Correct use of throttle and RPM control

[bongodriver]

Check tests of airplanes u are talking about. At lower RPM you cannot go faster than at higher RPM. In both Spit and P51 you need 3000rpm to gain max speed. All best performances for the Merlin engine are at 3000.

 

If DCS is correct. Our engines quit after a bit of abuse while we have IRL reports of way longer abuse and just engine damage not loss. For example I've read that R2800 was tested on WEP 2300hp for 24h (constant operation). Engine was sitting on the ground. Test concluded engine was damaged but it didn't quit.

 

Sure we don't have a radial, but I am pretty sure those engines should start running rough and loose power and not just die 100% of time.

 

I agree entirely there, stuff still needs work here without doubt, I'm still amazed at how the 109 seems to run like factory fresh despite poring black smoke that indicates it is losing all it's oil yet a single puff of white will kill the Spitfire.

[some1]

First of all....mate....my opinion is based on the physics I experience in the real world, yours seems to be based on the sim, now we need to establish if we have found a bug.

 

Nope. The analogy with manual transmission with a fixed number of gears is completely wrong. You don't have to "shift gears" by pulling the RPM lever back. The prop governor automatically changes prop pitch as you go faster maintaining the optimal conditions, which, as I said, is more akin to CVT transmission in a car.

 

But like in a car, to achieve top performance, you need to use the max power output from the engine, which is at peak power RPM. That's not always in the highest gear, when your RPM is lower, even if the torque is higher. The difference is that in a car with a few fixed gears, you have to change engine RPM to change speed, while in the air with constant-speed prop, you can just set the RPM to optimum and forget about it.

 

Here's an example Merlin power graph (first link from google). As you can see, any setting below 3000RPM gives you less horsepower than the same setting at full RPM:

 

 

In the German airplanes with Kommandogerät you don't even have a choice, throttle full forward = max designed RPM. In some western airplanes there was also a mechanical link that prevented the pilot from moving throttle forward without moving the prop RPM, so he couldn't set full power without full RPM as well.

 

Of course flying at max power setting all the time is the fastest way to kill your engine, and that's what Yo-Yo said.

Edited January 5, 2017 by some1

[ViFF]

Bongodriver, take a look again at what Yo-Yo said:

 

the worst idea is to use 3000 +18 at slow turns fighting. :-)

 

In other words, it is a very bad idea to be in this power regime (3000 +18 ) while in a flight regime that does not provide good airflow for cooling system (slow turns fighting)

 

He never said that 3000 +18 does not provide the greatest output from the powerplant.

 

S!

[bongodriver]

Nope. The analogy with manual transmission with a fixed number of gears is completely wrong. You don't have to "shift gears" by pulling the RPM lever back. The prop governor automatically changes prop pitch as you go faster maintaining the optimal conditions, which, as I said, is more akin to CVT transmission in a car.

 

But like in a car, to achieve top performance, you need to use the max power output from the engine, which is at peak power RPM. That's not always in the highest gear, when your RPM is lower, even if the torque is higher. The difference is that in a car with a few fixed gears, you have to change engine RPM to change speed, while in the air with constant-speed prop, you can just set the RPM to optimum and forget about it.

 

Here's an example Merlin power graph (first link from google). As you can see, any setting below 3000RPM gives you less horsepower than the same setting at full RPM:

 

But like in a car, to achieve top performance, you need to use the max power output from the engine, which is at peak power RPM. That's not always in the highest gear, when your RPM is lower, even if the torque is higher. The difference is that in a car with a few fixed gears, you have to change engine RPM to change speed, while in the air with constant-speed prop, you can just set the RPM to optimum and forget about it.

 

Here's an example Merlin power graph (first link from google). As you can see, any setting below 3000RPM gives you less horsepower than the same setting at full RPM:

 

 

In the German airplanes with Kommandogerät you don't even have a choice, throttle full forward = max designed RPM. In some western airplanes there was also a mechanical link that prevented the pilot from moving throttle forward without moving the prop RPM, so he couldn't set full power without full RPM as well.

 

Of course flying at max power setting all the time is the fastest way to kill your engine, and that's what Yo-Yo said.

 

I didn't say you shift RPM like gears, I said the way power is transferred is like transitioning through gears and that is true and no different to automatic or manual transmissions, mechanically a governor is doing something like a CVT but the similarity ends there, the whole CV prop system is more akin to a manual transmission because the operator selects the RPM, if you like we can compromise and agree that selecting RPM is more like switching to a separate CVT transmission instead of a gear.

So why have a selectable RPM? why not just have a governor that keeps the engine constantly at 3000 RPM?

