Saying goodbye to warbirds

[hitech]

You are mistaken batz, best glide on a plane with an eng out is course pitch.

In fact my RV8 has a areobatic prop that will go to course pitch on loss of oil pressure to lower drag in a glide.

HiTech

[Tumor]

Originally posted by hitech
.
In fact my RV8 ....

HiTech

Oh, well aren't WE just special!

[rshubert]

Originally posted by hitech
You are mistaken batz, best glide on a plane with an eng out is course pitch.

In fact my RV8 has a areobatic prop that will go to course pitch on loss of oil pressure to lower drag in a glide.

HiTech

Did you build it, or buy it?  Tail wheel or tricycle?

[mwjayd]

glasses wrote ......The loud boom you would hear would be from you body hitting the gorund at 750mph while a second later the pieces of the me163 would follow earthward.

Simply the 163 didn't have the structural integrity to sustain transonic flight, the Me262 however was said that it broke the sound barrier while on dives regularly,in what condition I don't know......................... ............................. ............................. ............

 I broke the sound barrier before.....no wait that was wind ...sorry bout that..

[flakbait]

Yup, coarse-pitch (low-rpm) is the best for gliding. Because the prop angle is closer to 90º (inline with the flight path) there's hardly any drag. Fine pitch (high-rpm) means the prop is somewhere around 30º, creating a mess of drag if the engine goes out. In fact, with the prop at 30º on a dead engine, you'll end up with the prop turning the engine!

Most turbo-props, twins, and some aerobatic AC these days have a large spring in the prop dome. The purpose of that spring is to regulate RPM with oil pressure. When oil press and the spring force are equal, you get whatever RPM you set. If the oil pressure suddenly drops to zilch that spring will drop the regulator down to the stop. As a result, it auto-feathers the prop. John Deakin did a nice write-up on props, with pictures, over here...

http://www.avweb.com/news/columns/182082-1.html


As for the sound of the engine increasing pitch as speed goes up, uhhh no. That doesn't happen. You may be going faster and faster, but the engine isn't. It's still turning over at 40" of manifold pressure and 2,400 RPM. Because the engine doesn't increase speed, there's no change in the engine pitch. It isn't a car with an automatic gearbox that changes gears every so often. The engine-to-prop gear-ratio is fixed, and can only be changed on the ground. Since there aren't any gears to change, and the engine speed stays the same whether you're at 120mph or 400mph, you won't hear any change in the sound. Now if you hit WEP you will hear a change, because the manifold pressure increases with WEP.



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Flakbait [Delta6]
Delta Six's Flight School
Put the P-61B in Aces High

[beet1e]

Originally posted by flakbait
Yup, coarse-pitch (low-rpm) is the best for gliding. Because the prop angle is closer to 90º (inline with the flight path) there's hardly any drag. Fine pitch (high-rpm) means the prop is somewhere around 30º, creating a mess of drag if the engine goes out. In fact, with the prop at 30º on a dead engine, you'll end up with the prop turning the engine!

hehe, flakbait. That reminds me of the 109 I used to fly. No not Me109, but Gröb 109 motor glider. I would stop the engine in flight and feather the prop. The aircraft would turn a hell of a lot better with the engine stopped - no gyroscopic force resisting the turn. When I wanted to restart the engine, I'd turn on the ignition, unfeather the prop to the FINE position, and nose down to 95 knots. Just as I hit 95, the prop would start to turn and the engine would fire up.

[Kweassa]

Um...

 So then, what of those over-revving things?

 In what situations would a pilot over-rev his engine then?

 Or, did that not happen at all, if the pilot just sets his RPM to a certain amount? Is over-revving a problem only when the RPM governers are damaged or so?

[ViFF]

There are two situations I can think of where a pilot can over-revv his engine:

In fixed pitch props it would be pretty easy.. prop blades don't change angle, so as airspeed increases (like when in a dive) the airstream will windmill the prop and shoot engine rpm's above red line limits. It all depends what angle the blades are fixed at to boot. Varies with different a/c. Some are fixed to low speeds like in trainers or banner towing a/c, some for higher speeds like the Hurri Mk1a or older ww1 a/c.

Same with Variable pitch (VP) props which are not CSVP. These types have a direct linkage from the blade angle to the Prop lever in the cockpit, the pilot has to constantly monitor the RPM gauge and manage the prop angle as airspeed increases/decreases to avoid over revving or underevving.

