What are you supposed to do?

[Kweassa]

I always get confused about these situations...


 Question:

 You are flying a P-51D.  You have engaged in a very steep dive at 100% throttle, and the speed is now passing over 500mph. The pressure from the air is too much to take, and despite the constant speed unit your RPM level starts to grow.

 What are you supposed to do in this situation?



(1) Lower the prop pitch, leave the throttle there. This will slow down the rotation of the props and help prevent your plane from overheating.

(2) You shouldn't lower the prop pitch. By lowering the prop pitch under those circumstances your prop will attempt to slow down at a situation where slowing down is impossible in the first place. Therefore there is a danger you might damage the prop governers. Another problem is by lowering the prop pitch you actually decrease the drag of your plane in such a deep dive, which will in turn speed you up even more, and thus put even more pressure to your plane.

 Therefore, you should throttle back first, and then attempt to get out of your dive without changing the prop pitch.

(3) Lower both your prop pitch and throttle. First lower the throttle, and then lower the prop pitch. This shouldn't give you any problems if the airframe can withstand the speed - you can continue diving.

(4) You should attempt to get out of the dive touching both the prop pitch and the throttle.

 
 
 
 Which one is it?

[BaDkaRmA158Th]

Engine 5% full left or right rudder.
Angle your plane 45 degree,and try to pull back.

If you apply left rudder,pull your stick to the right & back, this will force the nose to the left,your left wing up,your right wing down.
you will slow enough to recover, also works great bleeding E from a quick landing to a slow one.


Maby not what you were askin' but..yeah.

[OOZ662]

He's asking about engine settings related to damaging the engine and propeller equipment in a high speed dive.

[dtango]

Constant speed props operate under the principle that if speed increases then prop pitch increases (blade angle of attack).  If speed decreases then prop pitch decreases.

For a constant speed propeller like the Hamilton Hydromatic on the P-51D set to automatic control when airspeed increases the propeller governor should automatically increase the blade pitch in order to increase the prop torque.  This makes it harder to turn the prop and thus keeps the engine at the RPM setting desired.  

If in a dive and the forward velocity gets high enough so that the propeller reaches the mechnical stops for increasing blade pitch then a condition for engine overspeed can occur.  

The propeller blade is nothing more than a wing that produces thrust via a lift force as the relative airflow strikes the blade at some angle of attack.  Unlike a wing the propeller motion through the air produces both a forward velocity and rotational velocity perpendicular to each other which creates this resultant relative airflow at some aoa to the propeller.  The aoa is controlled by blade pitch.  At some given forward velocity the aoa of the blade becomes negative which then causes the propeller to windmill and turn even faster.  For a constant speed prop blade pitch is increased to offset this.  However If the propeller reaches the mechanical stops for blade pitch then the prop will turn faster than the set RPM because blade aoa cannot be increased to slow the shaft rpm down.

In this case the pilot should reduce manifold pressure and pull out of the dive to slow the aircraft down.
 
In this case I don't think changing engine RPM setting will do anything because the prop governor is already sending signals to increase load on the engine by increasing blade pitch.

Tango, XO
412th FS Braunco Mustangs

[Puck]

Steps to take in this situation:

1. Drop Bombs.  Think them into the target you were actually trying to hit, which is 1,000 yards to the left and behind.

2. Fire rockets wildly on the off chance they might hit something.

3. Yank back as hard as you can on the stick.

4. Attempt to fire wildly into the air, never actually realizing your guns took off with your wings.

5. Crash.  Some unsuspecting victim will get the proximity kill.

6. Get on channel 200.  Accuse said victim of cheating, being a dweeb, and threaten to hunt them down and kill them repeatedly.

7. Ignore all invitations to enter the dueling arena to facilitate the execution of said threats.

8. Wash, rinse, repeat.

[Puck]

Originally posted by dtango

 
In this case I don't think changing engine RPM setting will do anything because the prop governor is already sending signals to increase load on the engine by increasing blade pitch.

Tango, XO
412th FS Braunco Mustangs

Aren't the engine and propeller somewhat had-linked together with metallic gears?  I'm not aware of any kind of clutch arrangement.  

That assumes WWII fighters used constant speed props, and I'm not sure of that either.  Engine management tended to focus on manifold pressure and propeller RPM.  If they were constant speed that would become a single variable equation.

Gas turbines use CVPs.  There is a limited range of RPMs the engine will spin, from there you control thrust with pitch.