You can drive your car at 40mph in 1st gear or you can drive your car at 40mph in 3rd, which would you choose? that is the point if you want to optimise things you need to change gear, look at your graphs again and see for yourself that the difference in horsepower between 2850 rpm and 3000 rpm is minimal and certainly puts less strain on the system, you really don't need to be at max power output to reach max speed if you can optimise the propellers efficiency, it's no coincidence that the book climb RPM is 2850.

German stuff is a different kettle of fish as we all know they summoned magic from Satanic demons so god only knows what we will compare it to...:megalol:

[bongodriver]

Bongodriver, take a look again at what Yo-Yo said:

 

 

 

In other words, it is a very bad idea to be in this power regime (3000 +18 ) while in a flight regime that does not provide good airflow for cooling system (slow turns fighting)

 

He never said that 3000 +18 does not provide the greatest output from the powerplant.

 

S!

 

it's a bad idea to fly at 3000 and +18 boost beyond 5 minutes regardless of the speed, if you become reliant on these power setting then the enemy won't need to worry for so long.

 

let's agree that everyone is right and just have a different way of seeing things.

[AOG]

Your both correct and if you allow me to put my lecturer hat on I'll explain why. Let's start by looking at how a constant speed prop works works on a modern aircraft and then I'll talk about what the Merlin brings into the mix. Here's what's happening. When you pull back on the rpm lever the prop governor increases the pitch of the blades until the set RPM is reached. Increasing the pitch of the blades increases the AOA and the lift (thrust) produced by the prop. Since you don't get something for nothing it also means that the torque required to spin the prop goes up and the engine has to work harder. From an indication perspective, on a modern engine when you pull the prop back you will see the MAP rise because the engine is working harder. If your not careful you can overload the engine. That's why pilots are taught that after takeoff to reduce the throttle first and then pull back the prop. Reduceing the MAP first gives you the headroom you need to adjust the prop without blowing up the engine. The key relationship to remember is that if you hold the prop RPM steady the higher the MAP the greater the AOA on the prop and the more thrust it generates. Now let's talk about what the Merlin brings to the mix. In addition to the prop governer, the Merlin adds a MAP regulator. The MAP regulator will open and close the throttle to hold the selected MAP setting. What that means is that when you pull back the prop the MAP regulator will automatically reduce the throttle to hold the selected MAP setting. When we throw that into the relationship we talked about earlier it means that the prop governer will end up with a finer pitch for the same prop setting then it would have on a modern aircraft. That also explains why on a Merlin you get peak power at peak rpm. I hope this helps

 

AOG

[Schwarzfeld]

... its a really good idea for all of you discussing this topic (if you have not already, and are not a licensed pilot already) to go spend the money to log at least one flight hour with an instructor in a fixed pitch prop aircraft, then do the same for a continuous pitch/manually adjustable pitch prop aircraft (preferably one with forced induction if you can afford it) to experience it for yourself first-hand.

 

Flying a fixed pitch prop (what we all learn in starting out) we find that our particular prop just has a sweet spot where it works 'just right' or everything just jives. Outside of that RPM range, its jittery, unhappy, lossy in power, or tends to get out of hand quickly, and we feel like we don't have good 'grip' with the throttle during a descent, say.

 

So, do yourselves a favor and go try it in a real aircraft, it will vastly improve your flying in DCS across the board, my two cents.

[Zimmerdylan]

Check tests of airplanes u are talking about. At lower RPM you cannot go faster than at higher RPM. In both Spit and P51 you need 3000rpm to gain max speed. All best performances for the Merlin engine are at 3000.

 

If DCS is correct. Our engines quit after a bit of abuse while we have IRL reports of way longer abuse and just engine damage not loss. For example I've read that R2800 was tested on WEP 2300hp for 24h (constant operation). Engine was sitting on the ground. Test concluded engine was damaged but it didn't quit.

 

Sure we don't have a radial, but I am pretty sure those engines should start running rough and loose power and not just die 100% of time.

 

That's my point in my post. It seems that in the DCS Spitfire, even minimal overworking results in a catastrophic failure. And although running the engine at lower speeds and higher RPM or whatever isn't good for it and will lead to failure (According to Chuck's guide we're looking at minutes, not seconds), in DCS it's almost instant catastrophic punishment.

I am not going to argue physics or the power of the Merlin engine, or it's failures. But common sense tells me that if this were actually an issue for the Spitfire in reality, it would be documented somewhere, or the pilots in documentaries would point it out, or even the knowledge that the Germans exploited such a big weakness. But I have searched and found none of this. Plus, if it were that bad that you could only climb for a few seconds before your engine started dying, there would have been a fix pretty quickly as in war, you cannot afford such a weakness. So although physics plays the biggest part of all of this, common sense plays just as big a part in figuring out the limitations of this plane.

