my thoughts on propeller pitch

[Lololopoulos]

after playing this game for about 2 weeks now I think i have a rather firm grasp on what propeller pitch does, more specifically 109's, cus that's what I spend most of my time in.

You do not really understand something unless you can explain it to your grandmother.

So, IMHO, this is how I would explain propeller pitch to my grandmother.

Since I ride my bike to classes everyday, I'd like to compare propeller pitch with a bike that has gears.

So when you start from a stop on ur bike, you'd want to start with a low gear, which corresponds to 12:00 pitch in 109.
As you start to pedal harder and start to accelerate, you will eventually reach a point where u're pedaling very fast but not going any faster, in other words, the wheels now go faster than you can pedal. This condition corresponds to when you're in straight level flight and your engine is at almost 2500 RPM.
Now on a bike, you need to shift your gear up and you will immediately feel the force on the pedal and with relatively little amount of force, you can go faster. In our 109 counterpart, you would coarsen/decrease the prop pitch to make the blades catch more air, as a result, the engine's RPM goes down, with relatively little stress on the engine, the airplane goes faster.

Same analogy applies to climbing a hill/gaining altitude.
On a bike when you're trying to go uphill with some initial velocity, at the bottom of the hill, you would opt for a high gear to use off the remaining inertia, and then work your way down on the lower gears to maintain a optimum pedaling force. Towards the top of the hill, you would probably be in a low gear and pedaling very fast just to keep going.
In our airplane counterpart, as you start a climb from level flight, you start from a coarse pitch, depends on how fast your initial velocity is. Then as you gain more altitude, you slowly increase your propeller pitch to maintain the optimum engine RPM, between 2100 to 2400RPM. Towards the top of ur climb, you will be in a very fine pitch close to 12:00.

Please, everyone, feel free to point out anything you find incorrect. I wrote this to offer my own perspective on propeller pitch to help people who still don't understand it.;)
Lololopoulos

[louisv]

Watch this video and download the Spit book, also at A2A. In the book you will find the pitch (expressed in RPM...) for each phase of the flight and a lot of other things. Its the Spitfire owner's manual !!! So RTFM and fly away...(I'm sure there is a 109 book too, just not at A2A. Google is your friend).

In the video, there is the constant speed propeller, think of it as an automatic variable propeller as compared to the 109's manual variable. With a CSP (like the Rotol on the Hurricane), the RPM is maintained whatever the throttle setting. Not with a manual variable, like the one on the 109. But the principle of the variable whether it is the DH 2 speed (also on the video) or the Rotol CSP, or simply variable, the principle remains the same.


http://www.youtube.com/watch?v=psYL7thQj6M

[5-in-50]

The bike analogy is fine, but doesn't account for Constant Speed Units, which aren't present in CoD anyway.

[Skoshi Tiger]

The bike analogy is fine, but doesn't account for Constant Speed Units, which aren't present in CoD anyway.

Rotol Hurricane and MKII Spitfire have CSU's

Cheers!

[louisv]

The bike analogy is fine, but doesn't account for Constant Speed Units, which aren't present in CoD anyway.

There is the Rotol hurricane

Yeah the bike analogy is good actually, max fine pitch is not max speed, from the video...

[CaptainDoggles]

In the video, there is the constant speed propeller, think of it as an automatic variable propeller as compared to the 109's manual variable. With a CSP (like the Rotol on the Hurricane), the RPM is maintained whatever the throttle setting.

Within reason of course. The RPM governor has limits, and can't maintain e.g. 2000 RPM if you throttle down too much.

[JG53Frankyboy]

Rotol Hurricane and MKII Spitfire have CSU's

Cheers!

& Ju87, Ju88(what is wrong), He111, G.50, Br.20

[Lololopoulos]

Thanks everyone for ur opinion.
I have one question though, so in the video is it mentioned that how fast the airplane will be going depends on 2 factors:
1. pitch or how much air the propeller grabs
2. the rpm or how fast the propeller rotates.
theoretically more pitch and more rpm combined will make the airplane go faster

This is what I don't understand, in the constant speed propeller take off run, doesn't a fine pitch counteracts the high rpm? intuitively for me, if the pitch is fine then it doesn't really matter much how fast the propeller spins anymore because the prop doesn't get much air anyways??

Can someone point me in the right direction?

[adonys]

they are in a symbiotic (!) relation : the more air the propeller bites, the slower till be the RPM.

think about it in this way: if you have the engine without any propeller, and working at maximum power, the shaft gets to a certain RPM. it will NEVER went higher than that by itself (unless helped by an external force).

now, when you fit the propeller on max fine (biting the smallest amount of air), some engine force will be spent on pushing that air, so the force rotating the shaft will be smaller, therefore the RPM will go down. The more air bites, the slower it will rotate.

the exception is when you are diving, because while diving, the finest propeller setting will expose the max propeller surface against the airflow going up (as your aircraft is going down), and that will apply the biggest external force on the engine's shaft, making it exceed its maximum self obtained RPM, and breaking the engine.. that's why in dives you have to set your propeller on max coarse, to offer its smallest profile to the airflow going up, in order to not over-rev your engine.

