Pitch?

Thanks for all your replys, now I’m starting to understand how works.
The links are also very interesting and I will be having a read soon.
The sea plane thing is interesting I always thought they were float planes just to reduce weight of a heavy landing gear, I didn’t know about the need for long take off runs.

Chris

Its like trying to pull away in your car in fifth gear. Its also why the early speed record planes were sea planes – they had fixed coarse pitch props and they could use a much longer take off run at sea than they could with a normal runway on land.

Thanks for all the answers and photos they are great, so when the airplane is at altitude, the air is less dense, thus the prop needs a more aggressive angle to the air (coarse pitch) to actually ‘bite’ into the thin atmosphere?

So when pilots of for instance the Mk1 spit forgot to change back to fine pitch for take off, why would the aircraft struggle to take off, was it related to air resistance on the prop or the motor unable to achieve the correct RPM?

I may have further confused myself if I have the coarse and fine the wrong way around.:confused:

Fine Pitch ….= High engine RPM
Coarse Pitch = Low engine RPM

So if you try to take off in course pitch the engine revs will be way down and you may not get enough thrust before you run out of runway 🙂
That is why the early spit and hurris with 2 blade fixed pitch props had a longish take off run because the prop pitch had to be a fairly coarse compromise between take off performance and cruising revs.

Chris, these sites will help you understand wobbly props

http://www.lynehamaviation.co.uk/documents/VPFirefly.ppt

http://flysafe.raa.asn.au/groundschool/propeller.html

Thanks for all the answers and photos they are great, so when the airplane is at altitude, the air is less dense, thus the prop needs a more aggressive angle to the air (coarse pitch) to actually ‘bite’ into the thin atmosphere?

So when pilots of for instance the Mk1 spit forgot to change back to fine pitch for take off, why would the aircraft struggle to take off, was it related to air resistance on the prop or the motor unable to achieve the correct RPM?

I may have further confused myself if I have the coarse and fine the wrong way around.:confused:

Here’s a Constant Speed prop itself – notice that there is no CSU or Beta valve attached to this! Effectively what you have in this end is a hoofing great spring which attemts to feather the propeller (turn the blades into the airflow), and oil pressure fed through the tube in the middle to the right hand side of the piston to progressively fine the propeller. As has been stated, the pilot requests the pitch of the propeller, and the CSU directs oil into (fine) or out of (coarse) the prop hub. Couple of pictures here: http://www.fly13.co.uk/Tug/VP%20Prop/Prop.htm

Starting up, the prop would be held in ground fine pitch (2-3 degrees), and as the engine runs up, the start locks disengage and allow the pilot to control the pitch. Fine is used for take-off and landing, and coarse would be used to change the speed of the aircraft in the cruise. (OK, it is a little more complex than that, but it’ll do). Further than the coarse range, we have an option to feather the prop (not on Spitfires….). Further back from Fine pitch, turboprop engines usually have a superfine and a reverse pitch…..

Hi Chris

That`s right, The DH Bracket propeller could be used in two position or constant speed mode, with CSU.
The pitch of a propeller needs to increase with both airspeed and altitude, as the air gets less dense. So a continuously variable system is always going to get the most efficiency from the engine-propeller combination.
A good example of this is the fixed-pitch wooden Watts propeller on the earliest Hurricanes and Spitfires, which was optimised for a certain altitude, perhaps around 10,000ft. This meant that the engine was at full throttle for take-off at only around 2200rpm, instead of the 3000 it was designed for.
The constant speed prop also has the advantage of being semi automatic. The pilot selects a climbing or cruising RPM, and the CSU, being a centrifugal governor device, keeps the engine speed constant with variation in speed and altitude, thus removing the need for the pilot to concentrate on the engine speed indicator.
That`s a very simple explanation, I hope it hepls.

Pete

It’s basically about having the blades at the best angle for the given engine revs and aircraft speed. Fine pitch was used for take-off and climb, with the blades at less of an angle (think of it as first gear in car terms), the pitch would then be more progressively moved to coarse, with the blade angle increased, as the speed increased. The advantage of an automatic “constant speed” unit is that it adjusts itself to the best conditions, a simpler “variable pitch” was operated by the pilot via a lever in the cockpit and this meant that the prop wasn’t always at it’s most efficient setting.

Nothing ‘basic’ about that question! 😀