Question about relationship between RPM and Throttle settings

[DaGengster]

Im a somewhat confused on what the RPM and throttle setting relationships are used for in terms their actual effect on flight. I looked for guides and stuff but none really explained this clearly

 

For example on the Yak 1, I read that the plane will automatically adjust the rotor pitch in order to keep the RPM setting. However I dont really understand what effect this has on the plane. How does the plane determine what pitch to use? Does high throttle and low RPM produce a low pitch? I cant imagine a situation where a high throttle and lower RPM will benefit anything. Though I guess a low throttle and high RPM might be good for fuel saving in high attitudes.

 

When flying in normal mode, it seems moving the throttle also rotates the dial for the RPM in a 1 to 1 fashion so is it ok to simply map both of these to the same joystick axis?+

 

TL:DR - what maneuvers would different RPM and Throttle settings be appropriate for? Like High RPM with Low Throttle etc

Edited December 18, 2017 by DaGengster

[=TBAS=Sshadow14]

youtube is your friend :D

 

[DaGengster]

youtube is your friend :D

 

 

 

Thanks for the reply.

I understand the "gearing" of the pitch but how does the plane, for example yak 1, determine what pitch to use? On planes that allow you to manually adjust pitch, its pretty simple, but how do you tell the plane to adjust the pitch when theres no option for it?

He mentioned for take off to increase RPM before throttle, so high RPM and low throttle makes the pitch low. So are these 2 settings the only things the plane look for in determining the pitch?

Therefore is the entire point of having a separate lever and knob so that they can be placed at different settings to adjust the pitch?

 

Idk, it feels like the German method of having the pitch and throttle be manually adjusted and the RPM simply as a dependent variable seems more straight forward (as opposed to the pitch being the dependent variable based on RPM)

Edited December 18, 2017 by DaGengster

[ZachariasX]

You want a piston engine to run constantly at a certain, optimal rpm for any desired power regime. Unlike electric motirs, combustion engines are rather inefficient beyond their optimal rpm. (They have gotten better in this regard, but still.)

 

You essentially do not care for the pitch of the prop in automatic setting. This pitch is just a function of your set rpm and airspeed, same as the gear in your automatic car is a funtion of your speed and depression of the throttle pedal. Low airspeed makes the „pitch low“ as well.

 

The idea is to not only run the motor at optimal rpm, but also having the prop run at an efficient angle of attack. At low rpm your propeller blades are more airbrakes than props and you see that by having a much longer take off run (if you make it off the ground at all). Having a manual prop setting like you suggested is only useful for take off conditions, where airspeed is very low and you want a fine angle („max. rpm“) on the prop. But as soon as you accellerate, the setting will become increasingly inefficient.

 

This can be seein in the Spitfire Mk.I (fixed prop and manual prop) and the Mk.II (automatic prop). Having automatic prop increased the planes performance masurably, despite the aircraft becoming heavier.

 

There are stories about pilots prefering manual prop pitch, deeming it giving „better accelleration“ as you can do things like immediately setting the prop to coarse and then adding full power. The gyro of the prop will then give you a bit added momentum to make „a leap forward“, but even with nonsense like this on average you will not be able to draw the same power from the engine as you could have in automatic.

[DaGengster]

Thanks for the explanation ZachariasX.

[Guest deleted@50488]

Just to add to the excellent posts above, I'd say that sometimes in the Axis aircraft that have an even more sophisticated system where the pilot just has to set the throttle, and the prop RPM is automatically set to an "optimum" RPM, varying the blade angle automatically...

 

The question is that sometimes I actually switch off the automation in-flight and use the manual prop pitch control, which is not CS ( Constant Speed ) but rather variable prop pitch. I think I can get better control under some circumstances, like when diving to run away from the guy on my 6.... ( if I can't climb... )

[Tomsk]

Just to add to the excellent posts above, I'd say that sometimes in the Axis aircraft that have an even more sophisticated system where the pilot just has to set the throttle, and the prop RPM is automatically set to an "optimum" RPM, varying the blade angle automatically...

