Can someone simply explain RPM and Boost?

So, I've been messing around with the Spit for a couple of days now, and although I have finally managed to get take off about 8 times out of 10, and struggle completely with landing, I realised that I don't fully understand the relationship between RPM and Boost.

Before people start flaming me, I have tried to research this, but end up being more confused!

I don't understand what the 2 controls functions are. One is usually called the throttle, and the other is called various things, I think, e.g. Engine RPM control, propeller controller, propeller pitch lever, etc. But, I think I have seen the latter being referred to as the throttle as well!

Can anyone point me to or give me a simple explanation of what is going on here?

For example, the engine can have maximum RPM and no Boost and it drops like a stone. Is the angle of the propeller blades variable? By the 'boost' control?

Any help appreciated.

Even though not a correct analogue the easiest way probably to think about it in simplistic terms is to think of it like gears and the accelerator for your car. Putting the RPM lever fully forward or in fine pitch is like first gear (putting it forward increases the RPM of the engine just like 1st gear) and as you move it back you are changing gear up towards 4th or 5th. The throttle lever (controls boost) is essentially your accelerator pedal. So take off needs 1st gear (fully forward) and you might change down (move the rpm lever forward a bit) to get extra performance for climbing (like a car on a hill) or acceleration (overtaking). However just like a car if you leave it in too low a gear (full forward RPM level) and belt around at full throttle (full boost) the engine will overheat. Obviously if you notice you are overheating you fully open the radiator intake (to improve the airflow over the radiator but this causes drag and will slow you down) and ease off on the accelerator (reduce boost) and change up a bit (move the rpm lever back) and cruise for a bit straight and level. You can also do the equivalent of engine braking for say landing by reducing boost - foot off the accelerator - and putting the RPM level forward - changing down. Once you've taken off, just like a car you change up by moving the RPM back and throttle back to cruise.

The analogy breaks down in places for aircraft especially constant speed propellers but might help get you started.

Thanks a lot, Stonehouse. I'm going to think about that concept tomorrow. Fingers crossed!

So, I've been messing around with the Spit for a couple of days now, and although I have finally managed to get take off about 8 times out of 10, and struggle completely with landing, I realised that I don't fully understand the relationship between RPM and Boost.

Before people start flaming me, I have tried to research this, but end up being more confused!

I don't understand what the 2 controls functions are. One is usually called the throttle, and the other is called various things, I think, e.g. Engine RPM control, propeller controller, propeller pitch lever, etc. But, I think I have seen the latter being referred to as the throttle as well!

Can anyone point me to or give me a simple explanation of what is going on here?

For example, the engine can have maximum RPM and no Boost and it drops like a stone. Is the angle of the propeller blades variable? By the 'boost' control?

Any help appreciated.

- Boost:

Controlled by the Throttle. Relates to the amount of fuel you are allowing to enter the engine. If you fly the P51 Mustang this is similar to Mannifold Pressure.

- RPM.

Controlled by the engine RPM lever. Indicates engine speed turning the constant speed propeller.

Since RPM is dependant on fuel driving the engine it is effected by the throttle (boost) setting. With the RPM lever you are setting a maximum RPM. As you throttle up the engine turns over faster until you reach the max RPM set by the lever.

It is important to use 'complimentary' boost and RPM settings so you do not overboost the engine (stressing the engine by putting too high a throttle setting compared with RPM). There is a list of boost and RPM settings in the Spitfire cockpit. Do not exceed the throttle setting for a corresponding RPM (see attached image below) for long lengths of time. [edit] Note +18 boost is not listed on the plaque - ideally you'll want to use 3000 RPM here. Overboosting is modelled in the P51 (which has the same Merlin engine as the Spitfire IX) but I'm not sure if it's currently modelled in the Spit right now.

Note on constant speed propeller - the Spitfire is equipped with a constant speed propeller which automatically adjusts the propeller pitch to maintain a constant RPM. Here we select the optimum engine RPM and propeller pitch is automatically adjusted (depending on airspeed) between a range from fine to coarse. This is different from aircraft equipped with a propeller pitch lever where fine or coarse propeller pitch are set by the pilot and RPM is variable (depending on speed).

Thanks a lot. That raises another question. What is propeller pitch? Is it the angle of the prop blades?

Thanks a lot. That raises another question. What is propeller pitch? Is it the angle of the prop blades?

Yes. Sorry, I should have written that above!

I'm afraid that explanation is very wrong.

The carburetor is built to maintain a specific mixture ratio of air and fuel in the manifold. In this sense it is passive; controlled only by conditions in the engine.

The throttle lever controls the boost, which is the air pressure in the manifold, which is why it's called Manifold Pressure (MP) on the Packard-Merlin engine (P-51D).