I have far more questions than answers on this one.

[dtango]

Originally posted by Puck
Aren't the engine and propeller somewhat had-linked together with metallic gears?  I'm not aware of any kind of clutch arrangement.  

That assumes WWII fighters used constant speed props, and I'm not sure of that either.  Engine management tended to focus on manifold pressure and propeller RPM.  If they were constant speed that would become a single variable equation.

Gas turbines use CVPs.  There is a limited range of RPMs the engine will spin, from there you control thrust with pitch.

I have far more questions than answers on this one.

For a prop in constant speed operation there isn't a direct RPM control for the actual engine shaft.  RPM's are controlled via the propeller governor that senses how fast the shaft is spinning and then adjusts the propeller pitch that either creates more force or less force for the engine to oppose.  The force created by the prop varies with the amount of "lift" (thrust) that it is producing.  The higher the aoa of the propeller blade, the more lift created, the more force needed to spin the prop.  The opposite is also true.  Using these forces the prop governor then maintains the engine RPM at the set speed by adjusting blade aoa/pitch as the shaft overspeeds or underspeeds the desired RPM.

This isn't the only control on the engine and manifold pressure / throttle still exists. Manifold pressure controls air intake into the engine for combustion.  How fast the crankshaft spins is partly a function of how much combustion is allowed in the engine.  But the crankshaft RPM is also regulated by the amount of force needed to spin the propeller which is controlled by the propeller speed governor.  Another way to look at it is that the manifold pressure controls the amount of power that is available to spin the prop while RPM/prop pitch controls the amount of power required to spin the prop.

Constant-speed propellers were an important aviation development which became commonplace very quickly in WW2.  Constant speed props improved aircraft performance by maximizing the efficiency of the propeller over much broader spectrum of airspeeds (specifically broader range of propeller advance ratios).  

One of the more famous adoptions of the constant-speed prop occurred in mid 1940.  The Bf-109E had an electrically operated constant speed unit which maximized it's performance and enabled it to outclass the Spitfire Mk I.  Around June the RAF implemented in mass the constant speed prop on the Spitfire Mk I which increased it's rate of climb by 730ft/min.  This boosted the performance of the Spitfire and enabled it to go toe-to-toe with the Bf-109E just in time for the Battle of Britain.

Here are two charts that show aircraft that used the Hamilton Hydromatic, one of the more prolific constant speed props during WW2.





Tango, XO
412th FS Braunco Mustangs

[flakbait]

What dtango said in his first post. Throttle back and start your pull-out, and don't touch that blue knob (pitch).

Pulling pitch back won't do anything; the force of airflow is already moving faster than the governor can possibly compensate for. You can't increase pitch because the prop blades are at their mechanical stop and going faster by the second. If your RPM is increasing, you're already in trouble. Leaving it be isn't a good thing either. As engine speed increases, so does friction. In this case, it increases to the point the bearing surface between the crank journals and block bearings heats up the oil. Leave it too long, and your oil takes on the consistency of half-frozen molasses. This bogs down the crank, which is already loaded up with an outrageous amount of torque. So, Mr. Crank meet Mr. Bearing at 3,000+ RPM. Engine barks to a stop, prop slams to a halt, crank gets ruined, crank bearings are gone, and you've got exactly zero oil pressure to control that Ham Standard or Aeroproducts prop! Meaning your "Constant Speed" propeller just became a fixed-pitch prop.

A classic case of back-driving the engine. John Deakin wrote about this many moons ago over on http://www.avweb.com in a very detailed article. You have to sign up to read it, but it's still there. Hit their Columns section and check the Pelican's Perch archive. Hell, anyone remotely interested in WW2 engines or props needs to sign up and read Deak's articles.


Puck,
US aircraft in WW2 used one of three constant speed prop designs. The Hamilton Standard, which was operated by engine oil pressure. The Aeroproducts, which had it's own oil pump and reservoir. And the Curtiss Electric, using a double planetary gear system coupled to an electric motor that used contact brushes and a slip-ring to get power from the aircraft generator. All three had their pluses, all three had their problems, but WW2 aircraft did have them.

If you want a real eye opener, take a gander at the Fw-190. That little bird used a doohickey called a Kommandogerat (sp?) which controlled prop pitch and mixture as well as throttle. One lever for three functions, in 1941!



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Flakbait [Delta6]

[Kweassa]

Ahhh.. don't touch pitch, throttle back down.

