Author Topic: Constant speed props II  (Read 1571 times)

Offline Straiga

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Constant speed props II
« on: May 07, 2004, 03:13:12 AM »
Well I was flying a 38 and prop rpm was set at 3000 and I nosed the plane over got it to compress and no increase in prop rpm now whats up with this. Everybody been saying that the prop will overspeed in this condition but I guess not. The rpm gauge goes to like 5000 rpm I think. But why didnt the rpm increase and whats the differance if it was set at 2000 and see an increase, but not at 3000 and no increase from there? Does anybody know or is this another bug.

Later

Offline hitech

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Constant speed props II
« Reply #1 on: May 07, 2004, 07:09:58 AM »
P38 has featherable props, hence they can continue to increase there pitch.

HiTech

Offline RRAM

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Constant speed props II
« Reply #2 on: May 09, 2004, 03:18:02 PM »
Hitech ,is there any multi-engined plane in AH with non-featherable props?.

I'm asking because I don't know if the bf110 had them IRL, and I guess that if it has them here, it had them in real life too :)

Offline Straiga

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Constant speed props II
« Reply #3 on: May 09, 2004, 04:41:47 PM »
Hitech,

 Im in the middle of researching the different blade stop on varius WWII fighters, when at idle power when the prop pitch is moved to the high pitch stop to see how much forward acceleration is produced.  
 Meaning that when the prop is in the normal prop range there is drag produced from the prop at idle power, but when the prop lever is moved to the full aft postion this drag is reduced because of blade angle deflection.  Because when a airplane has a engine failure and prop lever is moved full aft to the stops this will inprove glide angle and speed, because of reduced drag on the prop.

Offline flakbait

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Constant speed props II
« Reply #4 on: May 09, 2004, 07:10:13 PM »
Straiga, why you keep writing a dissertation to folks who know this stuff is beyond me. But since you asked about props, here's a quickie lesson...

The correct answer is a Curtiss Electric propellor. It had no mechanical stop at all since any blade angle change was done with an electric motor in the prop dome. Without a mechanical stop the blade can swing through a greater degree of angles than otherwise possible. Most (if I remember correctly) P-38 models used a Curtiss prop because it allowed full feathering without any oil loss. Plus the prop could swing through a much larger range of angles which would prevent over-speeding. Once you set a given RPM, the electric motor would rotate through a series of planetary gears at a ratio of 7,500:1 to change prop pitch. This pitch was held by a friction brake inside the motor. Given that the wiring and electrical motors back then weren't exactly great, a small short could cause all sorts of trouble. Unless the Curtiss was well taken care of by a very picky mechanic, the prop could run wild when the wiring or motor had a problem. When it worked, the Curtiss was probably the best prop on any WW2 aircraft. There was no wait time or lag when you throttled up; you had 2,700 RPM right now and it held that RPM setting regardless of what the engine did. Another good thing was the lack of any oil dependency. You could have the engine oil system full of holes, generators shot to bits, and the Curtiss would keep working as long as you had battery power. Once set, the Curtiss could hold a given RPM without any electrical draw at all. If you needed to change RPM all you had to do was kick the battery on for a moment, change RPM, and kick the battery back off.

Hamilton Standards and Aeroproducts props used oil, not only limiting them to a given pitch range, but also causing a rather severe problem if damaged. Later models of Hamilton had a provision to dump oil out from behind the prop dome to feather it in an emergency. B-29's were fitted with 14-foot Hamiltons that had a provision to dump oil in case you had to feather an engine.

Hamiltons were tied into the engine oil system, which meant that if you (somehow) dumped too much oil overboard the prop governor couldn't function at all. With no control over prop pitch the prop would run wild and damage or blow the engine. You also had to exercise the prop to make sure you got the oil sludge out of the prop dome before flight. Although most pilots didn't mind, the Hamilton had a throttle-up lag of a second or two between when you advanced the throttle for takeoff and when the prop responded by changing pitch to maintain RPM. On average, pilots liked the Hamilton because it was simple, had very few frills, and nearly always worked to perfection. Mechanics also loved the simplicity of how it was built. Oil simply pushed a control plate, attached to a bevel-geared cam, forward or backward to set a given RPM.