When you increase boost, it increases the air pressure in the manifold, which in turn causes the carburetor to introduce more fuel into the compressed air in the manifold. More air-fuel mixture means more fuel burning in the cylinders per stroke and more power output.

Reducing boost, reduces the density of the air-fuel mixture, and reduces power. If you reduce boost to negative, you are actually forcing the engine to suck the air into the manifold through the function of the pistons moving down through the cylinders during their intake strokes, pretty much like a non-boosted engine works.

The RPM lever sets the wished speed of the propeller, which directly coincides with the RPM of the engine, otherwise an increase in power would increase the RPM's, which we don't want.

This is done by matching the pitch of the propeller blades to the power output of the engine. If you think of the propeller having a flat disc where the propeller blades are, when the blades of the propeller are adjusted to be flat on the disc, we say the blades are pitched fine because they have 0 Angle of Attack (AoA) in their motion through the air, and when the blades are turned so that to increase their AoA, we say the pitch is corse.

So when you increase power through boost, to use up that power increase the propeller increases the pitch of the propeller blades, causing them to push more air, which uses more power and balances the power output vs RPM equation and maintains the set RPM of the propeller.

Only when the power output is very low--when you are on your landing approach and have reduced boost greatly to decelerate--will RPM's not keep up with their settings, because the blades are set to their minimum AoA and cannot reduce power consumption further, so the RPM's simply start to fall off.

What you want to pay attention to is the relationship between power output of the engine and the RPM setting of the propeller. If you've ever ridden a 10 or more speed bike, you may better understand the relationship. Adding too much boost to a low RPM setting is like trying to ride up a hill in a high gear. It requires an extreme amount of push on the pedals to maintain the motion of the pedals and the bike. The same thing happens, when you increase boost at low RPMs. The propeller increases the AoA of the blades to use up the increase in power output, which puts an extreme amount of stress on the engine.

The small chart on the inside of the cockpit Bounder posted tells you what relationships between boost, RPM, and activity (take off, climbing, cruising) you should not exceed, and for how long you may maintain them.

@ Imacken, just read about constant speed propellers and You'll get it. I strongly recommend Pelican's Perch article on the subject (and his other articles about engines and airplanes for that matter):

http://www.avweb.com/news/pelican/Pelicans-Perch-16-Those-Marvelous-Props-182082-1.html

@ Bounder - overboosting is there in the DCS Spit as well. Crank down the RPM to 2000 and boost up to max and see You engine going pop in less than 15 seconds :D.

Thanks a lot guys for your detailed responses.

I am still really struggling with this.

I liked the simple car analogy with reference to accelerator (throttle) and gears (RPM), but I don't see it.

For example, if I am in level flight with max RPM and full throttle, by the car analogy, if I reduced the RPM (go to a higher gear) then my speed should go up, assuming a constant throttle boost.

However, what happens is that the throttle boost drops with the RPM, and similarly increases with an increase in RPM.

So, I could see the analogy if the throttle and RPM lever were inpdependent of each other, but obviously they are not.

I think I'm missing something fundamental here.

Thanks a lot guys for your detailed responses.

I am still really struggling with this.

I liked the simple car analogy with reference to accelerator (throttle) and gears (RPM), but I don't see it.

For example, if I am in level flight with max RPM and full throttle, by the car analogy, if I reduced the RPM (go to a higher gear) then my speed should go up, assuming a constant throttle boost.

However, what happens is that the throttle boost drops with the RPM, and similarly increases with an increase in RPM.

So, I could see the analogy if the throttle and RPM lever were inpdependent of each other, but obviously they are not.

I think I'm missing something fundamental here.

It's not the best analogy, but think about it this way:

You are driving at 30 mph in 5th gear, at an RPM of 1,100. You put the pedal to the floor. What happens? Nothing. The car accelerates very slowly. On the other hand if you drive at 30 MPH in 3rd gear at a much higher RPM your car will react to the accelerator much more dynamically. In exchange of course for a higher fuel consumption.

Others explained the details perfectly above, but what you need to know:

Lower RPM - better mileage, less performance

Higher RPM - worse mileage and bigger stress, better performance.

The analogy with manual gears in a car is just plain wrong an confusing.

The constant-speed propeller operates like CVT automatic transmission in a car (https://en.wikipedia.org/wiki/Continuously_variable_transmission). It adjusts the engine load automatically to keep it at constant RPM, by varying the propeller pitch. It's like having an automatic gearbox in a car that has presets for maximum power (high RPM) and economy (low RPM).

Boost regulator operation is also a bit more complicated, it is not meant to maintain constant boost as the RPM changes. Also, the engine won't be able to maintain the same boost value through all RPM and altitude ranges.

