Easy Way to Explain MP in Prop Thrust

[your_dreamguy]

Hello,

I'm having some trouble explaining how manifold pressure plays into thrust when it comes to constant speed props. As we know, thrust is the force produced as the prop rotates through the air. The RPM increasing and/or an increasing blade pitch can give a higher thrust off the prop. However, when I'm not sure how to explain why Airspeed will increase if MP increases or vice versa. Does anyone have some simple explanations or examples I can give to my students?

Thank you.

 

[bigD]

I usually explain it like this:

When you change the MP in either direction, the propellor governor has to adjust the pitch accordingly to maintain a consistent RPM. So for example, if you're cruising at 23" and 2400 RPM, then increase the throttle to 25", the governor has to increase the pitch of the blades to keep the RPM at 2400. With the increased pitch, the prop is taking "a bigger bite" out of the air, and thus your thrust increases.

I've had one friend have trouble understanding why higher propellor pitch required more power to yield the same RPM, and I used a rowboat paddle analogy to explain it. It's easier to pull the oar through the water if the blade is more parallel with the direction of the boat - meaning lower pitch. The same analogy can be used to explain why a higher pitch results in increased thrust, etc...

I see a lot of people explain it by using angle of attack of the blades, but for some students, it's difficult to visualize - especially if you're not drawing it out for them.

 

[Deftone45075]

What BigD said, would be a good way to explain it. If what they want to know is why the plane will go faster with higher MP when the RPM stayed the same, than the above post nails it. It all depends on how far into it you want to go.

If they want to know why you use MP, tell them it's a lot cheaper than having a system to measure torque. If they want to know how MP relates to power, i'd tell them as the engine is running, the intake manifold is like a venturi. Before the engine is started, it would indicate atmospheric pressure. Once the engine is running and sucking in air through the manifold, the pressure will drop. However, as you increase throttle and demand more power, a larger volume of air will move through the manifold thus increasing the pressure.

 

[A Squared]

Not that big D and deftone are wrong, but I'd approach an explanation of the whole issue from a different direction.

I'd talk about MP and throttle position *First* before I talked about anything else.
WHen you start this way, it's real simple. The MP is an indication of how far open the throttle is. Liken the throttle to the gas pedal on a car. The farther you open it, the more power you get. the more power you get that faster you go. Closed throttle, low MP, low power; open throttle, high MP more power; that's the foundation that needs to be in place before the rest is added. (FOI building blocks and all that....)

Once the student understands Throttle, MAP, Power, *then* you can start talking about props, and how the the constant speed prop works, and how RPM also affects power.

Don't put the cart before the horse.

regards

 

[metrodriver]

Here is how I explained it: with the engine at idle , the throttle (plate) is closed, however there is just enough air passing through to sustain combustion. All the cylinders are sucking a vacuum in the intake manifold whenver the intake valves open, resulting in a low (intake) manifold pressure reading. When you open the throttle, more are is allowed into the intake manifold and the pressure reading will go up till almost ambient pressure at full power (slight loss from restrictions and sucking cylinders). How is this going to relate to the prop? The flyweights in the propgovernor are trying to keep the engine rpm constant (the rpm you selected). When you add power (increase MP) the engine wants to turn faster, the propgovernor allows more oil into the propdome and changing the blade angle to a bigger bite (higher pitch), resulting in a higher aircraft speed.
The flyweights on the propellor blades act as an opposing (balancing) force. If for whatever reason (turbulence, climb, decend) the load on the prop changes (the prop is trying to change its rpm), the flyweights in the prop will make smalll adjustments to the blade angle and the prop will stay 'on speed'. The prop governor is doing nothing now. It has set the rpm you selected, now it's up to the prop blades to find their blade angle for optimum performance.
Why does the MP change when you change rpm? You had the throttle set to allow a certain rpm by allowing a certain amount of airflow in the intake manifold. Now if you increase the rpm, but don't change the throttle position, the cylinders will try to suck more air than is available, and the result is a drop in pressure in the intake (lower MP reading), if you decrease the rpm, air will back up in the intake, resulting in a higher pressure (higher MP reading).
explaining prop pitch: see it as a screw or bolt. If you have a high pitch, the distance between the threads is large, one turn and the srew goes a long way into whatever it is going in to. Low pitch screws are those screws that just take forever to get into something.
I hope you can do something with this explanation. If you got more stuff like this, pm me. I hate to see people operating equipment who don't understand what's happening