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na85
12-03-2009, 01:57 AM
In my car it measures revolutions of the crankshaft, per second.

In an aircraft does it also measure this? Or does it measure revolutions of the prop? I was under the impression that most piston-engined craft had reduction gears between prop and crankshaft whereas turboprops mostly do not.

PanzerAce
12-03-2009, 02:53 AM
It measures engine RPM. And turbo props most certainly do have reduction gearing, since they are spinning ~3 times faster than even the hottest piston engines of the time were.

M_Gunz
12-03-2009, 06:06 AM
AFAIK these WWII props were also on reduction gearing though I don't know the ratios. I bet there's a few
members here who do though.

T_O_A_D
12-03-2009, 08:09 AM
Hmm, I've never seen a car/truck/motor cylce/boat/etc Tach measure per second.

Always RPM (Revolutions Per Minute).

What does your Tach show, at Highway speeds.

I'm guessing 2500revs give or take a couple Hundred either way.

If so it actually reads Per Minute.

I have only noticed RPM (Revs Per Minute) of the Engines on the Aircraft tach in the sim.

Some of them may have a gauge for Prop speed, But I don't use it, I rely on the Hud Text for Prop pitch.

Kettenhunde
12-03-2009, 08:33 AM
The tach in an airplane measures engine rpm.

Frankthetank36
12-03-2009, 08:54 AM
I always thought that WWII engines were connected directly to the propeller (like most modern civilian piston planes). If they were gear-reduced, then why are they only redlining at like 2700 rpm instead of around 6500 like a typical car? The purpose of designing engines with low redlines is to not overspeed the prop, but there is no point in that if you have reduction gears.

BillSwagger
12-03-2009, 09:24 AM
Originally posted by Frankthetank36:
I always thought that WWII engines were connected directly to the propeller (like most modern civilian piston planes). If they were gear-reduced, then why are they only redlining at like 2700 rpm instead of around 6500 like a typical car? The purpose of designing engines with low redlines is to not overspeed the prop, but there is no point in that if you have reduction gears.


Don't forget that an aircraft engine has more moving parts than your typical car engine. Even a V8 redlines at lower RPM than a 4 banger.
Rotory engines, and motorcycle engines can get revs above 12000rpm before they red line.

Now consider that many war birds have 12, 18 or even 24 cylinders or more and you decrease the red line value. It has less to do with over speeding prop gearing but i read something a while back where it is more possible to fry bearings in a dive because the pressure is on the back side of the rotation, where under normal rotation the pressure is exerted on the frontside of the rotation. In other words, in level flight your engine pushes the prop, but in a dive the prop wants to spin more than the engine rotates, so the prop is pulling the engine. The difference is where the pressure is exerted on the bearings. My wording might no be 100 percent accurate, just going off recollection.

Also the red line isn't typically 2700RPM thats just where the engine happens to provide the most torque. I think redline is closer to 4000rpms but on most engines this is tough to get near because the (CSP) constant speed prop keeps the revs in the green.

Bill

Art-J
12-03-2009, 10:09 AM
Originally posted by Frankthetank36:
I always thought that WWII engines were connected directly to the propeller (like most modern civilian piston planes). If they were gear-reduced, then why are they only redlining at like 2700 rpm instead of around 6500 like a typical car? The purpose of designing engines with low redlines is to not overspeed the prop, but there is no point in that if you have reduction gears.

I'm, not sure If I understood your last sentence properly (not a native English speaker here), but I think you're looking at the question from the reversed perspective.

Old aircraft engines revved up to, let's say 3000RPM, not because someone didn't want to overspeed the prop, but because it's quite difficult (if not impossible) to build a big engine with max RPMs as high as in the smaller one. Remember that when you increase the size of the object (an engine piston for example), its mass increases with the CUBE of the scaling multiplier, so It's highly unlikely that you're going to move twice bigger aero piston as fast the smaller one in the car (especially when we also recall that the inertia forces acting on a moving piston and trying to break the connecting rod, vary with the square of piston's speed).

Second thing: the most important aspect of choosing the diameter and speed of the prop (after thrust and efficiency) is the speed limit for the tip of a blade (usually not more than 0,85 M?) and not the revolutions of the engine, hence adding the proper reduction gear to make both of these devices work as efficiently as possible. If I recall correctly, when you design an airplane, you choose the prop after choosing the engine - not the other way around (If I'm wrong, I'm sure Kettenhunde will jump in and shoot me in a minute http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif)

Cheers - Art

Frankthetank36
12-03-2009, 11:09 AM
Interesting... For small private planes the explanation I heard for the low RPMs (those planes typically use four-cylinder engines) was that they didn't want to overspeed the prop. By overspeeding the prop, you get less efficiency since the propeller is slicing through air that has already been cut through.

Art-J
12-04-2009, 08:43 AM
In today's smaller engines maybe that's the case indeed - you never want to use more prop blades then necessary because each of them breaks the airflow condition for the next one (from theoretical point of view, the prop with one blade and counterweight on the other side has the highest efficiency of all, however it won't provide sufficient thrust). Nevertheless, on bigger engines, there are more important factors restricting both prop and engine revolutions.

On a side note, prop reduction ratio for RR Griffon 65 engine on Spitfire was 0,5102:1, for Griffon 69 it was 0,451:1, for P&W R2800 mounted on Hellcat it was 0,5:1, I suppose values for most of the other fighter planes were similar. Napier Sabre was an interesting exception from that rule - it's smaller pistons with very short stroke allowed it to run with much higher RPMs than "conventional" engines, and it required reduction gear with a ratio of 0,27something : 1.