It's not the best analogy, but think about it this way:

 

You are driving at 30 mph in 5th gear, at an RPM of 1,100. You put the pedal to the floor. What happens? Nothing. The car accelerates very slowly. On the other hand if you drive at 30 MPH in 3rd gear at a much higher RPM your car will react to the accelerator much more dynamically. In exchange of course for a higher fuel consumption.

 

Others explained the details perfectly above, but what you need to know:

Lower RPM - better mileage, less performance

Higher RPM - worse mileage and bigger stress, better performance.

Thanks for that.

I get that in respect of a car, but in that environment, you can hold the RPM constant and you have independent control over gears and 'power'. In the Spitfire you don't as I described in my previous post. The RPM goes up and down with the boost level.

If the RPM changes as you move the boost lever, it means that you're exceeding prop governor limitations. Maybe you move it too fast and it needs a short bit of time to adjust prop pitch and bring RPM to the original value. But it also can't move the propeller pitch past its limits, so if you throttle back and slow down enough, the RPM will drop anyway, as the engine is idling and doesn't make enough power to rotate that fast.

For the opposite situation (why changing RPM affects boost), read my explanation above.

Only reason I brought up the car analogy is that you wanted it simple. As a simplistic idea it does basically work but as I said it isn't really accurate and it breaks down for aircraft fairly quickly and things like constant speed props in particular. But it still does give you an idea eg if you are low throttle and the rpm lever is fully forward (1st gear) and you shove the throttle to max boost then just like redlining a car you'll likely break something and also the idea of engine braking by reducing throttle and increasing rpm (because it puts the propeller in fine pitch). If you are taking off then you need 1st gear - fully forward rpm lever and once you are up you "change up" by pulling back the rpm lever a bit, when you need acceleration you change down by pushing forward on the rpm lever and then the throttle etc etc. You are pretty much juggling the two to avoid over speeding/overheating your engine but still get the performance you want at that moment.

If you want the full story then I agree with whomever posted about the pelicans perch stuff, they are definitely the thing to read. Unfortunately pretty much if you really want to understand it you are going to have to read up on it as there are limits to how much you can simplify things. It's one of the nice things about DCS - you can read real pilot education material and use the knowledge properly in the sim and it works. A lot of past sims fudged stuff like constant speed props and engine management.

OK guys, I'm gaining a bit of knowledge about this although some of the references are very technical for me, and I would need quite a bit of time to grasp them!

I have attached a graph showing my observed speeds from level flight with various boost and throttle settings. Does it look correct? If so, then the car analogy seems to hold up as speed decreases as RPM goes up in the case of 0 and +4 boost. At +8, why is the pattern different? It is a similar case with +12 - until my engine blew!

The car analogy using basic gears is wrong. Very wrong. I'd forget about it if I were you.

some1's explaination using CVT is mostly correct if you want to keep it about cars.

The RPM lever in the Spitfire and Mustang controls the rotation speed of the prop. The throttle lever is how much fuel/air mixture (boost) you're giving the engine (power!).

The governor on the prop works by adjusting the pitch of the blades to keep the prop (and therefore engine) at the speed set by the RPM lever. The pitch of the prop is effectively your gearing and something you do not have direct control over on these aircraft.

As you give the engine more fuel/air (throttle), then for a given RPM setting, the prop will adjust to effectively gear the engine to maintain the same prop rotation/RPM.

The car analogy using basic gears is wrong. Very wrong. I'd forget about it if I were you.

 

some1's explaination using CVT is mostly correct if you want to keep it about cars.

 

The RPM lever in the Spitfire and Mustang controls the rotation speed of the prop. The throttle lever is how much fuel/air mixture (boost) you're giving the engine (power!).

 

The governor on the prop works by adjusting the pitch of the blades to keep the prop (and therefore engine) at the speed set by the RPM lever. The pitch of the prop is effectively your gearing and something you do not have direct control over on these aircraft.

 

As you give the engine more fuel/air (throttle), then for a given RPM setting, the prop will adjust to effectively gear the engine to maintain the same prop rotation/RPM.

Exactly.

What might help to understand the dynamics is to think about what would happen if you didn't have a constant-pitch propeller, that is, no prop governor. The pitch of the propeller would be fixed (or you would need a separate apparatus to manually adjust the pitch, which is what the governor does automatically).

Without the governor, and without manually changing the pitch of the propeller, when you increase boost (more air-fuel mixture = more power output) the engine and propeller would increase speed. This is basically what would happen in a car, when (without changing gears) you press down on the accelerator pedal. The engine gets more fuel and accelerates.

With the Spitfire's constant-speed propeller, the governor senses the increase in power output, which causes it to increased the pitch of the propeller, which causes the propeller to dig into the air at a greater angle, which pushes more air, which expends the additional power from increasing the throttle, by increasing the thrust the propeller is creating.