 

Indeed and manual for the DCS FW 190 D-9 says that it was actually standard to set the engine power on that aircraft by RPM rather than ata. Not certain if this was the case for earlier 190's also, but I certainly use RPM rather than ata because RPM doesn't vary by altitude whereas ata can. The technochat agrees, so in the A3 for example "maximum continuous" power is always 2300 RPM, and "full combat" (but not emergency) power is always 2400 RPM .. however the ata for those settings varies a bit depending on altitude and supercharger gear.

Edited December 18, 2017 by Tomsk

[kalbuth]

 

 

[...]

Does high throttle and low RPM produce a low pitch? I cant imagine a situation where a high throttle and lower RPM will benefit anything.

 

[...]

 

TL:DR - what maneuvers would different RPM and Throttle settings be appropriate for? Like High RPM with Low Throttle etc

I sometimes temporarilly use High Throttle & Low RPM when diving from low speed. Low RPM to avoid overrevving the engine (the actual RPM will jump anyway because the plane gains speed from diving) while the high throttle will keep pushing the plane, at least at the beginning of the dive when speed is still rather low. Not sure it really makes sense or if it is actually usefull, it simply seems logical to me to do so.

 

EDIT : the most important is to reduce the RPM when diving, imho, as often the RPM go to the roof far beyond safe levels. Pushing the throttle in the hope to accelerate a bit faster if probably wishfull thinking

Edited December 18, 2017 by kalbuth

[216th_Jordan]

Throttle gives the engine power, rpm is set by the rpm lever. Power can very roughly be regarded as P = w * M. ( Power = omega * momentum). Throttle basically sets the momentum. (it will differ on a petrol engine for different rpms, but this is not important right now) RPM sets w, sonin theory you get higher power by higher rpm.

 

As there arenother factors as most efficient engine rpm and prop efficiency this can and will vary. Just taking off a plane with low rpm is certainly not a very good idea as you can see.

Edited December 18, 2017 by 216th_Jordan

[ZachariasX]

Here, you go:

 

[CIA_Yankee_]

I have a question concerning constant-speed props:

 

If I'm coming in to land and want to slow down, I would drop my throttle to 0 so the engine idles... but what would be the optimal RPM setting to ensure the fastest speed loss? And most importantly: why?

[=362nd_FS=RoflSeal]

I have a question concerning constant-speed props:

 

If I'm coming in to land and want to slow down, I would drop my throttle to 0 so the engine idles... but what would be the optimal RPM setting to ensure the fastest speed loss? And most importantly: why?

At idle throttle and 100% RPM, prop pitch becomes fine which induces the most drag.

[CIA_Yankee_]

At idle throttle and 100% RPM, prop pitch becomes fine which induces the most drag.

 

That's what I figured... though where I'm a bit hazy is why does pitch becomes fine? Is it because by selecting max RPM then the governor will seek the pitch that offers the least rotational drag and spin the fastest, and thus will be the finest it can be (though, if I were to go in a dive and speed up with the throttle at 0, the pitch would become coarser to maintain the RPM set (which is actually the fastest SAFE RPM, to avoids the engine overrevving))?

 

And I assume the greater drag is caused by a finer prop pitch being "flatter" against the incoming airflow?

[Barnacles]

That's what I figured... though where I'm a bit hazy is why does pitch becomes fine? Is it because by selecting max RPM then the governor will seek the pitch that offers the least rotational drag and spin the fastest, and thus will be the finest it can be (though, if I were to go in a dive and speed up with the throttle at 0, the pitch would become coarser to maintain the RPM set (which is actually the fastest SAFE RPM, to avoids the engine overrevving))?

 

And I assume the greater drag is caused by a finer prop pitch being "flatter" against the incoming airflow?

A governer will simply make the pitch finer when rpm is lower than that set and vice verca. If there is no engine power and max rpm is set it will continue adjusting until either the pitch is fully fine or max rpm is achieved, because of airflow.

[ZachariasX]

At idle throttle and 100% RPM, prop pitch becomes fine which induces the most drag.

 

You can ignore that. When flying the pattern, you use a higher rpm to enable you to quickly use power should you need it. Besides, you're not piloting a glider. Even in descent, you will use substancial power to fly your approach. Enough to keep the prop from windmilling. You will not have your prop as airbrake, unless you fly a pattern like a pig.

