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*OT* Variable Pitch Propellers *OT*

PRB

PRB

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This feature of modern airplanes has always been interesting to me. I'm reading a book now called “The Flight of the Mew Gull”, by Alex Henshaw. He flew this airplane in a record setting speed run from England to Capetown in 1939, and flew in the British air racing circuit before that. Not only is this book a great adventure story, it's also a fascinating peek into the engineering of fast airplanes in the 1930s.

One of the interesting things about a fixed pitch prop is that as you go above its optimum airspeed, the air loads on the prop actually slow down the engine! This is amazing. It's why the need for a VP prop was realized in the first place. And it makes sense. If you imagine a fixed pitch prop rotating at some RPM, there is a certain airspeed at which the air mass will pass through the plane of the rotating prop such that the “AOA” of the wind across the propeller blades is 0, meaning the prop is slicing through the air with minimum resistance. I would think that the optimum airspeed for a given pitch/RPM combination is slightly less than this “zero-AOA” speed. At speeds above and below this optimum, the air loads on the prop evidently “bog down” the engine and prevent you from maintaining maximum engine revolutions.

During the air racing scene in the 1930s, a lot of time and effort was spent adjusting the prop's pitch angle on the ground in order to achieve fast speeds, then testing it in flight. Once adjusted, however, that's where it stayed for the entire flight. Then some engineer put an air bladder in the propeller hub, and when the air loads built up on the prop, pressure was exerted on this bladder, which bled air out, and moved a prop pitch adjusting mechanism. But there was no pitch setting for the pilot to muck about with, and it presumably only worked once, for once the air is gone, there was no way to put the prop back into course pitch. Then they came up with electrically driven pitch controls, with only two positions “take off” (coarse) and “fast” (fine).

The “constant speed” propeller system came about in the late 1930s (I think?) Now we're talking about a system that automatically adjusts the prop's pitch to maintain the RPM dialed in by the pilot. Space age stuff, right there.

Isn't this fun? :jump:
 
I would think that the optimum airspeed for a given pitch/RPM combination is slightly less than this “zero-AOA” speed.
Click to expand...

But at zero-AoA you're not going to be generating much thrust either. It's more likely to be where the prop is experiencing a relative airflow of around 15 degrees. Note the reason prop blades have twist on them is to maintain a fairly consistent relative airflow across the whole blade, allowing for the increase in relative speed from the hub to the tip.

Apparently the RAF being run to a budget in the 1930s only had two position props, i.e. you could change it in flight from a take-off setting to a cruise setting. Fortunately around 1939/40 some bright spark pointed out if you drilled a couple of holes in the hydraulic manifold you could make it work over the whole range between those two points!
 
More to it of course....speed is one thing, efficiency is another.

To extend range, a variable pitch prop is an absolute necessity. Think of it as a transmission in a car. You need lots of power initially, lots of grab at the air. Once in cruise, you don't need near as much, and as pointed out, the angle of the prop becomes a hindrance.

This is a factor in Helicopters also, since one can only adjust the rotors so far...which physically limits you to a certain degree of speed.

The Eurocopter x3 and the Osprey are attempts to get beyond that limitation.

Space age stuff, absolutely!
 
I think that the experiments with the variable pitch started from the early 30's. A good example can be Polish competition plane RWD-9 which had 2-position prop pitch set on ground in the German VDM propeller. One setting served for the competitions which demanded very high acceleration and very short take off, the second one was for the competitions where the high top speed was very useful.
Another one, the PZL.37 "Los" bomber had variable pitch set manually by the pilot. The angle ranged from 23 to 30 degrees if I am not wrong. So, it still was not contant speed prop and could not be feathered. The early Messerschmitt Bf-109's had manual prop pitch as well, which, I suppose, required some attention from the pilot to avoid overreving the engine on too high RPM vs prop blade angle. In Spring 1940 the automatic prop pitch was introduced to the E-1's and E-3's.
On the other hand, the Hamilton-Standard company introduced the constant speed props from the late 30's and these became a standard.

Lucas
 
With the Jelly poppers the fact that one of the airfoils (the retreating blade) eventually will be standing still to the airflow and cause instability, loss of lift and so forth limits speed severly.

For an aircraft even with a constant speed prop, at some point in say a steep dive, the airloading will drive the engine and shaft speed will actually increase!

