Author Topic: Constant Speed Props  (Read 3655 times)

Offline Straiga

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Constant Speed Props
« on: April 07, 2004, 09:07:41 PM »
I have noticed in AH, say your in cruise flight and set the prop RPM, say at 2000 RPM and manifold pressure set constant that when you push the nose over and increase the speed of the plane that the prop RPM will increase too! This dicription  is a fixed pitch prop not a constant speed prop like the ones used on WWII fighters

 When you set the prop RPM on a constant speed prop system, you move the prop handle to set the rpm then the prop governer maintains that rpm with either oil pressure through the the prop governer to set blade pitch or by counter weights.

The points is when the prop governer is set to a rpm setting and with an unchanged manifold pressure setting(No throttle change) what ever your going to do rolls, spins, loops, high speed nose dives, slow speed climbs the prop rpm will remain the same as it is set until a new rpm setting is set.

 I have also noticed that in accellerated stall condition the airplanes rolls to the inside of the turn when the stall accurs, this should be opposite.

 The lower wings has less angle of attack then the high sided wing when high Gs are pulled to a stalled condition the high side wing stalls the lower wing is still producing lift so the airplane rolls in the opposite directions of the turn. This is an accellerated stall condition.

Offline flakbait

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Constant Speed Props
« Reply #1 on: April 10, 2004, 01:30:50 AM »
Surprise! When you increase speed far enough to allow airflow to turn your prop, it becomes a fixed-pitch prop. Why is this? If you set 2,000 RPM and then dive to 450mph, the airflow is moving so fast that it pushes more air against the prop blades. This increases RPM. The prop governor then dials the prop pitch clear down to the stop in an effort to maintain last-set RPM. Unfortunately, you're going so fast that it has no effect because the governor can't feather the prop. So your prop is now fixed-pitch. Congrats! You melted the main crankshaft bearing and ruined the engine. See this article for a more thorough explination...

http://www.avweb.com/news/columns/186778-1.html

Also, your description of a prop governor is all wrong. A prop governor uses fly-weights on a central shaft to actuate an oil valve that controls oil pressure to the prop hub. At least on Hamilton-Standard props. The Aeroproducts (P-51's) used a system enclosed behind the prop hub, while the Curtiss Electric used an electric motor. By moving the oil control valve (or electric motor) the prop blades engaged a central shaft via a beveled gear. Only the Curtiss could go to full feather without dumping oil. Late-model Ham-Standards used an oil dump system that ported oil from behind the hub overboard in an emergency. Otherwise there was always enough oil behind the hub plate for idle-RPM.

Accelerated stall are not a "high/low" wing condition. An accelerated stall happens when you crank the wing AoA up so fast that the airflow over the wing becomes disrupted. One wing (usually the left) will stall out first because the prop wash is not even. It spirals off the prop and over the wings, creating uneven lift.

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« Last Edit: April 10, 2004, 01:39:38 AM by flakbait »

Offline Straiga

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Constant Speed Props
« Reply #2 on: April 13, 2004, 01:49:43 AM »
During my LOA for a P-51, this was demonstrated . The prop govener works the way its supposed to, set the RPM then it maintains the RPM Hence the name Constant Speed Props. We set a constant manifold pressure and a sett RPM we did power on stalls, rolls, dives, chandels you name it the prop maintain the same RPM.
 Pilots in WWII did not have time to adjust rpm all the time they usually set RPM and had there hand on the trottle for power when they need it. They didnt have to worry about RPM settings is was set already.
 This article you are mentioning is about prop pull and prop push on the were and tear on the bearing assemble and crank and piston. When Either the Engine pulls or Prop push on the engine assembly. It also talks about shock cooling in big radials on desent. What power and RPM settings to maintain a constant Temp on an normal asperated engine not as bad of problem on a supercharged engine they run hotter and you can maintain cooling better on desents.
There are also vibration settings to stay away from that would set up bad resonence in the engine and do major damage. But nothing said anything about prop over speed because of airflow through the prop disk. This is a problem on an fixed pitch prop but not on an constant speed prop.
 A constant speed prop is what it means constant speed. Prop pitch varies to maintain a constant speed to a set RPM. Airflow then pushes the prop into the crank (prop push) this is what the article is talking about where you ruin bearings and the such, but prop RPM remains the same. With a constant speed prop system you can shut down the engine and go to full feather anytime you want, when you start the engine and go to prop pitch again the accumilaters will take it out of feather so you can set the RPM again.

Offline Straiga

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Constant Speed Props
« Reply #3 on: April 13, 2004, 02:08:30 AM »
Accererated stalls are yes an ubrubt disturbance of airflowover the wing. You can stall an airplane at any airspeed any altitude and any angle of attack. But in a high "G" turn the lower wing is at a lower angle of attack then the high sided wing which has stalled first and the low wing climbs and rolls the airplane in the opposite direction of the turn. Power or power of no matter.
 This is  demonstrate to all my student pilots so they can demonstrate this to the FAA so they can get there Pilot certificate. Its in the PTS for certification.
 Its funny that the airplane either in a left or right hand turn  always rolls to the high sided wing every time. Try it.