[some1]

bongodriver, I was referring to your explanation here:

 

simple way to put it is that max prop RPM is like being in 1st gear, you get plenty of torque and pulling power but wont go very fast and lowering the RPM is like shifting up the gears.

using 3000 RPM in combat would be a terrible idea, much better to use Max climb settings and don't use maximum boost for too long.

 

which is completely incorrect. Of course reducing RPM when the maximum power is not needed is a preferable way to reduce engine wear, heat generation, fuel consumption and noise, that's why the lever was put there in the first place and that's why a pilot should reduce RPM for climb and cruise. But first: you don't reduce the RPM to go faster - it's not like shifting up the gear in a car (ok, selecting highest overdrive gear is a closer analogy). And second - 3000RPM in combat is not a terrible idea - it's the right way to do. Actually, putting high boost without increasing RPM puts more strain on the engine and forces it to work outside its designed envelope. Just look at the diagram I posted, the highest boost settings are allowed only with 3000RPM. So if you are in combat and you have a lower RPM setting than 3000 RPM then you either:

- don't use the full potential of your plane

- have to remember to increase RPM before trying to increase power

- risk more damage to the engine if you forget to increase RPM and move the throttle fully forward in the heat of battle.

 

German stuff is a different kettle of fish as we all know they summoned magic from Satanic demons so god only knows what we will compare it to...

 

German stuff adheres to the same laws of physics and it does the same things that English engineers wrote in the Spitfire manuals - just automatically.

[firmek]

...

Just look at the diagram I posted, the highest boost settings are allowed only with 3000RPM. So if you are in combat and you have a lower RPM setting than 3000 RPM then you either:

- don't use the full potential of your plane

- have to remember to increase RPM before trying to increase power

- risk more damage to the engine if you forget to increase RPM and move the throttle fully forward in the heat of battle.

...

Looking on the diagrams, does it mean that:

- boost < +7, any RPM setting is allowed

- boost >= +7 and <= +9, only when RPM >= 2850

- boost > +9, only when RPM is 3000

?

[some1]

Yes, but that's for different Merlin version than in DCS Spitfire, so the values are probably different.

Of course that doesn't mean the engine will seize immediately the moment you put too much boost for a given RPM.

[bongodriver]

bongodriver, I was referring to your explanation here:

 

 

 

which is completely incorrect. Of course reducing RPM when the maximum power is not needed is a preferable way to reduce engine wear, heat generation, fuel consumption and noise, that's why the lever was put there in the first place and that's why a pilot should reduce RPM for climb and cruise. But first: you don't reduce the RPM to go faster - it's not like shifting up the gear in a car (ok, selecting highest overdrive gear is a closer analogy). And second - 3000RPM in combat is not a terrible idea - it's the right way to do. Actually, putting high boost without increasing RPM puts more strain on the engine and forces it to work outside its designed envelope. Just look at the diagram I posted, the highest boost settings are allowed only with 3000RPM. So if you are in combat and you have a lower RPM setting than 3000 RPM then you either:

- don't use the full potential of your plane

- have to remember to increase RPM before trying to increase power

- risk more damage to the engine if you forget to increase RPM and move the throttle fully forward in the heat of battle.

 

 

 

German stuff adheres to the same laws of physics and it does the same things that English engineers wrote in the Spitfire manuals - just automatically.

 

No, not completely incorrect at all, maybe different to the way you see things but not incorrect.

 

I didn't say you reduce RPM to go faster, I said you reduce RPM to fly more efficiently as you go faster, I already mentioned doing 40mph in both 1st and 3rd gear, which do you think is better for the engine? either way you are doing 40mph but at some point 1st gear wont allow you to go any faster, same with a prop, at 3000 RPM you will hit a wall eventually, that's why you use max RPM to slow down.

We are getting into a pointless argument over minutiae here.

 

p.s. you aren't German are you? because the last bit was really just a joke, no need to explain the same laws of physics apply.

[firmek]

Yes, but that's for different Merlin version than in DCS Spitfire, so the values are probably different.

Of course that doesn't mean the engine will seize immediately the moment you put too much boost for a given RPM.

Thanks for the clarification. I didn't notice that those are from a different engine version. It's clear that those would be a recommended and not following them exactly wouldn't kill the engine immediately :smilewink:

[some1]

either way you are doing 40mph but at some point 1st gear wont allow you to go any faster, same with a prop, at 3000 RPM you will hit a wall eventually, that's why you use max RPM to slow down.

