General Avionics Questions

[Plazus]

This is for any of you pilots out there:

I have been wondering about the difference between manifold pressure and RPMs. I am aware that RPMs mean Revolutions Per Minute and can be adjusted by changing the pitch of the propellers. I am guessing that manifold pressure means just that- pressure (often measured in PSI) on the manifold in the engine. Here are my questions:

1. Do RPMs measure the number of revolutions of the propeller, or some other part in the engine?
2. How do we get "thrust"? Does more thrust mean more speed, climb rate, acceleration, etc.?
3. Is "thrust" directly proportional to RPMs? Or to manifold pressure? Or both? Meaning, if I were to increase the throttle (manifold pressure) I would get more "thrust"?

Here is another example of what I am trying to get at:

I'm flying in a P38 at 1,000 feet altitude. My manifold pressure reads 35 inches. My airspeed is sitting around 275 knots. I advance the throttle to 42 inches manifold. My airspeed rises to about 320 knots. My RPMs remain the same at both settings. However, from my understanding, advancing the throttle created more thrust and thus more speed. But if the RPMs of the propeller remains constant, what other factors are creating this additional thrust?

Wouldn't you want to increase RPMs of your propellers so that you get more thrust?

Discuss.

[kvuo75]

the pitch of the prop changed as theres more torque applied to it



lemme find these links I learned from. They're old, but explain nicely.  From John Deakin's old column "Pelican's Perch" at Avweb

John Deakin is a 35,000-hour pilot who worked his way up the aviation food chain via charter, corporate, and cargo flying; spent five years in Southeast Asia with Air America; 33 years with Japan Airlines, mostly as a 747 captain; and now flies the Gulfstream IV for a West Coast operator. He also flies his own V35 Bonanza (N1BE) and is very active in the warbird and vintage aircraft scene, flying the C-46, M-404, DC-3, F8F Bearcat, Constellation, B-29, and others. He is also a National Designated Pilot Examiner (NDPER), able to give type ratings and check rides on 43 different aircraft types.

Manifold pressure:

http://www.avweb.com/news/pelican/182081-1.html

constant speed props:

http://www.avweb.com/news/pelican/182082-1.html


they have all his columns archived also:

http://www.avweb.com/news/pelican/list.html

[dtango]

Plazus:

Specifically to answer your questions:
1) RPM is the crankshaft rotation of the engine.  The actual propeller RPM is typically gear-reduced to a lower RPM for various reasons (keeping tip speed from hitting supersonic speeds is one of them).

2) We get thrust by use of Newton's laws from accelerating a mass of air, in our case via a propeller.  More thrust does mean higher top speed, greater climb rate, etc.

3) How is thrust related to manifold pressure & RPM?  They essentially control the mass flow of air via the propeller and thus the amount of thrust produced.  For details, I posted the following awhile back:

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For constant-speed propellers another way of describing manifold pressure and RPM control is this:  manifold pressure controls the amount of power that is "available" to spin the prop while RPM/prop pitch controls the amount of power "required" to spin the prop.  The throttle (MAP) controls the amount of air that can be sucked in by the pistons and thus the amount of air available to the engine.  RPM controls the speed at which the crankshaft can spin and thus how fast the pistons are allowed to pump.  Along with fuel mixture (which is automatic in AH) these three controls determine the power output of the engine.

For a prop in constant speed operation there isn't a direct RPM control for the actual engine shaft.  RPM's are controlled via the propeller governor that senses how fast the shaft is spinning and then adjusts the propeller pitch that either creates more force or less force for the engine to oppose.  The force created by the prop varies with the amount of "lift" (thrust) that it is producing.  The higher the aoa of the propeller blade, the more lift created.  The more the lift, the greater the induced drag thus the more force needed to spin the prop.  The opposite is also true.  Using these forces the prop governor then maintains the engine RPM at the set speed by adjusting blade aoa/pitch as the shaft overspeeds or underspeeds the desired RPM.

What does have to do with aircraft performance?  It's related to the amount of thrust and power available to propel the airplane.  In flight, the efficiency of the propeller to convert engine power into usable power will vary by the propeller advance ratio, J.

J = airspeed / prop_rotational_speed * prop_diameter

For a fixed pitch propeller max efficiency of a propeller occurs at a specific value of J (specific airspeed & rpm).  Outside of these values the efficiency drops.  With constant speed propellers we're trying to maximize propeller efficiency over a range of airspeeds thus the maximum amount of power converted from the engine to flight.  The way this is done is setting the RPM constant and then automatically varying the propeller aoa with varying airspeed to maintain a J value that maximizes propeller efficiency.

How all this is related to fuel consumption is that the fuel consumption of the engine can be maximized by adjusting MAP (power available) and RPM (power required) so that power output of the engine is at a setting most economic for fuel consumption.

For a fixed pitch propeller, RPM is completely tied to MAP/throttle setting of the engine.  All AH WW2 aircraft are constant-speed props which means adjusting the RPM means adjustment of the blade pitch (aoa).
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Hope that helps.

[SgtPappy]

I just had an exam a couple days back on all the above. Only exam that I felt was worth my time! Btw, avionics refer to sensors and airborne electronics like radar, IR sensors, EM detectors and the such. Your questions are more about flight mechanics and aerodynamics. 