 

ONLY if your engine quits and you're REALLY piloting a glider, prop drag is an issue and you might want to feather the prop to maybe make it to the runway. But on single engine planes it less relevant than on twin/multi engine planes, where the engine drag veer you off course and make you hit the farm.

[busdriver]

At idle throttle and 100% RPM, prop pitch becomes fine which induces the most drag.

 

Just a heads up...the notion of 100% RPM is an artifact of the game's technochat. IRL you wouldn't find percent rpm, you'd look at rpm measured in hundreds e.g. 2400 or 3000 rpm. Generally speaking you'd move the prop control to "fine" pitch (if you think like a Brit) or "high" rpm (if you think like a colonial) prior to landing to allow for a quicker go around. IRL some folks do this upon entering the traffic pattern, some wait until short final. The CAF instructor pilots I know that fly the BT-13 and AT-6 teach engine out gliding with the prop at fine pitch/high rpm, IF it looks like you will undershoot the landing ground THEN select coarse pitch/low rpm to reduce the drag of the prop.

 

I'm not terribly familiar with the prop governors on WWII fighters. But modern single engine airplanes don't have to capability to feather the prop. So if the governor fails the prop goes to fine pitch/high rpm. Again to permit a go around if required. There is an exception, single engine airplanes with an "aerobatic" prop. With an aerobatic prop if the governor fails the prop goes to a coarse pitch/low rpm state to prevent an overspeed (the worst case being the governor failure happens whilst diving to perform a loop or merely regain energy). Airplanes with an aerobatic prop have sufficient power to go around even with the prop at coarse pitch/low rpm.

 

The corollary is that with a prop that can feather, a failure of the prop governor will drive the prop to a coarse pitch/low rpm (the reverse of the non-feathering prop).  

 

Perhaps DakPilot can share his insight.

[Lensman]

Indeed and manual for the DCS FW 190 D-9 says that it was actually standard to set the engine power on that aircraft by RPM rather than ata.

 

However Erich Brunotte claims that he only used the ATA gauge when piloting the fw 190:.

 

https://www.youtube.com/watch?v=pvKs9VLUcCg

[ZachariasX]

 

 

The CAF instructor pilots I know that fly the BT-13 and AT-6 teach engine out gliding with the prop at fine pitch/high rpm, IF it looks like you will undershoot the landing ground THEN select coarse pitch/low rpm to reduce the drag of the prop.

 

Wow. Getting creative in dire moments. Then again, operating old engines make one familiar with procedures unheard for the common ones.

[Warpig]

Wow. Getting creative in dire moments. Then again, operating old engines make one familiar with procedures unheard for the common ones.

I personally like to switch to manual prop pitch for the same reasons when landing. A prop on 100% fine pitch makes a great air brake.

[busdriver]

Wow. Getting creative in dire moments. Then again, operating old engines make one familiar with procedures unheard for the common ones.

 

I reckon so, I've not bothered to check their flight manuals' for the WWII method. I don't fly those airplanes. But at our annual Single Engine Safety day the Chief AT-6 IP was challenged by another AT-6 pilot about this technique. The IP replied, "it's how we teach, it's up to you to decide how to fly the airplane." These airplanes don't typically get above 10,000' (+3000 meters), usually below 1000 meters. So putting the prop control to fine pitch/high rpm is predicated on trying to get the motor started. Having to try and stretch the glide would be a scary proposition.

 

Interestingly enough accident reports show (at least in the US), single engine pilots dealing with an engine out generally overshoot their desired touchdown point (land long) and often with excess speed. I have a friend that experienced a blown oil line on his N3N in July 2016, he was long and fast.

[Dakpilot]

I am mostly familiar with Hamilton standard Props, generally if you have governor failure (most likely from low/lack of oil pressure in the prop system) this will lead to complete lack of control of the prop, which will end up at the low(fine) pitch stops around 16-18 degrees.