Aeroproducts also used oil, but didn't tap it off the engine oil system. They were completely self-contained so any engine damage wouldn't screw up the prop's operation. The regulator, oil, governor, oil lines, and valves were all contained in the engine nose. Only a direct hit on the prop itself would cause loss of operation. Unfortunately, if you look at a cut-away of any Aeroproducts unit, you'll find a horrifically complex nightmare of gears, cams, hydraulic pistons, and splined drives that would give the average mechanic migraines. Like the Hamilton, it had a mechanical stop the blades couldn't rotate past. But since it wasn't dependant on engine oil, crews tended to think it would be a bit more survivable in any event of oil loss.

You can get a LOT more info from America's Hundred Thousand on all three prop types, along with a cut-away view of what they looked like on the inside.



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Offline Straiga

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Constant speed props II
« Reply #5 on: May 10, 2004, 12:39:42 AM »
Flak bait,

 I do agree with what you say its sound like your the prop expert.

 The P-51 has the hammilton props and govener on it from the factory. These props were able to reach a blade stop pitch point. With an engine failure the prop can be move to a high pitch angle which had very little drag from the props. While the prop was windmilling in the airstream when the engine failed it provided enough oil pressure from the engine to get the prop to the high pitch stops. This was designed to give the airplane better glide distances in case of an engine failure. Now could this prevent prop overspeeds.

Offline joeblogs

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I think the question is more basic
« Reply #6 on: May 24, 2004, 09:08:03 PM »
The engine doesn't overspeed unless you overcome the propeller governor. Assuming the equipment is in working order and you have a constant speed prop, that won't happen.

Certain pre-war planes did not have constant speed units and it would be easy to damage the engine in a dive. Remember, the original spitfires had wooden props...

-Blogs


Quote
Originally posted by flakbait
Straiga, why you keep writing a dissertation to folks who know this stuff is beyond me. But since you asked about props, here's a quickie lesson...

The correct answer is a Curtiss Electric propellor. It had no mechanical stop at all since any blade angle change was done with an electric motor in the prop dome. Without a mechanical stop the blade can swing through a greater degree of angles than otherwise possible. Most (if I remember correctly) P-38 models used a Curtiss prop because it allowed full feathering without any oil loss. Plus the prop could swing through a much larger range of angles which would prevent over-speeding. Once you set a given RPM, the electric motor would rotate through a series of planetary gears at a ratio of 7,500:1 to change prop pitch. This pitch was held by a friction brake inside the motor. Given that the wiring and electrical motors back then weren't exactly great, a small short could cause all sorts of trouble. Unless the Curtiss was well taken care of by a very picky mechanic, the prop could run wild when the wiring or motor had a problem. When it worked, the Curtiss was probably the best prop on any WW2 aircraft. There was no wait time or lag when you throttled up; you had 2,700 RPM right now and it held that RPM setting regardless of what the engine did. Another good thing was the lack of any oil dependency. You could have the engine oil system full of holes, generators shot to bits, and the Curtiss would keep working as long as you had battery power. Once set, the Curtiss could hold a given RPM without any electrical draw at all. If you needed to change RPM all you had to do was kick the battery on for a moment, change RPM, and kick the battery back off.

Hamilton Standards and Aeroproducts props used oil, not only limiting them to a given pitch range, but also causing a rather severe problem if damaged. Later models of Hamilton had a provision to dump oil out from behind the prop dome to feather it in an emergency. B-29's were fitted with 14-foot Hamiltons that had a provision to dump oil in case you had to feather an engine.

Hamiltons were tied into the engine oil system, which meant that if you (somehow) dumped too much oil overboard the prop governor couldn't function at all. With no control over prop pitch the prop would run wild and damage or blow the engine. You also had to exercise the prop to make sure you got the oil sludge out of the prop dome before flight. Although most pilots didn't mind, the Hamilton had a throttle-up lag of a second or two between when you advanced the throttle for takeoff and when the prop responded by changing pitch to maintain RPM. On average, pilots liked the Hamilton because it was simple, had very few frills, and nearly always worked to perfection. Mechanics also loved the simplicity of how it was built. Oil simply pushed a control plate, attached to a bevel-geared cam, forward or backward to set a given RPM.