 

Here is a flaw in your reasoning. While it is true that finer pitch (higher prop RPM) produces more drag, this is more than offset by the engine capability to produce more power at higher RPM. The whole problem of choosing the right propeller size, shape, pitch and optimum RPM for a given aircraft is complex and is a headache for the aircraft designers - not the pilot who later flies it. The correct propeller profile is chosen to not hinder performance at high power output, especially in a high performance airplane like Spitfire. But i haven't seen any aircraft in which a pilot has to reduce engine RPM to achieve top speed.

 

Cruising is a different story, but then you don't need all the power the engine can produce and don't need the high RPM. Your priorities are fuel consumption and engine longevity.

 

Also, it's not a common practice in real life to increase engine RPM in order to slow the aircraft down. The reason why you're instructed to set RPM lever to highest RPM during landing is not to increase drag, but to prepare it in case of go-around when you will need full power again. At landing speeds and power, setting 3000 RPM in a Spitfire doesn't make any difference, the engine would barely turn at 2500 RPM anyway.

[bongodriver]

Here is a flaw in your reasoning. While it is true that finer pitch (higher prop RPM) produces more drag, this is more than offset by the engine capability to produce more power at higher RPM. The whole problem of choosing the right propeller size, shape, pitch and optimum RPM for a given aircraft is complex and is a headache for the aircraft designers - not the pilot who later flies it. The correct propeller profile is chosen to not hinder performance at high power output, especially in a high performance airplane like Spitfire. But i haven't seen any aircraft in which a pilot has to reduce engine RPM to achieve top speed.

 

Cruising is a different story, but then you don't need all the power the engine can produce and don't need the high RPM. Your priorities are fuel consumption and engine longevity.

 

Also, it's not a common practice in real life to increase engine RPM in order to slow the aircraft down. The reason why you're instructed to set RPM lever to highest RPM during landing is not to increase drag, but to prepare it in case of go-around when you will need full power again. At landing speeds and power, setting 3000 RPM in a Spitfire doesn't make any difference, the engine would barely turn at 2500 RPM anyway.

 

Minutiae again, I grant my statement that max level speed is not achievable at max RPM is wrong but I had myself confused with the thinking of continuous power settings for long term performance as opposed to short 5 minute bursts, if you were in the level cruise and changed only the RPM to max then the aircraft will slow down.

I feel you are on a quest to argue with me and I just can't be bothered for much longer, you are attributing statements to me that I simply have not made, check my post #17 on page 2 and just agree that we are on the same page for the most part.

the key point being that a CV prop system is remotely similar to gearing in a car (automatic or manual) I was only trying to help out the original poster of this thread to give a basic idea.

furthermore an RPM of 3000 at +18 boost is not combat effectiveness but more like combat desperation, it just seems to me much more effective to manage the aircraft better and use your smarts and use max setting to get you out of trouble.

[Schwarzfeld]

.... the whole purpose behind the obscene amount of supercharger boost available in these WWII piston props had less do with the ability to turn & burn (few could, the Spit was one that technically 'could') than it did with the boost being there to allow them to fly at altitude and therefore accompany bomber formations, and also gain altitude with which to dive on an opponent.

 

You'll notice any of these aircraft can safely be taken off of even a short field without the use of anything near full throttle.

 

If you've any experience building sports car motors, you understand the concept of minimum RPM for boost or nitrous/water-meth injection; the motor needs to be moving a sufficient volume X of air already at ambient pressure, and getting it combusted and outta the way before you start feeding it additional atmospheres, or in the case of nitrous, simulated additional atmospheres, but effectively the same thing.

 

Prop pitch is indeed maxed out for landing per these planes' flight manuals for that reason mentioned above, go-around, its to set you up for faster throttle response.

 

It is absolutely not common practice to decelerate the aircraft on it's prop (to my knowledge, in my world, anyway), thats like engine-braking in a sports car - its really not a good habit unless the engine is expendable.

 

I personally use that mod I linked above and those ranges marked on the gauges appear to correlate to the Spit's specified engine RPM and boost limitations in general, and I've never experienced any engine failures at all in DCS.

 

You guys both sound like you could use some practice controlling the airspeed of the plane using the stick, not the throttle. Pull up to decrease speed, push down the nose to increase speed, and use the throttle to control your climb or descent angle. This applies less and less the higher the power to weight ratio of the acft in particular is, but the fundamental holds true regardless.