Depending on phase of flight this can (does) lead to a 'runaway prop' with often (very) catastrophic prop failure, generally you would feather the prop (and cut the engine, quickly) which used separate 'feathering' oil reservoir +- 2 gallon,  but some early types relied on engine oil pressure, so with engine failure you could get into trouble very rapidly.

 

Some later systems have autofeather which detects governor issues and goes direct to feather in event of failure, many (modern) single engine aircraft have prop system that will err to high(coarse) pitch stops (by counterweights/springs) in event of failure, electrically controlled props have different systems again

 

There are so many different types of propellor systems (and manufacturers) that it is hard to generalise (and they can vary with multi and single engine applications) without being called on it, so out with those manuals people!    

 

On approach (standard op procedure) you would want to be fully fine pitch, this gives best control, more drag to slow, and most response for acceleration in case of go-around or miscalculation

 

Cheers,  Dakpilot

[CIA_Yankee_]

Awesome answers, everyone, thank you.

 

As for why I asked, well I was just wondering if there was a way to use RPM as a brake. Obviously if I really needed to lose speed rapidly I'd rely on slips, but I was just in the "ok, I need to lose speed, what's the most logical RPM setting" mindspace. Naturally, I wouldn't rely on just the prop itself to really slow me down.

 

Anyway, thank you for the answers.

[Warpig]

Awesome answers, everyone, thank you.

 

As for why I asked, well I was just wondering if there was a way to use RPM as a brake. Obviously if I really needed to lose speed rapidly I'd rely on slips, but I was just in the "ok, I need to lose speed, what's the most logical RPM setting" mindspace. Naturally, I wouldn't rely on just the prop itself to really slow me down.

 

Anyway, thank you for the answers.

If you wanted to get real crafty, you could open the radiator vents to help slow you down as well.

[Tommy_Atkins]

Being a simple fella that I am it is all still a little confusing. I have never messed around with the RPM and only play with the throttle. What are the best times to actually manage them well?

[Guest deleted@134347]

2 hours ago, Tommy_Atkins said:

Being a simple fella that I am it is all still a little confusing. I have never messed around with the RPM and only play with the throttle. What are the best times to actually manage them well?

depends on the aircraft.

 

- German/Axis planes typically have a fully automatic prop governor, however the automation can be disabled and you can control the prop pitch manually. The german ac's allowing for that are bf-109's and  fw190 Antons (D9 dora doesn't have such feature). Typically you won't use the manual mode unless the engine is degraded and you want to keep a certain RPM at a certain manifold pressure (like 2,000 rpm at 1ata or something like that), i.e. you're trying to make it back to the base on a damaged engine..

 

- in Allied planes you can combine throttle and rpm levers and move them together, however, usually, you won't get the maximum power out of the engine that way, i.e. you'll either run too low RPM's or too low of manifold pressure since the rpm and manifold pressure lever positions don't get to their maximum and/or recommended combat/cruise/whatever power values when they are 'aligned'. For example in P51 the RPM goes to 3,000rpm and manifold to 61" (i think) to get the 'maximum' power, however at that rpm (and a particular high air speed) the tips of the prop are close to breaking the sound barrier creating all sorts of turbulence that slows the plane down. So, it's recommended to lower the rpm's to 2,700 while maintaining the maximum manifold pressure, and the only way to do so is to have two separate levers so you can achieve the recommended values for their respective axis. 

 

- soviet planes are similar to the allied ones. In Il2 for example, if you look at the recommended engine values you'll see that you can't achieve them by linking the throttle/rpm levers (rpm lever is actually a knob in Il2). ..

 

i hope it makes sense..

 

edit:  also, with the allied/soviet planes, when you lower the rpm's and throttle together (linked) the engine will take  much longer to spool back up to get to the target rpm's when you throttle up, which is rather dangerous during the dogfight/engagement. That's why it's a lot more efficient (and safer) to leave the rpm lever at the target setting and just use the throttle to control the engine power.

 

Edited December 23, 2022 by omicron_21

[Skycat1969]

 

1 hour ago, Tommy_Atkins said:

Being a simple fella that I am it is all still a little confusing. I have never messed around with the RPM and only play with the throttle. What are the best times to actually manage them well?

