Spitfire Props

It’s a very good display at the Fort. The pilot abandoned his aircraft as it was ‘vibrating’. Here’s the hole vacated by the blade.

I also have a page on the late Jay Simpsons Thunderbolt P47 also on display in Fort Perch, but thats a different topic.

Modern GA aircraft idle at around 1200 engine rpm deliberately, this to avoid plug fouling

They actually idle at about 650, though you wouldn’t tend to run it at that, you would as you say keep it about 1200, but that isn’t idle 😉

One Second in the Life of a Racer

by Tom Fey

The Unlimiteds go flashing through the racecourse, engines howling, air shearing, heat waves streaming. Four hundred eighty miles an hour is 8 miles a minute, and the elite racers take about 70 seconds to cover the 9.1 mile Reno course. If you could take a souped P-51 racer flying the circuit at Reno, slow time down, and examine just one second, what would you find?

In that one second, the V-12 Rolls-Royce Merlin engine would have gone through 60 revolutions, with each of the 48 valves slamming open and closed 30 times. The twenty four spark plugs have fired 720 times. Each piston has traveled a total of 60 feet in linear distance at an average speed of 41 miles per hour, with the direction of movement reversing 180° after every 6 inches. Three hundred and sixty power pulses have been transmitted to the crankshaft, making 360 sonic booms as the exhaust gas is expelled from the cylinder with a velocity exceeding the speed of sound. The water pump impeller has spun 90 revolutions, sending 4 gallons of coolant surging through the engine and radiators. The oil pumps have forced 47 fluid ounces, roughly one-third gallon, of oil through the engine, oil cooler, and oil tank, scavenging heat and lubricating the flailing machinery. The supercharger rotor has completed 348 revolutions, its rim spinning at Mach 1, forcing 4.2 pounds or 55 ft³ of ambient air into the combustion chambers under 3 atmospheres of boost pressure. Around 9 fluid ounces of high octane aviation fuel, 7,843 BTUs of energy, has been injected into the carburetor along with 5.3 fluid ounces of methanol/water anti-detonant injection fluid. Perhaps 1/8 fluid ounce of engine oil has been either combusted or blown overboard via the crankcase breather tube. Over 1.65 million foot pounds of work have been done, the equivalent of lifting a station wagon to the top of the Statue of Liberty.

In that one second, the hard-running Merlin has turned the propeller through 25 complete revolutions, with each of the blade tips having arced through a distance of 884 feet at a rotational velocity of 0.8 Mach. Fifteen fluid ounces of spray bar water has been atomized and spread across the face of the radiator to accelerate the transfer of waste heat from the cooling system to the atmosphere.

In that one second, the aircraft itself has traveled 704 feet, close to 1/8 mile, or roughly 1.5% of a single lap. The pilot’s heart has taken 1.5 beats, pumping 5.4 fluid ounces of blood through his body at a peak pressure of 4.7 inches of mercury over ambient pressure. Our pilot happened to inspire during our measured second, inhaling approximately 30 cubic inches (0.5 liter) of oxygen from the on-board system, and 2.4 million, yes million, new red blood cells have been formed in the pilot’s bone marrow.

In just one second, an amazing sequence of events have taken place beneath those polished cowlings and visored helmets. It’s the world’s fastest motorsport. Don’t blink!

From

http://www.enginehistory.org/rolls-royce.shtml

.

RPM gauge on the Spitfire is propellor RPM, not engine RPM.

Engine power is set with reference to the boost gauge, you can set a constant power and make the RPM gauge move by moving the propellor lever.

(Edited to add that this is a pilot’s (and a trainee Spitfire pilot’s at that) and not an engineers view…comments below have prompted me to return to my manual and deepen my understanding;))

Its engine RPM – 0 boost at 2150 Engine RPM. Max engine RPM, 3000 RPM. TSD 292

It’s a very good display at the Fort. The pilot abandoned his aircraft as it was ‘vibrating’. Here’s the hole vacated by the blade.