Cheers - Art

koivis
12-04-2009, 09:28 AM
The main limiting factor for any kind of piston engines' rpm is the piston speed. This is easy to calculate:

Piston speed = stroke x rps x 2
(of course you have to use meters and revs/sec)

Examples:
Junkers Jumo 213 had a very high piston speed for WWII engine

0,165 m x (3250/60) rps x 2 = 17,9 m/s

For comparison, a modern "high-revving"http://forums.ubi.com/groupee_common/emoticons/icon_rolleyes.gif automobile engine, Honda Accord 2.4L I4 (K24Z3)

0,099 m x (7100/60) rps x 2 = 23,4 m/s

As you see, the difference is not that great, and also, aeroplane engines are designed to run long periods of time at max rpm. I doubt your Honda would like running at 7100 for too long...

As a sidenote, while F1 engines rev 18000 rpm easily, their piston stroke is also VERY short. This puts the piston speed somewhere between 25 and 30 m/s.

M_Gunz
12-04-2009, 11:54 AM
Smaller engines run at higher revs in general though not a hard and fast rule.
I had a 750 V-twin MC that redlined at 7000rpm, I had a 650 inline 4 piston that redlined at 10000rpm.
Those were just cruisers. I've seen small engine crotch rockets that redlined 12000 and 13000rpm.

Long ago I talked to a man with his own homebuilt that ran with a car engine and he showed me one of
the pistons and explained that it was extra thick/heavy just because "you don't want a piston to sieze
up there". Special pistons in a normal car block. He said he ran at 3000 pm or "like a Volkswagen
going about 35".

You want to compare those motors to car engines... there is an Aussie I've seen on TV who put a Merlin
in an old hotrod. It took a lot of modifying but it does drive. Pass everything but a gas station.

PanzerAce
12-06-2009, 04:34 PM
Originally posted by BillSwagger:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Frankthetank36:
I always thought that WWII engines were connected directly to the propeller (like most modern civilian piston planes). If they were gear-reduced, then why are they only redlining at like 2700 rpm instead of around 6500 like a typical car? The purpose of designing engines with low redlines is to not overspeed the prop, but there is no point in that if you have reduction gears.


Don't forget that an aircraft engine has more moving parts than your typical car engine. Even a V8 redlines at lower RPM than a 4 banger.
Rotory engines, and motorcycle engines can get revs above 12000rpm before they red line.

Now consider that many war birds have 12, 18 or even 24 cylinders or more and you decrease the red line value. It has less to do with over speeding prop gearing but i read something a while back where it is more possible to fry bearings in a dive because the pressure is on the back side of the rotation, where under normal rotation the pressure is exerted on the frontside of the rotation. In other words, in level flight your engine pushes the prop, but in a dive the prop wants to spin more than the engine rotates, so the prop is pulling the engine. The difference is where the pressure is exerted on the bearings. My wording might no be 100 percent accurate, just going off recollection.

Also the red line isn't typically 2700RPM thats just where the engine happens to provide the most torque. I think redline is closer to 4000rpms but on most engines this is tough to get near because the (CSP) constant speed prop keeps the revs in the green.

Bill </div></BLOCKQUOTE>

The amount of "stuff" in an engine has next to nothing to do with engine redline. What matters (in the case of aircraft engines, especially old ones), is reciprocating weight, metallurgical quality of all the bolts, and the engine stroke. All that being said, engine stroke is the prime determinant of engine redline. All else being equal, an engine with a smaller stroke will (generally) be able to be spun faster without problems.

Koivis shows the math above for this. (though I will point out that F1s don't *quite* hit 25m/s, since they have to run at that all the time.


I think some people are confusing programmed fuel cut redlines and CSP redlines with actual mechanical redlines. CSP RLs are for max power, fuel cut is for reliability, but mechanical redline is the point at which stuff attached to the crank starts becoming DEtached.

Skoshi Tiger
12-06-2009, 06:33 PM
One of the limiting factors for propeller RPM, is the that if you spin the prop too fast the tip will break the sound barrier, causing all sorts of problems. This is why they have the reduction gears between the motor and the prop.

The speed of sound is about 343 M/s or 20580M/minute in dry air at 20C.

Now if we have a one metre diameter propeller the maximum RPM it can spin at without the tips breaking the sound barrier is a bit more than 6500RPM.

If its 2 Metres than it's about 3274RPM. If it's a F4U with a 4.06 Metre prop then it would break the sound barrier if it prop goes over about 1612RPM.

Now most light aircaft engine RPM would be limited by the HP that they produce and get no where near the maximum propeller RPM.

When we get High performance engines we have to have the motor running at it's most efficient RPM and gear down the prop so that it doesn't over speed. To soak up the extra power they needed to put on extra blades and uses variable pitch props.

Now there are many many more factors but this is one of them.

Cheers

Caveat: Any simple mathematical misscalculations or logic errors are proudly of my own making!

M_Gunz
12-06-2009, 10:38 PM
You need to add forward motion of the plane to that. Make that one side of a right-angle triangle and other side
is the length the prop tip moves, the hypotenuse will be the tip travel distance. Then you have to compare that
to the speed of sound at altitude since it decreases with alt. At 720kph forward motion alone is 200 m/s.

Skoshi Tiger
12-07-2009, 02:03 AM
Originally posted by M_Gunz:
You need to add forward motion of the plane to that. Make that one side of a right-angle triangle and other side
is the length the prop tip moves, the hypotenuse will be the tip travel distance. Then you have to compare that
to the speed of sound at altitude since it decreases with alt. At 720kph forward motion alone is 200 m/s.


Which is why they don't let mugs like me work out the reduction ratio's for high performance prop planes! http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

Thanks for the correction, I'll leave the maths for someone else with a more through understanding of he physics and doesn't want to bolt their aircraft to the carrier deck!

Cheers!