In the simplest of analogies, this is kind of like asking when to shift gears in your automobile or on your bicycle. When you're driving the car, what are your driving conditions and desired speed? Are you pulling a trailer? Will you be going up (or down) any steep hills? Think of 100% RPM like first gear -- it will get you moving but you don't want to drive down the highway with the throttle pedal mashed down to the floor and engine screaming at redline. Soon in, you'll need to upshift for overall efficiency. Decreasing (and increasing) aircraft RPM is functionally similar -- it allows the engine to make the most efficient use of power in combination with speed, altitude and throttle.

[dbuile]

For those that have not seen it, this is a gem!  

 

[Charon]

1 hour ago, Skycat1969 said:

 

In the simplest of analogies, this is kind of like asking when to shift gears in your automobile or on your bicycle. When you're driving the car, what are your driving conditions and desired speed? Are you pulling a trailer? Will you be going up (or down) any steep hills? Think of 100% RPM like first gear -- it will get you moving but you don't want to drive down the highway with the throttle pedal mashed down to the floor and engine screaming at redline. Soon in, you'll need to upshift for overall efficiency. Decreasing (and increasing) aircraft RPM is functionally similar -- it allows the engine to make the most efficient use of power in combination with speed, altitude and throttle.

 

This is misleading when describing constant speed props, which almost all WWII planes in the sim have. Put your car in 1st gear and try to hit highway speeds and you'll over-rev your engine, and the rev limiter will cut your fuel, to prevent damage. But 100% RPM is perfectly appropriate when trying to hit your top speed in a plane.

 

A constant speed prop is more like a CVT that allows you to select the sort of performance you want. You can put it in "eco mode" (0% RPM) and cruise around making only a tiny bit of power, or you can put it in "sports mode" (100% RPM), or anywhere in between. Choose RPM based on the performance you need or expect to need, and with reference to the limits in the manual.

 

It is an accurate analogy with regards to variable pitch props. Switch your prop to manual mode, very fine pitch, and try to take off, and you're likely to destroy your engine (much like you would if you floored your car in 1st gear after disabling the rev limiter).

[Skycat1969]

Good points. Yes I was thinking of specific WWII aircraft and not all aircraft. My bad.

[357th_KW]

8 hours ago, Tommy_Atkins said:

Being a simple fella that I am it is all still a little confusing. I have never messed around with the RPM and only play with the throttle. What are the best times to actually manage them well?

 

I think in general you should think in terms of more rpm = more power.  Engine limits and fuel economy require that we pull the rpm back in cruise.  So at takeoff, on landing (in case of a need to go around) and in combat you're going to want max power, and thus max rpm.  Many engines have an intermediate setting (often called climb or combat) to use when you want more, but it's not an emergency.  Outside of that, in order to preserve the engine and your fuel, pull back to the max continuous setting for your aircraft (or even further for better fuel economy).

Edited December 24, 2022 by 357th_KW

[Tommy_Atkins]

6 hours ago, 357th_KW said:

 

I think in general you should think in terms of more rpm = more power.  Engine limits and fuel economy require that we pull the rpm back in cruise.  So at takeoff, on landing (in case of a need to go around) and in combat you're going to want max power, and thus max rpm.  Many engines have an intermediate setting (often called climb or combat) to use when you want more, but it's not an emergency.  Outside of that, in order to preserve the engine and your fuel, pull back to the max continuous setting for your aircraft (or even further for better fuel economy).

 

 Thanks, that was really the answer I was looking for. I guess I should keep an eye on the oil temp as well just in case??

 

Thanks and seasons greetings. ???

[CountZero]

Also lower rpm on max trottle will give you better level speed on some altitudes, so if you run from some one or your chasing someone in strait line or shalow dives you would use best rpm for speed, what is on some airplanes for example 2550 insted 2700 or 2850 insted 3000rpm and so on... you would need to know whats good for airplane you use, this dosent help in climbs for that max throttle and RPM is best rate.

[Tommy_Atkins]

What is the default RPM setting for most aircraft, or is it different from one plane to another?

[Skycat1969]

It is not the same for all aircraft types, for many reasons including they don't all have the same motors and propellers.

