Why a need for PSRU

[Bavafa]

For those aeronautical engineers, mechanics are plain knowledgeable folks:
I was wondering what is it the limiting factor that we need to reduce the prop speed to under 3000 RPM? Is it a limitation of the props, or is it a part of aeronautical engineering/physics of the property of the prop to be more efficient at below 3000 RPM?

Thanks in advance

[gasman]

tip speed.....................

[John Clark]

actually. The RPM number is, of course, a function of the prop diameter.

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA

[rv9av8tr]

It's not only the prop tip speeds, which creates high forces and drag on the prop, but also internal centrifugal forces in the engine. I forget the exact Lyc limitations, but it's something like max 110% (2700 + 270) for some time frame, "requires" engine inspection. Those poor pistons reversing directions puts a lot of force on the "knee bone connected to the thigh bone".

[bkthomps]

tip of prop cannot break sound barrier, so max RPM is derived from diameter of the prop

[rv6ejguy]

If we are talking about PSRUs on auto engines (since most Lycomings don't have gearboxes) it is really to allow the engine to produce rated hp while allowing the prop to operate at it's most efficient rpm/mach number.

Direct drive auto engines usually offer pretty lame performance and a poor power to weight ratio.

[pierre smith]

Originally Posted by gimp2x

tip of prop cannot break sound barrier, so max RPM is derived from diameter of the prop

props broke/break the sound barrier on every take-off, revving 2850 with the IO-520 Conti's....ear-piercing sound and IIRC, those 86" props.

Best,

[Bavafa]

Originally Posted by gimp2x

tip of prop cannot break sound barrier, so max RPM is derived from diameter of the prop

I guess, this is the sort of information I was curious about? knew it should not be higher RPM but did not know why?

I know these are more of a hypostatical questions however..

given the above reason? would it be permissible to have it a higher RPM but shorter prop, perhaps much shorter to eliminate this sacred rule. Do we lose all the efficiency? Or if we were to reduce the RPM even further then our standard 2700 in Lyc, can we extend the length of the prop and get the same efficiency.

[elippse]

This whole concept that there is some mythical diameter above or below- which the efficiency of the prop drops-off is back in 20s or 30s technology. Try this: Tom Aberle at Reno in 2003 did 220 mph qualifying with a two-blade. In 2004 he did 240 with a 58" three blade at 250 rpm less than the two-blade of the previous year. In 2009 he did 252 at the same rpm as the two-blade with a 56" four-blade. I designed a 6-blade, 50" prop for a friend with a Blanton V-6 to turn 4200 rpm without the 75 lb PSRU. It turns 3200 rpm static and should come close to the design 4200 rpm. I'm now in the process of designing an 8-blade prop that may give over 7000 fpm.
It's all a matter of keeping the tip Mach reasonable, say 0.85, designing the planform to keep the critical Mach over the blade less than 0.75, and having enough blades to give the required mass-flow to get the efficiency. I've also designed a two-blade prop that turns 7250 rpm, and a three-blade prop that turns 9000 rpm, both for UAVs. Believe very little of what people say about prop diameters, rpm, and number of blades. And contrary to what some may say, prop design is not a black art practiced by the light of a full moon with various incantations and the tongues of newts. True prop design is very scientific and predictable.

[David Paule]

It's not just tip speed... generally speaking, moving more air slower through the prop is more efficient than less air faster. That means larger diameter and slower turning. But the approaching Mach number limits the diameter. Slow the rpm and the diameter can get bigger.

For a particular airplane, the lower the diameter, the shorter the landing gear and the less the whole plane weighs. Some of the trade-offs.

If you can find a copy of Frank Tallman's excellent book "Flying the Old Planes," he makes a great comparison between the performance of WW I airplanes and '60s general aviation airplanes, and comments that the slow-turning props of the old planes give them a decided performance advantage over the new ones, for the same horsepower.