Cheers: T
 
djwaglik said:
I think that the experiments with the variable pitch started from the early 30's. A good example can be Polish competition plane RWD-9 which had 2-position prop pitch set on ground in the German VDM propeller. One setting served for the competitions which demanded very high acceleration and very short take off, the second one was for the competitions where the high top speed was very useful.
Another one, the PZL.37 "Los" bomber had variable pitch set manually by the pilot. The angle ranged from 23 to 30 degrees if I am not wrong. So, it still was not contant speed prop and could not be feathered. The early Messerschmitt Bf-109's had manual prop pitch as well, which, I suppose, required some attention from the pilot to avoid overreving the engine on too high RPM vs prop blade angle. In Spring 1940 the automatic prop pitch was introduced to the E-1's and E-3's.
On the other hand, the Hamilton-Standard company introduced the constant speed props from the late 30's and these became a standard.

Lucas
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I think variable pitch propellers started much earlier than that - around 1910 on airships. I think you're thinking about constant speed propellers, which are similar, but different.
 
Some bush aircraft of the 20 and 30's had ground adjustable pitch props. The pilot/mechanic got out his wrench and moved the fixed pitch to a new value, depending for instance how much low speed thrust was needed for that short strip....

T
 
Ground adjustable have been around as long - longer actually - than heavier-than-air aircraft. That's why they were made of wood - they could be `adjusted` by filing, sanding and shaving, or cutting a new one. The science of propellers was years ahead of the aircraft they hung them on.

The definition of variable pitch usually means in-air-adjustable by the pilot. It wasn't needed in early aircraft because the speed range was too low to need to worry. But as aircraft started to get faster the need increased, driven mainly by air-racing between the wars.
 
I would like to point out on rotorcraft that the aerodynamic limits of the blades are different as the rotor blade is used for different purposes than a propeller. I am not a fixed wing guy, so I cannot help with that. With aircraft propellers though as you generate forward airspeed they are not the load bearing structure of the aircraft (the wing is) so as you approach their stall point, or max efficiency it is not such a big deal. Most props spin at a couple thousand RPM, and speeds at the tips can sometimes reach the speed of sound. As far as a rotor goes the rotor disc itself is the primary load carrying structure of the aircraft, and also the blade chord profile is thicker than a regular prop. For this (following is for the H-60, so disregard for other communities) the 258RPM puts the rotor tips at about .7 Mach static. As you increase airspeed though at some points the rotor blade tips can reach to speed of sound as well just like the prop. Now the interesting part to that is because the rotor disc is what carries the aircraft you can create an imbalance at high speed with the retreating blade known as retreating blade stall. Some of the things they have put in the design of rotor blades is really interesting. There is a slight twist as you look spanwise down the blade, but hardly noticably unless you are right up on it. Along with sweeping the tip cap trailing edges 20 degrees to reduce sound/improve efficiency. The japanese have taken this one step further and put what I refer to as reverse winglets on the end of the blades to improve efficiency further. I included a link to a photo as I think those tip caps are pretty cool.

Other than that is about all I have to add. I find the whole prop discussion pretty interesting in itself. I am hoping to go work on some turboprops birds later in the year finally. It is always a double edge sword when you talk about adjustabilitiy in aircraft. Cause the added weight/complexity of the prop hubs and drivetrains add huge maintenance/reliability implications. The adjustable blade pitch does allow fixed RPM's, and a wider flight regime. As far as aerodynamics and origins behind it though I am clueless.

http://img2.blogs.yahoo.co.jp/ybi/1/3d/1e/gala8357/folder/961405/img_961405_46940845_1?1230870669
 
Multiengine prop planes also have a great need for the ability to feather to meet engine out climb and controllability issues associated with engine failure.

With the advent of turboprops which use internal mass flow for cooling rather than relying on external air velocity, reverse became a more practical device, though some piston installations (such as DC6) were so equipped. Such props becme complicated in design and operation as several pitch locks and additional controls are necessary to provide sufficent safety and reliability.

The early RR Dart powered F-27 (and similar YS-11) had only a ground fine to use prop the flattened prop disk to provide aerodynamic braking. Later model aircraft I flew such as the DHC-7 and Lockheed L-382G (Herk) had truly reversable pitch props.

Dang, my Supercub on wheels, skis or floats has truly spectacular takeoff acceleration with it's large and finely pitched Borer prop, but the cruise performance is equally less exciting. Some 180 hp units do have constant speed units, at additional weight and perhaps still inferior prop efficency for takeoff. But the cruise is much improved.

Airplanes are tradeoffs, optimized for some specific mission. Less "good" away from that design parameter.

T
 
fliger?? Didnt the B-337 have some those incredibly complex props attached to the Waspmajors they used? and wasnt that one of the main problem area that existed with that plane??
 
Brakes of the era were really not very good, and have lagged behind heavy and high performance aircraft till the advent of the current carbon brakes as used on the 747-400 and contemporaries. Hence the interest in reverse thrust as an aid in stopping. Yes the 337 did have rather complex props, but probably wasn't the major issue with the plane. The turbo compound engines reached about the limit of maintainable sophistication and were right at the reliability boundary.