 

If you use mods you might be interested in this in your virtual cockpit:

 

[BlitzPig_EL]

All somewhat different, and all differ on flight conditions.  Some are faster in level flight with a bit lower RPM, some like higher RPM.

Most Allied (except Tempest and Typhoon) like to run in the mid 2000 rpm range (2500 to 2800) or so.  Just keep flying and trying it out and you will get a feel for it.

 

As an example, one of the harder aircraft to extract performance from is the P40, owing to it's, umm, odd engine limits.  That said when I fly it using full manual management and my buddies in the BlitzPigs run it on "automatic" setting, I will run away from them, and seriously out perform them in air to air situations.

 

These are not easy things to learn, it is a simulator after all, but once you get a grasp on it, it's very rewarding from a performance standpoint, and also one of personal accomplishment as well. 

[Tommy_Atkins]

Thanks. I must admit I'm still a little confused by it all. Maybe it would be easier to ask, what have I been missing out on by NOT managing my throttle?

[Skycat1969]

I think that in SP mode RPM management is more of an academic exercise, which can be very satisfying in of itself. And for some aircraft you can get a lot more out of the aircraft if you take control of all facets of engine management and if you know what you are doing (as BlitzPig_El illustrated very well discussing the P-40, and I mentioned at the Steam forums using the P-47 as an example). 

[Dragon1-1]

In fact, in P-47 it's kind of mandatory, or you'll break the main journal bearing. Windmilling the prop in a radial engine is a bad idea. If this happens, the forces acting on the gearbox invert, and this causes the main bearing lubrication to fail. I know you can kill a P-47 that way both in Il-2 and in DCS, never had this happen with German or Soviet radials, but German ones have automatic RPM, at least. I don't know if this is not modeled in the Soviet ones, or if they used different bearings that didn't have this problem.

Edited December 25, 2022 by Dragon1-1

[Charon]

30 minutes ago, Dragon1-1 said:

In fact, in P-47 it's kind of mandatory, or you'll break the main journal bearing. Windmilling the prop in a radial engine is a bad idea. If this happens, the forces acting on the gearbox invert, and this causes the main bearing lubrication to fail. I know you can kill a P-47 that way both in Il-2 and in DCS, never had this happen with German or Soviet radials, but German ones have automatic RPM, at least. I don't know if this is not modeled in the Soviet ones, or if they used different bearings that didn't have this problem.

I don't believe it's possible to kill the jug this way in Il-2. I can't recall it ever happening to me, and I just tested (D-28) with a VNE dive from 30,000ft to 4500ft, minimum throttle and maximum RPM. I got a warning about overcooling, but no other issues.

 

And, FWIW, I suspect DCS dramatically overmodels this, and Il-2 gets it more correct. While it's a real problem, the stories I've heard suggest it's a long-term problem, not an acute problem. Allegedly, some old-time DC-6 (n.b. same engine) operators used to descend this way: "full RPM, throttles closed, and stuff the nose down to high speeds, even redline speeds. Several old-timers have told me they used to do that, with no known harmful effect, and good records of few master rod bearing failures" and it's "a standard technique for an emergency descent".

 

Further evidence comes from the P-47 manuals itself: they warn the pilot of detonation, of overspeeding the turbo, of turbo collapse, etc; they warn the pilot against over-cooling in a let-down. But windmilling the prop? I can't find any warning against this, in any version of the P-47 manual, and if it was indeed so easy to break the engine, I'd expect one. P&W's "The Aircraft Engine and It's Operation" (1947, and aimed at civilians) advises against high RPM and <20", but I don't recall any indication there that it'll break an engine in seconds, like detonation can.

 

So while it's a real problem, I don't believe DCS's decision to model it as an immediate failure is correct.

 

 

Edited December 25, 2022 by Charon

[Skycat1969]

The IL-2 P-47 used to seize up at random times for me, usually at cruise speed egressing from the target and already halfway home. At some point I noticed that the oil temps will redline quickly if I don't watch them on takeoff; and while the engine seems very robust to operating at full throttle and 2700 RPM I think high oil temperature is an easy to miss cause of engine damage. Use the oil cooler shutters.