CSVP uses an indirect linkage, they still have a prop lever next to the throttle, but it sets an RPM  as opposed to a BLADE ANGLE like in the older generation variable pitch props. CSVP has been the standard since WW2, so thats a no brainer. The biggest advantage of a CSVP over the old VP is that it reduces pilot workload. All the pilot has to do is move the prop lever to set an RPM, and the prop governor will do it best changing the blade angles to keep that RPM as he transitions through various airspeeds.

Now in both VP & CSVP the prop governor has an angle limit to how far it can change the blade of the prop to compensate for the extra speed.

Its modelled verywell in Aces High as well. Take a Mustang up to 26k, make sure your at max RPM (2,400 i think ?) and start diving with WEP. Somewhere around 550-600 IAS you'll see that the RPM actually rises beyond the MAX you usually see, and you will hear the audible increase in engine RPMS.  Thats the point where the prop governor has reached its MAX coarse pitch available (somewhere around 35-45 degrees in actual blade pitch) to compensate for the increased airspeed windmilling the prop.

The other case I can think of (and this is not moddeled in AH) is if the prop governor is damaged, and the blades are stuck at a certain angle. Overreving would then occur much like in a fixed pitch prop. This is modelled in WW2OL and IL-2.

[ViFF]

Just as a point of interest, I should note that AH does NOTmodel the interaction between manifold pressure and prop rpm. Over revving an engine is not the only thing a pilot needs to worry about, but also about over torqueing an engine.

When you increase pitch on a blade (reduce rpm's), its harder for the engine to turn the prop, cause of the higher angle of attack between the blade and the airstream its plowing through. In a real aircraft, you can set a certain MP/Boost setting (for piston engines) or N1/2 setting (for turboprops). If you horse around with the RPM lever you see how the engine responds...

Increase RPM (reduce pitch) and the engine MP will drop (less torque on the engine cause the prop causes less drag/lift due to lower angle of attack as it plows throgh the air around it).

Decrease RPM (increase pitch) and the engine MP will rise (more torque on the engine cause the prop is trying to plow through more air).

[joeblogs]

It is possible to overspeed a plane with a constant speed propller in a very steep dive.  

All that it takes is to push the propeller governer to the stop.  Then the remaining excess power will drive the engine faster.

That's why manuals for WWII US planes include instructions on RPM management in a dive.

Yes it would be far worse with a fixed pitch propeller.

-Blogs

Originally posted by beet1e
Zanth, you are absolutely WRONG. The way AH has it modelled is correct. And that's because most of the WW2 planes had constant speed, variable pitch propellers. I have had to explain this so many times that I saved the explanation, and reproduce it below in blue text, extracted from an old BBS post.

Many of these planes had constant speed variable pitch propellers. As power is increased, the pitch of the propeller blades increases. The load on the prop also increases so the set speed is maintained. In a climb, the prop would tend to slow down. The prop governor detects this and adjusts the blades to a finer pitch - again, to maintain the set speed. In a dive, loading on the prop is reduced, the pitch increases. I believe that early Spitfires did not have the VP prop, and the pilot would have to throttle back in a dive to avoid over-speeding the engine. The VP prop is able to soak up that power by adjusting the pitch, and no need to throttle back.  If throttling back, the only time the engine RPM would decrease is when the pitch of the blades reaches minimum and the throttle is closed still further.

If you’ve ever flown a real plane with fixed prop, and then move onto something with a CSVP prop, believe me, you would never want to go back! It makes engine management so much easier. You just have to remember to increase throttle to maintain manifold pressure as the plane gains altitude. But you’d have to do that anyway even with a fixed prop, as power/RPM decreases with altitude.

For me, the way Aces High models CSVP props is one of its good points. I wish WB would too.

Over to funked to explain how CSVP governors work - something to do with lead weights sliding along the prop shaft in response to centrifugal force?  Geez, I’m lost already.

Two of my former Dutch squaddies (Lapwin & Zepher) took a ride in a real P51 - I have the film they made. And when Lapwin landed he made the comment that the engine note remains unchanged in dives/pullups. I don't think he had believed it until that day, and as he walked towards the camera he said "Scrmbl was right". Of course I was right!

Now, if we could only get them to calibrate the magnetic compass the right way round instead of backwards, we would be in good shape. The compass is calibrated backwards in both AH and WB, but is correct in IL2.