Offline HoHun

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Constant Speed Props
« Reply #4 on: April 13, 2004, 11:59:45 AM »
Hi Straiga,

>The prop govener works the way its supposed to, set the RPM then it maintains the RPM Hence the name Constant Speed Props.

From the Pilot's Notes for the Spitfire II:

"(i) Constant-speed airscrew.- At maximum r.p.m. 3,000 , the throttle must be 1/3 open.  The pitch control need not be brought back to reduce r.p.m., the range of pitch is enough to hold down the r.p.m. at any airspeed."

Note that this is at reduced throttle. Add more throttle, and the propeller will overspeed.

Regards,

Henning (HoHun)

Offline Tilt

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Constant Speed Props
« Reply #5 on: April 15, 2004, 07:26:14 AM »
Quote
Originally posted by HoHun


"(i) Constant-speed airscrew.- At maximum r.p.m. 3,000 , the throttle must be 1/3 open.  The pitch control need not be brought back to reduce r.p.m., the range of pitch is enough to hold down the r.p.m. at any airspeed."

Note that this is at reduced throttle. Add more throttle, and the propeller will overspeed.

Regards,

Henning (HoHun)


If the pilot has separate pitch control (rather than pitch control from a governer driven by rpm setting) is it really a constant speed prop? isn't it really a variable pitch prop?

The difference is between a pilot that has to balance throttle and pitch (ignoring mixture) to arrive at optimum rpm and boost whilst ensuring that neither exceed limits and a pilot that sets rpm and then varies throttle to set boost confident that neither will exceed max setting whilst always able to reach it.
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Offline Tilt

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Constant Speed Props
« Reply #6 on: April 15, 2004, 07:33:21 AM »
Quote
Originally posted by Straiga
But in a high "G" turn the lower wing is at a lower angle of attack then the high sided wing


Still tryin to get my mind round this............... I am going to have to draw it........ I always thought that the lower wing had the higher angle of attack and was marginally slower than the higher wing. (assuming the turn is maintained in the horizontal plain)

Hence I believed the lower wing lost lift...............
« Last Edit: April 15, 2004, 07:47:41 AM by Tilt »
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Offline Tilt

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« Reply #7 on: April 15, 2004, 08:03:40 AM »
This may explain something .........


Quote


Students are taught to avoid steeply banked turns at low altitude. If you overshoot the extended centreline on a turn from base to final, there is a tendency to “cheat” by applying inside rudder to increase the rate of turn – which requires opposite aileron to maintain the bank angle. The skidding turn tends to make the nose drop requiring back pressure on the control column.

In an extreme case, the result can be a full back control column with full opposite aileron and full inside rudder. The inside wing will stall first resulting in a sudden incipient spin. This is sometimes referred to as an “under the bottom stall”.

A top-rudder stall or “over the top stall” can occur when the aircraft is slipping. The aircraft should roll towards the higher wing at the point of stall.


From elsewhere it seems that the direction of roll is more dependant upon the element and direction of "cross control".

We all know we can induce a very unstable but high rate of turn thru ruddering with a turn (ie down wards)and compensating for lift with aileron and increased elevator. It seems this induces a different roll direction upon stall to that of a more conventional turn where rudder is used to kep the nose up.

I wonder if the use of a twisty grip rudder influences the type of  cross contol used when in combat.
« Last Edit: April 15, 2004, 08:23:50 AM by Tilt »
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Offline mars01

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Constant Speed Props
« Reply #8 on: April 15, 2004, 09:40:53 AM »
Quote
Surprise! When you increase speed far enough to allow airflow to turn your prop, it becomes a fixed-pitch prop. Why is this? If you set 2,000 RPM and then dive to 450mph, the airflow is moving so fast that it pushes more air against the prop blades. This increases RPM. The prop governor then dials the prop pitch clear down to the stop in an effort to maintain last-set RPM. Unfortunately, you're going so fast that it has no effect because the governor can't feather the prop. So your prop is now fixed-pitch. Congrats! You melted the main crankshaft bearing and ruined the engine. See this article for a more thorough explination...



AHHHHH so this is why in IL2 when you are in a steep fast dive you hear the RPMs increase.  It is a very cool affect and heightens the immersion factor.

Offline flakbait

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Constant Speed Props
« Reply #9 on: April 15, 2004, 09:58:34 AM »
Quote
Originally posted by Tilt
If the pilot has separate pitch control (rather than pitch control from a governer driven by rpm setting) is it really a constant speed prop? isn't it really a variable pitch prop?