One a similar topic, I have been finding out about a Spitfire that crashed in Birkenhead Park during WW2, the pilot bailed out and landed on the roof of a Maternity hospital in Liverpool allegedly.

The cause of the crash, a prop became unattached. The remains were excavated in 2007 and are now on dispay in Fort Perch Rock, New Brighton. Thew whole room is filled with pieces of the plane.

Attached is image of engine and prop boss from the plane.

Ask what?? haha

I have to say though brilliant how the the thread has panned out.

one apparently simple question leading to engine and rotor rpm, reduction ratios, circular velocities and forward speed components.

If said teacher were to read this thread though, I’m willing to bet the thought “I wish I’d Never asked” has passed through their head 🙂

Moggy already correctly stated the Merlin reduction gear ratio for most Spitfires is 0.477:1.
The idle speed however is normally specified as 1200 rpm. The engine will happily run down to about 600, but when hot the oil pressure drops at this speed, and the supercharger volute fills with unburnt fuel.
I too have never seen an aircraft which has a tachometer reading propeller rpm unless it is direct drive, ie Harvard.

Pete

Huh…

If the tacho on a Spit reads airscrew RPM it’s news to me….. I can’t think of any a/c with this other than a helicraptor.:eek: My offhand recollection of the reduction ratio of the Spit was about 2.8:1.

If the prop RPM was 3000 ,it would have a tip speed of 1688 ft/sec,appx. M1.5,standing on the ground..!
Out of interest,anyone got the diam. of a Mustang prop..?

Yes, but be careful

Most piston engines seem to idle at about 800rpm, but that’s engine revs. On a Spitfire 800 engine revs would only be turning the prop at 380-390 rpm.

Modern GA aircraft idle at around 1200 engine rpm deliberately, this to avoid plug fouling. Here there is no reduction gear so prop rpm is the same

Moggy

Ah, I see what Sycamore has done – he has factored in the forward speed vector to get the total velocity of the blade tip, not just the rotational component. Definitely one to feed back to teacher!

Of course, if the aircraft is stationary – and 800 rpm would suggest an engine at idle, as Moggy said – then this isn’t a factor.

Isn’t it amazing how many real experts come out of the woodwork when things get technical. Love it.

Prop tip speed at 3000 ERPM.1431 PropRPM at 460 mph/400KTS is 1045.5 ft/sec,or .9359 Mach..

Ok, Ok…I’m but a pilot and not an engineer…it works for me to think of it that way… Perhaps I shouldn’t make such bold statements without checking my manual. For me, if it’s within the governed range, I move the propellor control lever and the RPM changes:)

Bold pilots ….

Ok, Ok…I’m but a pilot and not an engineer…it works for me to think of it that way… Perhaps I shouldn’t make such bold statements without checking my manual. For me, if it’s within the governed range, I move the propellor control lever and the RPM changes:)

The uncropped single MK IX blade sitting on my desk is an RA 10046/RTS and it measures from base of hub to tip 60.5″

Blimey! That beats my ball-point pen that maxes out at 5 and 7/8in from end to end :D.

Seriously though, how big is your desk, Mark ?

(Apologies for uncorrected drift of thread)

I have two blades on my desk, one each end and both from MK923, a Rotol and a new Hoffman that has been scuffed at the tip….and three computers.

Mark

RPM gauge on the Spitfire is propellor RPM, not engine RPM.

I would check that. 🙂

Mark

Actually, the information about diameter is more useful for me than the length of one blade because the rotation is still about the very center of the prop, so the distance from the center, i.e. the radius is what I needed. The blade itself, I assume, would be shorter than the radius of the circle defined by the rotation of the prop because the blade (again I’m assuming) is attached to some sort of central shaft.

I’m lost but this is what was said

RPM gauge on the Spitfire is propellor RPM, not engine RPM.

I am prepared to bow to your better knowledge but it does make it difficult for me to comprehend why, if maximum engine revs are 3,000 rpm (see above) and the reduction gear takes the prop back to about 48% of engine revs, it was normal for a Spitfire panel to feature a 0-5,000 rpm rev counter.

:confused:

Moggy