[bw_wolverine]

they are in a symbiotic (!) relation : the more air the propeller bites, the slower till be the RPM.

think about it in this way: if you have the engine without any propeller, and working at maximum power, the shaft gets to a certain RPM. it will NEVER went higher than that by itself (unless helped by an external force).

now, when you fit the propeller on max fine (biting the smallest amount of air), some engine force will be spent on pushing that air, so the force rotating the shaft will be smaller, therefore the RPM will go down. The more air bites, the slower it will rotate.

the exception is when you are diving, because while diving, the finest propeller setting will expose the max propeller surface against the airflow going up (as your aircraft is going down), and that will apply the biggest external force on the engine's shaft, making it exceed its maximum self obtained RPM, and breaking the engine.. that's why in dives you have to set your propeller on max coarse, to offer its smallest profile to the airflow going up, in order to not over-rev your engine.

A very good explanation. I think this is something a lot of people miss and end up over-revving their engine as a result.

[CaptainDoggles]

Thanks everyone for ur opinion.
I have one question though, so in the video is it mentioned that how fast the airplane will be going depends on 2 factors:
1. pitch or how much air the propeller grabs
2. the rpm or how fast the propeller rotates.
theoretically more pitch and more rpm combined will make the airplane go faster

This is what I don't understand, in the constant speed propeller take off run, doesn't a fine pitch counteracts the high rpm? intuitively for me, if the pitch is fine then it doesn't really matter much how fast the propeller spins anymore because the prop doesn't get much air anyways??

Can someone point me in the right direction?Propellers and the mathematics behind them are quite complex, but we can do a crude analysis.

An approximation of the thrust produced by a propeller can be made by computing:

Thrust = [efficiency factor * power] / speed

For takeoffs, you want to accelerate to liftoff speed in as short a distance as possible, which means you want as much thrust as you can get.

Looking at the above equation, there are only 3 terms. Speed is very small as you begin the takeoff roll and the efficiency factor is determined by the design of the prop.

That leaves engine power as the only remaining variable we can manipulate. Since the engine develops more horsepower at high RPM than it does at low RPM, you want to run the engine at high RPM so that you can produce the most thrust.

Going a little deeper: Power is computed by multiplying shaft torque by rotational speed. So to increase power you want the prop to spin faster.

Therefore you want to go to fine pitch.

------

I really dislike the "car analogy" that many make, but if it helps: In a car you accelerate from a standstill in lowest gear. This is akin to putting the prop at fine pitch.

[Art-J]

Ehm... Sorry, Cheesehawk, but I'm sure Cpt.Doggles' formula is right :D. It's a simplified, basic formula for prop thrust, obtained from the definition of prop efficiency, which can be found in any book about props... or on the same page You linked to, just a couple of lines below the formula You quoted. Note the formula being used for thrust estimation in the numerical example at the bottom of the same page.

Cheers - Art

[Lololopoulos]

Propellers and the mathematics behind them are quite complex, but we can do a crude analysis.

An approximation of the thrust produced by a propeller can be made by computing:

Thrust = [efficiency factor * power] / speed

For takeoffs, you want to accelerate to liftoff speed in as short a distance as possible, which means you want as much thrust as you can get.

Looking at the above equation, there are only 3 terms. Speed is very small as you begin the takeoff roll and the efficiency factor is determined by the design of the prop.

That leaves engine power as the only remaining variable we can manipulate. Since the engine develops more horsepower at high RPM than it does at low RPM, you want to run the engine at high RPM so that you can produce the most thrust.

Going a little deeper: Power is computed by multiplying shaft torque by rotational speed. So to increase power you want the prop to spin faster.

Therefore you want to go to fine pitch.

------

I really dislike the "car analogy" that many make, but if it helps: In a car you accelerate from a standstill in lowest gear. This is akin to putting the prop at fine pitch.

thanks a lot!!
when it comes to things that are unintuitive, looking at a math formula is very very helpful, because it gives u an idea of what is important and what is irrelevant. Good job with the condensed version of the formula and explanation!!

[CaptainDoggles]

Sorry Doggles, I am sure your formula is off.

No need to apologize, cheesehawk. The formula I posted is perfectly valid (if a bit simplistic). You'll notice I used the phrases "a crude analysis" and "an approximation". In real life we must also account for components of thrust not in line with the direction of motion, rotation of the fluid within the slipstream, etc. The formula you posted doesn't account for these either. It all depends on how much effort we are willing to invest and how precise we want our approximation to be.

The concept of delta-v added to the flow is not easily discerned from in-cockpit instrumentation. Horsepower on the other hand is at least related to engine RPM, and we have a tachometer in the cockpit :cool:

[CaptainDoggles]

Ok, you're right, I wasn't looking at the crude or approximate in your statement, I got distracted by the simplistic formula, haha. I was looking for something precise, which takes propller diameter into account, as well as the delta V (how do you type those in Windows anyways?). The formula I posted DOES take diameter into account, but the diameter term is hidden away in the efficiency term. I type Δ by going into the character map. It's not in the ASCII character set so I don't think it has an ALT+XXXX code.

[Lololopoulos]

delta is a greek letter, so in windows, you would go to control panel and add the greek keyboard and Δ is the D key. After you add the greek keyboard, you can switch around different languages by pressing alt+shift.
i work with multiple languages all the time, this is the way I would recommend to type Greek letters.