Aces High Bulletin Board
General Forums => Aces High General Discussion => Topic started by: dtango on July 05, 2001, 04:18:00 PM
-
I've been performing some engine off glide tests to determine drag on AH aircraft. I've noticed that after I shut the engine off my prop continues to spin which adds thrust and throws off my glide tests.
I notice that I can reduce the RPM with engine off and that appears to help but the prop never completely feathers. The RPM indicator still shows the shaft spinning. It actually even increases as I glide at higher speeds.
Is there a way to totally feather the prop in normal flight without having someone to shoot out your engine or running out of gas?
[ 07-05-2001: Message edited by: dtango ]
-
When you say RPM, you do mean RPM don't you, and not manifold pressure?
I'm no expert, but it goes like this:
The throttle controls how much air the engine can draw (manifold pressure); and this governs motor power output.
The Keypad +/- keys control prop pitch, and therefore shaft RPM, and this governs how engine power is applied.
I don't recall ever reducing RPM (prop pitch) enough to bring the prop to a standstill even when gliding engine off (when it stops on a dead motor, it's because the motor has siezed), but I do agree with your observations that it can make a considerable difference to the glide slope.
-
Yes I do mean controlling the prop pitch to reduce RPM and not manifold pressure. Didn't know if there was some way to feather the prop totally so I could remove that from the equation. Otherwise I'm not sure how to account for the amount of thrust that the spinning prop is still producing.
-
I guess only twin-engine fighters and bombers usually got system which allowed prop feathering ?
Edit: P-47 with Curtiss Electric prop could feather it and even brake with reversed thrust in landing and in divebombing in test flights.
[ 07-05-2001: Message edited by: Staga ]
-
Staga is correct. Virtually all single engine fighters do not have a control for feathering a prop. The reasoning behind it is that if the engine dies, you are going to want to leave the plane as soon as possible, rather than try to glide for any distance. Ditchings were generally performed only when the fighter near a friendly base or too low for the pilot to bail out.
-
Originally posted by dtango:
I've noticed that after I shut the engine off my prop continues to spin which adds thrust and throws off my glide tests.
I notice that I can reduce the RPM with engine off and that appears to help but the prop never completely feathers. The RPM indicator still shows the shaft spinning. It actually even increases as I glide at higher speeds.
[ 07-05-2001: Message edited by: dtango ]
The prop isn't producing thrust if the engine is off. It's producing a whole bunch of drag. The air hitting the prop is turning it, like a windmill. Hence it turns faster as you glide faster.
[ 07-05-2001: Message edited by: Thrawn ]
-
btw here's a pic for this topic:
This plane (http://www.kolumbus.fi/staga/Pics/reverse.jpg) is using reversed thrust in landing in airshow (220kb).
-
Even a constant speed prop will stop when the engine quit. Shut off your engine, if u keep your gliding fast enought, the prop will still be wind milling. If you can slow down the plane enought, the prop will stop.
In constant speed prop, pulling the lever all the way will definatly help gliding but will not always stop the prop rotation.
-
Kewl Pic Staga ... but that's a Turbo Prop Cresco...
I know hardly any piston single that havs a full feather prop...
-
Staga, as far as I can see on the background of the pic, I think they are using our Mindanao terrain, aren't they? ;)
Cheers,
Pepe
-
Call me crazy, but I'd just use the wheel brakes and skip the whole process of trying-to-flare-with-zero-visability-near-a-crowd-thanks-to-a-reversable-prop-and-oil-injector and all that.
:)
-
blank screan for general news.. just inducing a redraw.
Pay no attention.
-
My question is this, since I fly Lancs and B-17s a lot....when the engines fail/die, do the props automatically feather?
-
lol Staga. I though that was a screen shot from "IL2" of an aircraft firing it's nose guns ;)
Westy
-
Originally posted by dtango:
I've noticed that after I shut the engine off my prop continues to spin which adds thrust and throws off my glide tests.
[ 07-05-2001: Message edited by: dtango ]
My props stop when I switch the engine off.
Tilt
-
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. :D
-
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>
-
Wow, that is whacky. So some props stops while others don't. Is there some explanation for this?
-
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
-
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. :)
-
you can TAC... just activate the individual engine control (shift 1, 2, 3 or 4) and hit E... the engine will stop and feather.
-
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.
-
http://207.5.89.218/erc/tutorials/navigation/turboprops/cturboprops.htm (http://207.5.89.218/erc/tutorials/navigation/turboprops/cturboprops.htm)
-
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 :
(http://home.cfl.rr.com/rauns/constant-speed-prop.jpg)
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.
(http://home.cfl.rr.com/rauns/sig-frenchy1.jpg)
[ 07-10-2001: Message edited by: SFRT - Frenchy ]
-
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.
-
Check out John Deakin's excellent column on CS props (http://www.avweb.com/articles/pelperch/pelp0016.html) over on AVWeb.
His other columns on recips (http://www.avweb.com/articles/pelperch/pistonlinks.html) are pretty much recommended reading as well. When you are done with that, read the rest (http://www.avweb.com/articles/pelperch/perchlinks.html), 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 ]
-
Crap, I keep confusing the Feather term. I meant reduce RPM on one of the engines.
Thnx for correction Quackboy :)
-
Same thing Tac :) just hit Shift 1 (etc.. ) and reduce RPM on that 1 engine
DW6
-
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 ]
-
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
-
I've read a story of B-17 pilot who lost one engine but decided to NOT feather it because it would tell all the near LW fighters "Hey guys I lost one already!"
One other reason to feather prop is because in some smaller planes (at least Mitsubishi MU-2 twin turboprop) un-feathered prop brokes airflow around wing thus making plane watermelon to fly.
-
duck.. that doesnt work for me. It reduces rpm for both engines, not just one.
-
Originally posted by Tac:
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. :)
Not sure if I should resurrect this thread or not but I think this is a question that is easily answered...
This should happen Tac because a spinning prop that is not producing thrust has more drag on it than a stationary prop. Thus if you feather the prop or stop the engine there will be less drag than if you reduce the RPM and throttle back to 0 MP.
Bluemako
-
Tac:
If i hit Shift-1 and then pull my throttle back to 0, only 1 engine will be affected.. same thing for shift-2 etc..
you can use this to turn off individual engines as well
i use it to turn b17's on the runway.. they're pigs on the ground
kraquen
-
Thanks Mako, that makes sense. Try and land a 38 with engines out, its unreal that the thing wont lose E even with full flaps down..it will literally hover above the runway :)
Kracquen: You can lower MAN on individual engine yes, but can you lower RPM on an individual engine? I fly level (no autopilot) and press shift-2 and press the - key on numpad.. no effect. You'd think the plane would noticibly bank to a side when one engine is putting out less power.
-
My prop always stops when the engine stops. But that's usually when i'm -6 altitude and a integral part of the airframe, I noticed a bit of drag a second before prop stopped tho :D