but by the cost of increased power required for the blower. The second effect is the effect of additional air heating that decreases manifold air density.

 

THat's why the desired point of speed shifting is the point where two power curves for low and high blower speed intersect.

[Nybble]

I was hoping you would show up here Yo-Yo

 

I think it's a common mistake both in Russian and English community naming SPEED shifting as STAGE shifting.

 

That's cleared it up for me at least, it was the one function I didn't fully understand. Thanks for that!

[Buznee]

I think it's a common mistake both in Russian and English community naming SPEED shifting as STAGE shifting. The point is that blower rotor is switched to higher or lower speed using a gearbox and a clutch almost as in a car.

THen, the term "critical altitude" that is the same as "full throttle altitude " is a term for the altide where the blower begins to be unable to produce necessary MP.

Critical altitude depends on the MP that is used as a reference and on the engine rpm. The higher is rpm for the same MP the higher is the crytical altitude. The higher is MP for the same rpm the lower it will be.

 

 

And answering the question - how could it be that after switching to the second speed the power remains the same - the higher speed compresses the air more but by the cost of increased power required for the blower. The second effect is the effect of additional air heating that decreases manifold air density.

 

THat's why the desired point of speed shifting is the point where two power curves for low and high blower speed intersect.

 

Thanks so much Yo-Yo! I was thinking about it a bit more after I posted and remembered that the supercharger has to be working harder. The blower efficiency also is a variable I would imagine. Heating and larger compression ratio must play into that.

 

Yo-Yo would you be able to address the operational concern of how you go about effectively transitioning between low and high when in the auto setting? Do you let the engine go to high manifold pressures when it switches since the throttle is all the way forward? Any adjustments on the prop before during or after the transition? Or do you lower the power, manually switch, trim it out and switch back to automatic? I'm trying to understand how real world pilots go through this transition.

 

Maybe it doesn't matter because the blower is designed to switch where the horsepower remains the same at full throttle? So even though the MP goes way up the engine is not producing any more power so it's actually a smooth transition?

Edited December 28, 2013 by Buznee

[BitMaster]

Hey,

 

there are dozens of youtube videos showing you the original US-Army training videos how that thing wants to be treated... for most AC used in WWII.

 

If it doesn't behave like it should, complain !

 

I got my "how to ramp start" the P-51 from a youtube video from the 1940`s... not a DCS tutorial

 

Bit

[Buznee]

Link please? I've had no luck finding real world p-51 training videos or videos regarding supercharger ops or transition