Feathering the prop

[Weave]

Whats the big deal? Everyone knows the prop's primary function is to keep the pilot cool. You can tell because the minute it stops spinning he starts sweating.  

[GunnerCAF]

Originally posted by Tilt:


My props stop when I switch the engine off.

Tilt

I think some aircraft props stop, and others spin.  I noticed last night when I turned off the engine in a LA7, the prop stopped.  

Gunner <CAF>

[dtango]

Wow, that is whacky.  So some props stops while others don't.  Is there some explanation for this?

[Rocket]

It depends on the make of the prop on whether or not it is featherable.  Several fighters had featherable props along with the bombers.  HTC researched the props and implemented the correct ones AFIAK to the planes that are featherable.  Just because it is a fighter doesn't mean that it can't be feathered  

s!
Rocket

[Tac]

you cannot feather individual engines in multi-engined planes.

Another interesting fact, get to 20k, turn off P-38's engines. Props will stop. Shift-X and see how long your speed lasts. Now try the same thing with MAN to 0 (end even try it with Man at 0 and RPM at the lowest setting)... from what i've seen the 38 keeps MORE speed and glides longer with engine off than with engines on and spinning (with no MAN and lowest RPM setting).

Dunno if this is what should happen, but I think its hilarious.  

[Duckwing6]

you can TAC... just activate the individual engine control (shift 1, 2, 3 or 4) and hit E... the engine will stop and feather.

[Fastbikkel]

A constant speed prop. What does this mean?
Rotating at a constant speed? I cannot imagine a system like that. What happens when you open up the throttle??

It makes more sense to me if the pitch is constant, but then the word "constant speed" doesn't make any sense.

Hope you can clear this for me.


Greetings,


JG5FaBi.

[Staga]

http://207.5.89.218/erc/tutorials/navigation/turboprops/cturboprops.htm

[SFRT - Frenchy]

Originally posted by Fastbikkel:
A constant speed prop. What does this mean?
JG5FaBi.

This is a homemade simple drawing that I'm using for my students :

   

It's like riding a bike. When you are riding on the flat, you have selected a long gear and you are gently paddeling, achieving good speed. As soon as you hit a hill, you start to feel that you need to apply a lot more pressure on the pedals, and you eventually end up shifting to a lower gear. Then your legs are turning like crazy but you "efforlessly" climbing the hill at slow speed.

Your body sensing an increase in force needed is the same as the job of the "fly weights". The gear selected is the "angle at wich the prop bites the air".

You select your prop to turn at a fixed RPM (Prop RPM, not engine RPM), you cruising flying straight and level. You start to climb, your prop will start to slow down like you would in a bike climbing a hill. The Fly Weight will spin slower on himself and will "shrink". This will make the Pilot Valve go up or down, allowing :

- oil from the engine to flow to the prop hub
or
- oil from the prop hub to flow back to the oil sump.

The oil coming from the engine flowing into the prop hub will push a piston who will make the blade angle varie in one way. If the oil is leaving the prop hub, a loaded spring will push the piston the other way, making the blade angle varie the other way.

No electronics, simple physic stuff. Awsome conception isn't it?

The system needs the oil pressure from the runing engine, if you have an oil leack, the oil will leave the prop hub for sure. In some plane, like the Mooney, it will put the prop at "high RPM", slowing down the plane.
In a twin like the Seminole, it will "feather" the engine (low RPM), creating way less drag. (Trust me, your legs will say thank you)  

Hope that helped.

 

[ 07-10-2001: Message edited by: SFRT - Frenchy ]

[dtango]

Fastbikkel:

Constant speed prop does mean that prop spins at a constant RPM.  

Good references from Staga and Frenchy.

Without getting into the flight dynamics, a fixed pitch prop has maximum efficiency at only 1 airspeed.  A variable pitch prop greatly enhanced a/c performance since you can achieve maximum efficiency with at prop at various airspeeds and not just one thus enhancing the powerplant performance.

Throttle setting on a constant-speed / variable-pitch prop relates to offsetting the drag produced by the prop at different pitch angles and keep the prop spinning at a constant RPM.  Higher airspeed-->more prop pitch, more prop pitch-->more drag, more drag-->more torque needed to maintain constant RPM for prop.  The throttle is related to the oil pressure some way as well to vary the blade-pitch for different speeds.

That's my simpleton understanding of it.

[ft]

Check out John Deakin's excellent column on CS props over on AVWeb.

His other columns on recips are pretty much recommended reading as well. When you are done with that, read the rest, a goldmine for any aviation enthusiast.

I think I need to prepare a cut & pasteable version of this post to keep with me at all times. I find myself posting it in every forum I participate in, sooner or later.  

If I got those UBB tags right it's a small miracle.  

Edit:There are no miracles!

Cheers,
  /ft who, unfortunately, has no reason to plug Mr. Deakin's articles other than the fact that they're impressive

[ 07-10-2001: Message edited by: ft ]

[Tac]

Crap, I keep confusing the Feather term. I meant reduce RPM on one of the engines.

Thnx for correction Quackboy  

[Duckwing6]

Same thing Tac   just hit Shift 1 (etc.. ) and reduce RPM on that 1 engine

DW6

[funkedup]

Frenchy nice drawing.    

And you hit the nail on the head  - different prop designs will go to different pitch when oil pressure or electric power is lost.  I have not taken the time to figure out how each prop in AH should behave but I have a feeling HTC have done so.  

FT thanks for the links.    

[ 07-11-2001: Message edited by: funkedup ]

[ft]

Originally posted by funkedup:
And you hit the nail on the head  - different prop designs will go to different pitch when oil pressure or electric power is lost.  I have not taken the time to figure out how each prop in AH should behave but I have a feeling HTC have done so.    

Singles - fine pitch when the oil pressure goes. You can always get the most power in fine pitch (power=2*pi*RPM*torque IIRC, don't care to do the arithmetics right now  ) which is nice when the sh*t hits the fan up there. If your prop control unit goes, you won't wan to fly long-distance anyway.  

Multis - Feather as soon as you loose oil pressure. As Frenchy said, there is not enough rudder trim in the world to counter the torque from a windmilling prop. And quite possibly not enough thrust either...

When you shut down the engines the props latch at say 50 degrees or so since a recip might just beat itself to death trying to rev up with a feathered prop. Turboprops OTOH usually go to full feather (except for that weird french one-turbine design I guess if ever used in anything but helos  ), check out the A/C on the ramp next time you visit an airport.

Of course, nothing is ever without exception. The B17 for example had a feathering system that sometimes wouldn't do the job leaving the prop unfeathered and windmilling. A windmilling prop with no oil pressure eventually overheated, seized... and tore loose. And if it was an inboard often hit the fuselage, causing massive damage. They updated the system eventually, I think from electrical to hydraulical with pressurised feathering oil containers or the other way around.

Cheers,
  /ft