If you understand this, then you will realize that you can produce the same thrust at different RPM's. Considering a specific thrust, at a lower RPM the propeller will have a greater pitch to produce that thrust. At a higher RPM, the pitch will be lower.

The difference for the engine is, the greater the propeller pitch, the more torque required to turn the propeller, the greater the stress on the engine, while the lower pitch will cause less stress on the engine, but the engine and propeller will need to running faster to produce the same thrust.

In neither case will the engine necessarily be burning more or less fuel. That would be against the physics. To create x-thrust, you will always need to burn y-fuel, not taking the difference in friction caused by a faster or slower running engine, or the stress caused by torque.

Also remember, because you have a supercharger, you can be burning the same amount of fuel (same power output, same thrust) at different engine RPM's, simply by increasing the amount of air pressure (boost) the engine receives.

And this is where engine management comes into play, balancing friction and torque, speed and cooling, to realize the optimal thrust output. Too much torque and the engine blows. Too much speed is equally bad, but you shouldn't be able to turn the engine too fast per se, because the governor on the constant-pitch propeller prevents that. But you can turn the engine faster than necessary to reach the thrust you require for the speed and/or climb-rate you desire, which tends to overheat the engine, which if you are flying too slow to cool the engine, will again damage the engine as well.

This is why you have those rules posted in the cockpit. They give the the guide lines to safely manage the engine.

Thanks so much for all your detailed responses. I'm trying to get my head around them all!

Can I ask, by looking at my table of speeds in post 15 above, am I right in saying that in level flight, it is only boost that affects speed? Different RPMs in the range 2000-3000 don't seem to have much effect on speed.

Lots of craziness here in examples and "pretend this pretend that" etc. As a former instructor for many years I recommend starting from the beginning with official documentation. This chapter should give you a solid basis of information for what you are asking about. http://https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/09_phak_ch7.pdf

Hope this helps! Feel free to PM me with additional questions

Thanks so much for all your detailed responses. I'm trying to get my head around them all!

Can I ask, by looking at my table of speeds in post 15 above, am I right in saying that in level flight, it is only boost that affects speed? Different RPMs in the range 2000-3000 don't seem to have much effect on speed.

For the same boost, increasing rpm does increase power delivered by the engine and thus the speed should be higher.

There are many inaccuracies throughout​ this thread, best if you read that FAA document that acdelta57 linked.

Lots of craziness here in examples and "pretend this pretend that" etc. As a former instructor for many years I recommend starting from the beginning with official documentation. This chapter should give you a solid basis of information for what you are asking about. http://https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/09_phak_ch7.pdf

 

Hope this helps! Feel free to PM me with additional questions

Thanks for that. Very useful article, I learned a lot. However, I'm on the verge of giving up on the Spit. I've spent the best part of 4 days trying to master landing, and no matter what advice I follow, or do the training module, or look at videos and do a whole heap of trial and error, I just can't get a landing with out bounce. Totally soul destroying.

Think I'll go back to the A-10C. Much easier!

Don't give up on her but sometimes a much needed break is just as important as flying. She is a handful

Thanks for that. Very useful article, I learned a lot. However, I'm on the verge of giving up on the Spit. I've spent the best part of 4 days trying to master landing, and no matter what advice I follow, or do the training module, or look at videos and do a whole heap of trial and error, I just can't get a landing with out bounce. Totally soul destroying.

Think I'll go back to the A-10C. Much easier!

A noob spit pilot here but let me try giving some advice. Speed. Less speed. And this:

https://www.digitalcombatsimulator.com/ru/files/2291919/

Easy gauge labeling coupled with lower speed on landings helped me immensely.

The gauges in the spit are somewhat unconventional and different to what we are used to in other DCS planes, but make sure you're having the landing speed and descent rate written down on a post-it and stuck to the display.

And if you fail, just go slower and slower and slower. I had the same problems but once I allowed myself to go slower on landing, it just "clicked"... :)

Thanks for that. Very useful article, I learned a lot. However, I'm on the verge of giving up on the Spit. I've spent the best part of 4 days trying to master landing, and no matter what advice I follow, or do the training module, or look at videos and do a whole heap of trial and error, I just can't get a landing with out bounce. Totally soul destroying.

Think I'll go back to the A-10C. Much easier!

Nooo! Don't give up! You will regret it. Also flying the WWII birds will make you a (MUCH) better jet pilot. Trust me on this.

I have yet to crash a Spit on a normal landing (though I did dip a wing or two, and my takeoffs leave much to be desired) and I actually find her an easier bird to do a three pointer with than the Mustang. (Though once she sets down, thats when the problems begin)

One thing that helped me A LOT was following Matt Wagner's take-off and landing video exactly. When I say exactly I don't mean to the km/hr, watching the gauges might work for a lot of people but I think it doesn't work for the newbie pilot. I mean the procedures and what they look like when you look out of the cockpit.