 

Some1 is correct, reducing engine RPM by varying the prop pitch is not a top speed aid; that just throws a ton more stress on the motor and it gets hotter. You should be able to hit max speed at full prop pitch (as you set at take off and landing) by running the throttle up to max continuous RPM - though you will reach it more rapidly if you throw boost up to max climb or max T/O.

 

Either way, this argument is really kind of pointless and glosses over some really core fundamentals of flying, in my opinion.

[bongodriver]

Personally I believe the problem is people getting too hung up on the comparison to cars an misinterpreting things, you can slow down on the prop without any problems (it's not a car) and no it's not the primary means of slowing down but it sure does help with all that drag it produces.

 

been flying in real life for over 20 years.

[ViFF]

 

Sorry to digress, but I have a question about the attached graph:

 

Is there a reference in the pilot's handbook regarding what is the minimum RPM that should be used at the higher boost settings?

 

From what I can see in the graph I have a hunch its hinting that from +7 to +9 boost the RPM should be at least 2850, whereas from +9 and upwards you should be a max RPM (3000)

 

Is there a reference for this in the pilot's operating manual?

 

 

S!

[bongodriver]

ENGINE CONTROLS 19.Throttle.—The throttle lever (33) is gated at the climbing boost position. There is a friction adjuster (31) on the side of the quadrant. The mixture control is automatic and there is no pilot's control lever. 20. Propeller control (i) On early aircraft the speed control lever (35) on the inboard side of the throttle quadrant varies the governed r.p.m, from 3,000 down to 1,800. (ii) On later aircraft the propeller speed control is interconnected with the throttle control. The inter-connection is effected by a lever, similar to the normal speed control lever, which is known as the override lever. When this is pulled back to the stop in the quadrant (the AUTOMATIC position) the r.p.m, are controlled by the positioning of the throttle lever. When pushed fully forward to the MAX. R.P.M, position it overrides the interconnection device and r.p.m, are then governed at approximately 3,000. The override lever can be used in the same way as the conventional propeller speed control lever to enable the pilot to select higher r.p.m, than those given by the interconnection. It must be remembered that the interconnection is effected only when the override lever is pulled back to the stop in the quadrant; indiscriminate use of the lever in any position forward of this stop will increase fuel consumption considerably. At low altitudes (and at altitudes just above that at which high gear is automatically engaged) the corresponding r.p.m, for a given boost with the override lever set to AUTOMATIC are as follows:

 

Boost (lb/.sq.in.)R.P.M.

Below +3 1,800-1,850

At +7.. 2,270-2,370

At +12 (at the gate) .... 2,800-2,900

At +18 (throttle fully open) .... 3,000-3,050

(iii) A friction damping control (46) is fitted on the inboard side of the throttle quadrant.

[Schwarzfeld]

Personally I believe the problem is people getting too hung up on the comparison to cars an misinterpreting things, you can slow down on the prop without any problems (it's not a car) and no it's not the primary means of slowing down but it sure does help with all that drag it produces.

 

been flying in real life for over 20 years.

 

I'll likely never have as many flight hrs as you do, but slowing on the prop is a pilot's-call-thing and/or owner of the plane's-call thing as far as I'd hazard to guess. I've got more time building sports cars than working on planes, and while engine braking a car is a bit different than prop pitch braking in a plane, if I owned that plane, I'd not want that done on principle unless it were a life or death emergency. I also wouldn't let someone engine brake my built-motor sports car, because thats what brakes are for.

 

If its your machine, you do what you want, and everyone flies/drives differently, sims are great for that too. As far as people whining about how the DCS Merlin (Spit edition) engine is somehow too fragile vs. the P-51D's model of Merlin as seen in DCS, I think we can all agree (I hope) that its failure to operate the engine within its specified limits at speed/altitude per flight manuals...

 

... arguing over what, how and why to use prop pitch is like arguing over how specifically to land plane X at airfield Y in crosswind Z - everyone is gonna do it their own way - its like sex, it works best for everyone in their own way.

[bongodriver]

I don't know why the argument started other than someone really took offence at a comparison to a car gearbox (which is perfectly valid), all I did was try to help out a poster who clearly hasn't got a grasp on the subject yet and listening to a bunch of experts probably has provided as much help as if he had asked for sex tips.

I'd assume engine braking is also a drivers call thing too and airbrakes are not so common on the aircraft we are dealing with so slowing on the prop is kind of unavoidable to an extent.

 

I think the Spits merlin is modelled a bit too fragile but this is still beta and may well get changed, I have read in some Spitfire tests that even the 5 minute limit was allowed to be exceeded up to 15 minutes as a further emergency limit which is interesting.