Aeroproducts also used oil, but didn't tap it off the engine oil system. They were completely self-contained so any engine damage wouldn't screw up the prop's operation. The regulator, oil, governor, oil lines, and valves were all contained in the engine nose. Only a direct hit on the prop itself would cause loss of operation. Unfortunately, if you look at a cut-away of any Aeroproducts unit, you'll find a horrifically complex nightmare of gears, cams, hydraulic pistons, and splined drives that would give the average mechanic migraines. Like the Hamilton, it had a mechanical stop the blades couldn't rotate past. But since it wasn't dependant on engine oil, crews tended to think it would be a bit more survivable in any event of oil loss.

You can get a LOT more info from America's Hundred Thousand on all three prop types, along with a cut-away view of what they looked like on the inside.



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Offline flakbait

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Re: I think the question is more basic
« Reply #7 on: May 25, 2004, 12:34:42 AM »
Quote
Originally posted by joeblogs
The engine doesn't overspeed unless you overcome the propeller governor. Assuming the equipment is in working order and you have a constant speed prop, that won't happen.

Certain pre-war planes did not have constant speed units and it would be easy to damage the engine in a dive. Remember, the original spitfires had wooden props...

-Blogs



Wrong. The engine WILL overspeed if you overcome the mechanical ability of the prop to change pitch. The governor will call for the highest possible blade pitch (feather or close to it), pushing the blades against a mechanical stop of some sort. Once they are against that stop the prop is not a constant-speed unit, it becomes fixed pitch. If you knew anything about props, you'd be well aware of this fact. Only the Curtiss can infinitely vary its pitch because it has no mechanical stop. The Hamilton uses oil to move a plate inside the prop dome, which in turn rotates a geared camshaft. Both of which have mechanical limits. The Aeroproducts used a piston inside the base of each prop blade to change pitch; the pistons have a limit on travel. Once that limit is hit it becomes a fixed-pitch prop. The propeller blades can't rotate any further, and as a result the prop begins back-driving the engine along the prop shaft. This can melt main crankcase and crackshaft bearings, boil the oil, and ruin the engine.

Run a search for the pre-cursor to this thread, started by Straiga. Or hit the link in my sig and visit Ground School. The information here and there is based off schematics of these prop types and discussions I've had with someone who has actually used all three in flight operations. Not to mention someone who's written more on the subject than most people alive today. The myth that a constant-speed prop can maintain any pitch at any speed no matter what is just that, a myth. Modern props can infintely change pitch, but this game isn't about modern aircraft or modern equipment.

Pelican's Perch; John Deakin used to write it. There you'll find descriptions and articles on manifold pressure, mixture, props, back-driving the engine with the prop, and several articles about actual warbirds flown by the Camarillo wing of the Commemorative Air Force. You'll also find a basic diagram of the Hamilton Standard prop's inner workings in a few of his articles.



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Offline Bodhi

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Constant speed props II
« Reply #8 on: May 25, 2004, 12:37:15 AM »
the only way the 38's props will fail too feather is if there is a complete loss of electrical power.  Which, because the 38 is has this wonderous brush system built onto the shaft is next to impossible or a complete loss of actual power (rare considering the generators are X2, or severing of the lines....  Now, that is null and void if the system is not maintained... then, the damn thing could be nightmare.
« Last Edit: May 25, 2004, 12:50:43 AM by Bodhi »
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Offline Bodhi

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Constant speed props II
« Reply #9 on: May 25, 2004, 12:43:38 AM »
here is a photo of the system broken down...

« Last Edit: May 25, 2004, 12:47:09 AM by Bodhi »
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Offline Bodhi

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Constant speed props II
« Reply #10 on: May 25, 2004, 12:46:42 AM »
here is one of the brush assembly itself.

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Offline Bodhi

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Constant speed props II
« Reply #11 on: May 25, 2004, 12:49:03 AM »
the actual prop motor....

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