The difference is between a pilot that has to balance throttle and pitch (ignoring mixture) to arrive at optimum rpm and boost whilst ensuring that neither exceed limits and a pilot that sets rpm and then varies throttle to set boost confident that neither will exceed max setting whilst always able to reach it.



If you have a direct pitch control, instead of an RPM control, then it's a variable pitch prop instead of a constant speed prop. I think the only AC we have that used this sort of system was the Bf-109. All the rest had a constant-speed.

Constand speed props set engine RPM to a given point, and through either oil or an electric motor the prop pitch is changed to maintain that RPM. Hence the reason over-speeding a prop can be a VERY bad thing for your engine. Variable pitch props only controlled prop pitch, not engine revvs, which made flying with this prop a lot like playing a three-handed piano. Adjusting pitch meant the engine speed would change, so you had to fiddle with the throttle and mixture to avoid over-revving it.

As for the under-bottom stall, it does happen. However it only happens when you crank in a lot of rudder at very low airspeed and rather high bank angles. Usually in AH the tendency is for the aircraft to stall on the left wing (right wing for the bass-ackwards engines). So you can dial in some rudder to increase turn speed without getting into a stall. However, I've never seen or had this happen to me and I do use a twisty stick. Then again, I don't like playing the low-speed stall-fight games some guys prefer. I've used some rudder to delay an oncoming stall in a turn more than once during practice. But I've never had an under-bottom stall occur. It might be a difference between AH modeling and the way a real aircraft handles. I'll check it in AH2 to see if this characteristic is present.

Striaga: The prop pitch in any aircraft is limited by a physical stop inside the prop dome. You can not get the prop angle below that stop, it's impossible without a feathering system. Also, when you increase airspeed with a low RPM setting, eventually you'll hit the point where the prop begins to drive the engine. When this happens you have a very finite amount of time to either firewall the prop lever or start dumping speed. 30 seconds was the limitation in most engines for this to happen, typically at 3,050 RPM and no higher. If you failed to take any action the crankshaft main bearing would melt, ruining the engine.

Now if you set 2,000 RPM with 48" MAN and proceeded to dive, the prop angle is set too high. As you pass a given airspeed, the engine/prop is no longer generating enough thrust to offset the increase in speed. So the prop starts turning faster (driving the engine with the prop shaft) while the governor tries reducing the prop angle. Once the adjustment plate inside the prop dome hits the fixed stop on the cam the prop becomes fixed-pitch. All WW2 era props except the Curtiss Electric had/have a mechanical stop. The governor can not adjust the prop angle any lower, and your engine speed starts climbing. This is why you saw the RPM gage climbing after setting 2k RPM and diving.

Try it in any constant-speed prop aircraft that can move faster than 200 knots. Pull the RPM back to 1,500 and dive the aircraft until you see the RPM climb. It can and will happen. Constant speed props have mechanical limits that most people never see simply because they aren't that dumb. No one wants to have their crank sieze up or plow a hole in the ground. Take the time and actually read that article I posted, along with several others Deak has done on props.


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Offline Tilt

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Constant Speed Props
« Reply #10 on: April 15, 2004, 10:30:57 AM »
Quote
Originally posted by flakbait
If you have a direct pitch control, instead of an RPM control, then it's a variable pitch prop instead of a constant speed prop. I think the only AC we have that used this sort of system was the Bf-109. All the rest had a constant-speed.



la5Fn pilots had to vary pitch and throttle (and mixture) separately.

la7 introduced the constant speed prop and latterly linked mixture to throttle control.

I believe numerous early war ac had variable pitch props but not constant speed props.......... I always thought early spits and hurricanes were without it.
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Offline HoHun

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Constant Speed Props
« Reply #11 on: April 15, 2004, 01:53:18 PM »
Hi Tilt,

>If the pilot has separate pitch control (rather than pitch control from a governer driven by rpm setting) is it really a constant speed prop? isn't it really a variable pitch prop?

The name "pitch control" is an anachronism. In other places in the manual, it's called "airscrew lever".

From the same manual:

"Airscrew.- This is a Rotol variable-pitch type governed by a constant-speed unit. The pilot's control regulates the engine r.p.m. through the action of a governor unit which controls the airscrew pitch according to the power output and in relation to the airspeed."

Note that the propeller itself is considered "variable pitch", but the entire system is "constant speed".

Regards,

Henning (HoHun)

Offline mars01

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Constant Speed Props
« Reply #12 on: April 15, 2004, 02:01:37 PM »
So is AH and AHII modeled incorrectly, since you don't hear RPMs increasing when you are in a steep 400+ MPH dive?  Like IL2 does.

Offline hitech

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Constant Speed Props
« Reply #13 on: April 15, 2004, 02:20:21 PM »
It does increase with speed.

HiTech

Offline mars01

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Constant Speed Props
« Reply #14 on: April 15, 2004, 02:29:16 PM »
I have not noticed the sound increase, I know you guys were still working on rpm and sound etc.