Today, the somewhat less sophisticated "C" series R2800 powered aircraft (DC6, C46) are still flying whereas the R3350 and R4320 aircraft fell out of commercial favor more than half a century ago.

Off to fly the dreamlifter.... later: T
 
The japanese have taken this one step further and put what I refer to as reverse winglets on the end of the blades to improve efficiency further. I included a link to a photo as I think those tip caps are pretty cool.
Click to expand...

The link's not working, have you got another one!?
 
On a related note, I'm now reading Spitfire, a Test Pilot's Story, by Jeffrey Quill. Good book, BTW. In it he speaks of the Spitfire's first flight:

“...Given the serial number K5054, it was fitted with a special fine-pitch propeller to ensure a safe take-off run and to minimize swing due to propeller torque during take-off...”

This I don't get. How would the pitch setting of the propeller affect torque? Surely the only things affecting toque are rpm and the size of the prop, no?
 
Torque is the turning force imparted by the propeller, if you think of it like a paddle wheel on an old river boat the coarser the pitch the more rotational force the prop will impart to the air stream. So using a fine pitch prop will reduce the problem, at the cost of limiting your top speed if you've only got one pitch setting! On variable pitch aircraft it's less of a problem although aircraft like the Sea Fury could flip you on your back if you suddenly applied full throttle at low speed even with the prop in it's fine position.

As an aside I know it was a problem on the later Seafires with the Griffen engines as one gear oleo would be noticeably more compressed than the other at take-off power, hence the move to a contra-prop on the last few models.
 
This is interesting. Torque is a rotational force. The power imparted by the engine against the crank shaft is torque. And Newton's third law says the airframe will experience a rotational force in the opposite direction.

A “fine pitch” prop, in the context of Quill's statement above, means the propeller blades are turned "sideways" to the wind, the “opposite” of a feathered prop, right? This would mean the engine would have to develop less power to turn the prop at a given RPM, which would equate to less torque. But it would also mean less thrust produced by the propeller, which does not equate to a “safer take-off”, which was the other reason Quill stated the fine pitch prop was used for the first flight.

I think my understanding of prop pitch may be backwards... If you can only attach a fixed pitch prop to your plane, and you desire a propeller better at take-off performance, at the expense of speed at altitude, then I thought you would want a prop angled such that more “bite” is taken from the air, i.e., one with blades angled at 45 degrees (more or less) right?
 
Paul:

A propeller is an airfoil and like a wing has an optimum AOA for maximum lift over drag. Rather than thinking of pitch of a static propeller one must think of the propeller as a corkscrew. The angle of the relative wind is the important factor. As the aircraft speed increases, the angle of the relative wind to the prop at any given rotational speed changes. So for a fixed pitch prop there will be just one true airspeed that is optimal. At low airspeed the AOA will be too high and more drag than lift (thrust), at too high an airspeed the AOA will be quite low and little lift will be produced.

T
 
But it would also mean less thrust produced by the propeller, which does not equate to a “safer take-off”, which was the other reason Quill stated the fine pitch prop was used for the first flight.
Click to expand...

It depends on what you call a safe take-off! As long as it's producing enough thrust to get airborne, with a safety margin, that's probably safer than having too much thrust.
Additionally with the prop fined off the engine should accelerate and decelerate faster allowing easier control of aircraft speed. I seem to remember* when I've flown aircraft with constant speed props it was set in the fine position (max RPM) before entering the circuit for this reason as it makes the go round safer.

*I think it was last in 2002 unless you count helicopters.
 
Whats confusing me about this whole conversation is what i'm seeing in fsx as to opposed what i believe is seen in real life..
in FSX: with props set at 100% pitch, you obtain maximum rpm, and at 0% pitch you obtain your minimum rpm.
in a real plane ( correct me if i'm wrong ) when the prop is set at 100% pitch, you have your highest amount of pitch on the blades, which creates greater friction on the engine and reduces the rpm, and when you have o% pitch you have the minimum amunt of pitch on the blade and minimum amount of friction on the engine, increasing the rpm. ( the blade is also flatter in relation to the air flowing through it ). Yet, afsd reports that when you have the prop levers at 0% your pitch angle on the blade is at maximum pitch which slows the engine down and when your prop pitch is set to 100% the blades are at minimum pitch, and the engine speeds up. However the confusing part is that, with the prop pitch set to 100% ( minimum pitch ) your able to achieve your highest speeds while at 0% pitch you lose speed and slow down.. Thats kinda bassackwards to me..
 
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