 Thanks for the replies.

[dtango]

Flakbait:  Totally agree on Deakin's articles.  Those Pelican's Perch articles are outstanding!

On a tangent for Kweassa, my guess is that a runaway prop worried pilots more than overspeeding the engine in a dive.  If the governor failed for a variety of reasons forces on the propeller would cause the pitch to go "flat" (low pitch).  The propeller would spin like crazy at very high RPM's since very little prop torque now opposed the engine power to turn the crankshaft (akin to a bicycler peddling like mad at the lowest gear but getting no where fast).  The runaway prop could lead to engine failure or prop blades to the entire prop sheering off - nasty things that make a pilot go "ahhh $%@#&!!".

Runaway propellers afflicted all manner of aircraft with no prejudice.  As examples the P-38 had one of those electrical governor types and a reduction or a loss of electrical power could cause the governor to fail resulting in a runaway prop.  Apparently the P-39 was infamous for having a flaky electric governor that caused the prop to runaway as well.  The Tempest had teething issues with it's hydraulic based propeller where the seals would fail and blow under pressure.  A runaway prop would ensue and the pilot would be greeted by bits of metal coming out the exhausts as the engine tore itself up!  Plenty of stories about US bombers exist with runaway props either from battle damage or just pure mechanical failure as well.

Anyway, thought folks might be interested!

Tango, XO
412th FS Braunco Mustangs

[flakbait]

dtango, I believe you're thinking of the Curtiss electric in regards to P-38 prop trouble. There's a reason for it, and a reason that transport/bomber crews along with fighter pukes weren't too fond of the Curtiss. You see, the Curtiss needs a lot of electrical power to operate, more than just the battery could provide. Crews didn't know this at first, nor did they know just how much of a difference a good prop mechanic could make in how it operated. So they did what they always did. Fire the engine up, put away the portable generator (putt-putt), taxi out to the runway and do their takeoff checks. This naturally included a prop check to make sure the Curtiss was doing what it was supposed to. However....

At low RPM the generator isn't getting "excited" enough to generate electrical power. Something that doesn't happen until around 1,800 RPM. As a result, the battery would get drained down when the crew performed a prop check. At takeoff this could really cause problems. As the aircraft engine wound up, the governor would signal a pitch change to the electrical motor and friction brake. Small problem: the engine wasn't turning fast enough to spin up the generator. Where'd the juice come from? Battery. Big problem: the battery had enough power to trip the friction brake, but not enough to drive that electric motor and change pitch. Result? The famed "Curtiss runaway!" Once pilots and mechanics realized what was going on they did things a bit differently. No more prop checks without the putt-putt running, no more maintenance checks on the prop without a substantial electrical source present, and the mechanic had to eyeball the brushes, slip-rings, friction brake, and contact points very carefully.

It did, however, have two fabulous perks. First, you didn't have to wait 2-5 seconds for oil pressure to come up before getting max RPM. You had absolute max RPM and you had it now. To boot, it could be operated either as a constant speed prop or as a fixed-pitch prop. Toggle the switch from "Auto" to "Manual" and you could set the exact pitch you wanted. Toggle it back, and the governor ran the show. Really a great piece of hardware when it was well taken care of.



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Flakbait [Delta6]

[AquaShrimp]

I read a book about a squadron of B25s in the Pacific.  If a B-25 was hit in the engine, and it began leaking oil, the pilots usually wouldn't make it back to base if he didn't immediately shut down the engine.  Apparently to feather the prop, you need a substantial oil pressure.  An unfeatherable prop at the very least would cause the aircraft to run out of fuel before making it back to base.

[dtango]

Great info flak!

Shrimp: The B-25 used the Hamilton Standard constant speed prop which was an hydraulic prop using oil.  The prop governor used the engines oil pressure to change the propeller's pitch.  Lose your oil then you lose the ability to change the propeller's pitch which is bad because you could either end up with a runaway prop (as discussed above) or a windmilling prop.  

Without oil your engine eventually quits too.  If your prop wasn't feathered after losing the engine then you would end up with a windmilling prop which produces substantial drag depending on how great the relative angle of attack on the prop is.  Feathering bascially aligns the prop blades so that the chord of prop is parallel to the forward relative airflow of the airplane which keeps the prop from windmilling and creating a bunch of drag that would worsen the performance of an already hampered aircraft flying without an engine.

Tango, XO
412th FS Braunco Mustangs