Just to add to how thrust is created:

If you look at at the cross-section of a propeller blade, it will be just like that of an airfoil. As it moves through the air it creates lift by accelerating air in front of the blade faster than the air behind the blade. There are a great many theories as to how lift is created in the subsonic regime, but my books tend to all explain it as the air resisting flow as it moves over the top of the airfoil. By resisting this change, it consequently gets 'squashed' over top of the airfoil (front of a prop blade) as it moves over the maximum thickness of the airfoil and speeds up as all subsonic  Newtonian fluids do when flowing through a compressive resistance. The bottom of airfoil (behind the prop blade) is shaped only such that the air will simply pass by, slowing down slightly. The more air you can get above the airfoil, the more will resist a compression, and the faster the air can accelerate (don't mistake this for a Venturi effect which requires a device to physically surround the air as it passes through, compressing it as a consequence).



This difference in airspeed create a force, or thrust. Of course, the higher the RPM, the more thrust you can create, but the efficiency of the props degrade with speed - or more specifically with the advance ratio, given by dtango. So increasing the pitch will increase lift, BUT will also create drag. Whenever lift is created, induced drag is created as well. So changing the pitch to a more coarse one at higher speed will slow down the prop (lower the RPMs), but it will create thrust more efficiently. Hence, cruise conditions call for making the prop more coarse in pitch.

[Plazus]

Thanks for the responses, guys! Much appreciated. So basically the way I'm reading it is: Advancing the throttle means that you are creating more available power. Adjusting the RPMs means that you are controlling the power required to spin the prop. Ideally at, say in a WW2 aircraft (or in game for that matter), you would want a war emergency power setting to be maximum available power with maximum required power to spin the prop (staying within the sound barrier). This in turn means that the props will exert more force in the air, creating thrust, and thus getting speed/climb/acceleration etc.?

Max available power + max RPMs = more force

More force = more speed/climb/acceleration etc.

[Plazus]

So by increasing the throttles, the prop would naturally spin faster? So by countering that, you increase the prop pitch, thus bringing the RPMs of the prop within sound barrier. But with the increase in available power and increase in prop pitch, the props will exert more force and have more thrust.

I think I have an analogy that might explain this. Let's compare riding a bicycle to flying an airplane. Increasing the throttles would be equivalent to pedaling faster on the bike. So to keep from pedaling too fast, you "adjust the RPMs" by shifting up in gears. Increasing prop pitch would be like preventing your prop from "pedaling too fast" while getting that thrust that you need.

[dtango]

Yes generally speaking to all your clarification questions Plazus. 

[dtango]

SgtPappy- you're coming along in your aero my friend! 

[icepac]

Now if they would just model what happens when you run 60 inches MAP at sea level and low rpms......

[Tupac]

Now if they would just model what happens when you run 60 inches MAP at sea level and low rpms......

Detonation? Or will the engine blow up?

[Plazus]

This difference in airspeed create a force, or thrust. Of course, the higher the RPM, the more thrust you can create, but the efficiency of the props degrade with speed - or more specifically with the advance ratio, given by dtango. So increasing the pitch will increase lift, BUT will also create drag. Whenever lift is created, induced drag is created as well. So changing the pitch to a more coarse one at higher speed will slow down the prop (lower the RPMs), but it will create thrust more efficiently. Hence, cruise conditions call for making the prop more coarse in pitch.

SgtPappy,

To further explain how force/thrust is created, could you show me some diagrams of what the prop pitch would look like with a high RPM setting versus a lower RPM setting? I'm not sure what "coarse" looks like in terms of prop pitch.

Thanks!

[OOZ662]

I don't remember the term for "anti-course" atm, but a "course (aka high) prop pitch" means one that is closer to putting the prop blade itself parallel to the desired thrust vector (also can be seen as maximizing the surface area pounding the air, or on the same plane as the aircraft's wings...looking side-on at the prop, you'd see the paddle shape) whereas the opposite of that is...the opposite. The prop "flattens (lowers)" and whiffs through the air, lowering resistance on itself but generating little thrust as a result (looking side-on, you'd see a thin, flat surface). Imagine a fully course prop as slapping the air while a low prop pitch karate-chops it, I guess. Of course, all the optimals for propellers exist between those two extremes.

[kvuo75]

I don't remember the term for "anti-course" atm,

"fine"

vs

"coarse"

[Ron]

A simple way could be to think of the prop pitch control as a variable gearbox.
Once set for a certain speed, It keeps the RPM constant by changing the blade angle (gear) to continuously compensate for changes in the planes speed, and the your changes in throttle. (The RPMs will lower irrespectively however, if you reduce the throttle way back).

Think of:-
High RPM - FINE PITCH (Low gear) to accelerate, and go up-hill.
Low RPM - COARSE PITCH (High gear) to cruise, and save fuel. Plus, in AH, it seems to nurse a damaged engine to last longer.

Manifold pressure is "roughly" linked to indicate the amount of power your engine is producing relating to throttle setting.
There is much more to it, but for the purposes of this game it  works.

Increase throttle = High MAP = high power
Decrease throttle = Low MAP = low power

Open the E6B in cruise, change the prop/throttle settings and see the effect on speed, and fuel burn.

_____________________________ _____________________________ _________________________

NB:

AH props look to be actually modelled as a constant speed prop, rather than a variable pitch prop as some of the WW2 A/C would have had..
Forget ATM. Like CTM, this is prop mechanics, and is way out of the scope of AH.
This is massively over simplified, but all you really need for this game.

[Weirdguy]

If I recall correctly one of the things that Zero pilots in the war would do is get extreme range out of their planes by reducing power and flying just 5 mph faster than stall speed.

That way they could get a long, long way from base to make attacks on American held islands, who actually thought it was a carrier striker.  It wasn't until later they found out that they were land based planes from bases they couldn't hope to strike back at.

It would make for a long, dull flight, however.  You fly slowly after all.

Also, it makes me think about the P51 and how it was regarded with such esteem for it's ability to get to Berlin, but the Japanese already had a plane that could do that years before.