[Grimm]

Originally posted by JB73
i have yet to be at 750 (close to sound barrier) in ANY plane in AH yet.

the closest i have gotten is 640 in a 163 @ 35,000 feet. the plane was fully compressed and the slightest touch of the stick sent me to the tower from too many g's.

JB73,

Here a stupid trick for offline horsing around.  

Take an AR234 with Rato pods,  Dont use the Rato.   start it in a climb and go do somethings around the house, return when you have alot of ALT.  

Put it full power in a medium to shallow dive and build speed.  Dont touch the stick go into compression and wait for the wings to break off.    

The Rato pods are still there,  Now hit the Rato and accelerate.  I think you will break 700 before you auger in....  

Its funny looking because the wings are gone, but the Pods are just hanging in midair.

[flakbait]

What ViFF said. (btw, max RPM in a P-51 is 3 grand)

For the most part it's fairly hard to over-rev the engine with a constant-speed prop. Starting your decent from cruise without adjusting power in a Cessna would probably do it, but WW2 fighters are a different story. Hamilton-Standard props had a gear/roller-cam arrangement that regulated RPM with oil pressure, while others had the spring system mentioned above. Curtiss Electric props are marvelous little doohickeys that use an electric motor to directly control prop pitch. When you set, say, 2,500 RPM there's two things happening...

For the H-S type props a governor-controlled valve opens, letting oil pressure on the outside of the dome-plate rise. When that pressure rises, the plate pushes back against the roller, rotating a sleeve via the cam on the ass-end of the prop dome. At the same time, oil is let out from behind the dome-plate. The sleeve has beveled gears on it that match up with the prop blades, and when the cam is moving backwards it sets high RPM. Oil pressure on the inside of the dome plate acts like an RPM "lock", via hydraulic force. Since you can't compress a given volume of oil into a smaller volume, pressure rises. Prop governors are driven by the engine, and as engine speed increases the governor wants to dump oil in behind the dome-plate to lower RPM. But, if that roller cam hits the stop, the blades are at full low RPM and speed is still increasing. After a certain point hydraulic "lock" takes over, and the prop is driving the engine. If RPM is already at redline, don't plan on getting away without any engine damage. With any H-S type prop you'd probably end up blowing the prop control oil line to the hub. Along with the valves and valve heads if this condition went unchecked. What about glycol lines? If pressure went unchecked, those could possibly blow too.

With spring-type props, the same thing happens. At high speed the governor starts letting oil out from behind the dome-plate while the spring is slamming that plate right down to the stop. But, unlike the H-S type, you don't have to worry about blown oil lines. You'd probably end up with some damage to the cylinder valves but beyond that not much would happen.

Curtiss Electric props give direct control over blade pitch, but had it even worse than the H-S. When it worked the Curtiss prop could set exact pitch, and RPM, however you wanted it set. It did this via an electric motor. But if the electric motor crapped out, or the wiring corroded, it would "run wild" because there wasn't any oil pressure used to control RPM. Electrically adjusted Hamilton props don't have this problem: If the motor conks out, the prop governor tries keeping the last RPM setting.

No matter which type of prop it is (even the new Hartzells and McCauleys), every one of them has the same problem. Once the mechanical limits of the prop governor, dome-plate, or pitchmotor have been reached the propellor is not a constant-speed prop anymore. It's a fixed pitch prop. Remember though, really bad things only happen when you over-rev the engine. And that takes at least 400mph or more in these aircraft. Some won't start to over-rev until nearly 500mph. Going up around 550-600mph makes all this a moot point; the prop blades probably would've shattered at that point from blade-tip speeds going super-sonic.


See the above link from AVweb for really detailed info.

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Flakbait [Delta6]
Delta Six's Flight School
Put the P-61B in Aces High

[Eagler]

set all sounds to 75-80%(max) and wind at 100%, crank up wav volume in windows

[Kweassa]

Hmm..

 Thanks for the explanation guys.

 I was really having doubts about whether a WW2 plane can just go merrily into a full throttle power dive.

 So, would it be safe to assume that while there may be different tolerance levels to certain speeds according to plane type, usually pilots would not think to use full throttle in steep dives and chases, for the sake of their engine?

[brendo]

The little aeroplance I fly has a fixed prop, so you have to be constantly managing the throttle to ensure that you do not over-rev and also still have best power out of the engine through various speeds.

My engine does sound like the Warbirds sim sounds. But the problem is nearly no WWII aircraft had a fixed prop in 1939.

Your engine shouldnt sound like it is going up and down like a car.

Gratuitous image below