Harvard Question

Moggy C and ChristiaanJ, thanks for showing me that I hadn’t completely ‘lost it’ with the passage of time the regarding tip speeds of the good ole’ Grey Lady.

contrailjj, another boring piece of info, again IIRC, the area where greatest noise is heard from a propeller is within a 15 dergee arc of the propeller tips, perpendecular from the propshaft, i.e. to the side. (hence the old one about ‘how do you identify a Shack co-pilot? He can only hear you with his left ear, How do you identify a Shack captain? He can’t hear you.’) Also IIRC, there was a Gannet AEW that was used a few years ago carrying out noise sampling trials. I seem to remember it was fitted with a VERY long probe from the (port?) wing leading edge. The mike was fitted to the end of this probe ajacent to the propeller tips. I think it was all part of the trials for the now defunct ‘Propfan’ development going on, both in the West and the East. Why defunct? Unacceptable noise levels. They were very efficient, but were also VERY LOUD.

25deg south, the rear propellers on the ‘Shacklebomber’ were about six inches less in diameter, but to ensure they still created equal thrust as the front ones, they were slightly ‘paddle blade’ shaped. Can’t remember if there was a difference in pitch. Sorry, just too long ago.

Camlobe- “I sat down and worked out what the tip speeds were for the front prop. Why the front? It was six inches larger in diameter than the rear. Why? To prevent the tips of the rear from churning through turbulent flow from the front tips.”
Funnily enough Handley Page had cracked that for the U.K. in the first World War for tandem contra-rotating props ( although its a pretty safe bet that it came earlier from the Germans) , albeit at opposite ends of a nacelle. I think the rear propeller had a slightly coarser pitch as well ?

just to add an extra descriptive…. having recently stood within 5 metres (without hearing protection) of a Mustang IV on final throttle push prior to shut down… the same ” trans-sonic snap” is quite evident to the attentive listener – albeit in the rather sexy form of a purrr … and I’m not talking about the sexy purr of a Merlin here. The Mustang IV (P-51D) with its considerably paddle-bladed prop – in flat pitch – on the ground, does generate the effect of which we type.

JJ

Moggy,
I’ll believe your figure… thousands wouldn’t.
Quick and dirty check just the same:
13ft diam = 40 ft circumference.
Prop rpm abt 1220 rpm
40×1220=48650ft/min=2920000ft/hr= 890km/h = 550 mph (slide rule accuracy).
Sorry for not trusting your maths.
At sealevel that’s about Mach 564/760 = 0.74.

As an ancient boffin, that’s the bit I wanted to add.
The tips themselves are not ncessarily supersonic, but the airflow around the tips becomes transonic, i.e., it’s supersonic over part of the blade profile. Describing the effect as cavitation is not all that far off, even if the term normally applies to ship propellers.

With typical propeller profiles, a “critical Mach number” of 0.75 (i.e., the speed where the airflow over the profile goes supersonic locally) sounds perfectly reasonable.

Please forgive me. (age, probably). 2750 rpm times point 4423 (reduction gear ratio) and diameter of IIRC thirteen feet gave a figure of around 500mph!! Somebody want to check my maths? It was twenty lots years ago.

camlobe

564 mph by my reckoning.

Moggy

Mustang Merlins with paddle blade props sound different from those with Albatross blades I have recently discovered….

Ah, let’s see.
Propeller tip speeds, different noises, cavitation. Hmm. Over the years, I’ve come across all the above in different applications.

Right. What shall I bore you all with first?

Let’s do tip speeds.

Pretty well covered already. For the same given propeller rpm, a larger diameter propeller will have higher tip speeds than a smaller diameter propeller. Next time you look at an aircraft fitted with a contra-rotating propeller, think about this. It doesn’t have the two opposite-rotating propellers on the same shaft just for ground clearance. It is also to prevent the propeller tips going supersonic and loosing efficiency due to turbulant airflow at the tips. Lost efficiency means less power of the engine converted into thrust produced by the propeller. This is because a large proportion of the power produced by the engine will be lost in overcoming the severely increased drag caused by the propeller tips going supersonic. Hope that’s easy enough to follow. As an aside, there was a single-seat turboprop aircraft (can’t remember all the juicy details, sorry. someone here will be able to supply) known or called the ‘Thunderscreech’. IIRC, the propeller tips were supersonic (by design?).

Cavitation.

Seen the effect of this on propellers. It is most notable as ‘plucking’ of the metal at the tip of the propeller just behind the leading edge on the rear face. Before I became aware of this in aviation, I thought it only affected ships and boats.

Different noises etc.

Any of you guys and gals out there will probably have noticed this at various, well attended airshows. Same engine, different exhaust, different noise. Now, be careful here. To make sure we don’t fall into the trap of confusing the ‘same engine, different propeller’ noise change with the ‘same engine, different exhaust’ noise change, we need to restrict what we are going to listen to. Easy one is probably DC3/C47 with and without cabin heat exchangers. The longer tailpipe of the heat exchangers gives a lower, deeper note. Similarly affects (or enhances, depending on your viewpoint) the Harvard/Texan. This is a bit like a pipe organ. The high notes come from shorter pipes, the low notes come from longer pipes.

With some of the more numerous types eg Merlin engined Spits, the three bladers will sound different than four bladers, even if they have the same exhausts. However, Merlins fitted with three blade props will sound different if they are fitted with different exhausts.

And to make matters worse, not all three bladed Merlins sound the same. Baffled yet? To give an example of what I mean here, listen to the following during ground running:
a Hurricane XII fitted with a Hamilton Standard metal bladed prop
a Huricane IIC fitted with a Dowty wooden bladed prop
a Spitfire II fitted with a Dowty wooden prop
the Lanc having a single engined ground run
All similar, but not the same.

P.S. On one of those long days waiting for that 42 ton, four engined, taildragging fighter (well, 8 Sqn was a fighter squadron, old boy) to return after a PLE (prudent limit of endurance i.e. how long can we stay up today?) I sat down and worked out what the tip speeds were for the front prop. Why the front? It was six inches larger in diameter than the rear. Why? To prevent the tips of the rear from churning through turbulent flow from the front tips. I digress. Please forgive me. (age, probably). 2750 rpm times point 4423 (reduction gear ratio) and diameter of IIRC thirteen feet gave a figure of around 500mph!! Somebody want to check my maths? It was twenty lots years ago.

Sorry to drag it on.

camlobe

Er Alex – its the prop……

I KNOW!!! 😀

Alex

Lets hope no one falls in the water then……

This discussion is reminiscent of an Eccles and Bluebottle exchange- only longer.

lol did u actually go out and try? 😉 😀

Alex

I regret to admit I didn’t. The calculations are quite simple so I modelled it. 😉

Moggy

It’s a long time since A-level Physics but I think the formula you need Moggy is Velocity = Radius times Angular velocity (in radians per second). Thus if the angular velocity stays the same the tip velocity will increase as the radius increases. Don’t know what that proves other than Moggy is right.

as far as I know the typical sound of the T6 is due to the very short exhaust (the sound is indeed different in Harvards, which have a longer exhaust stack). Btw every plane has a sound of its own..
The phenomenon u r talkin about is called cavitation, where the tips of a prop exceed the speed of sound and other than producing a really loud and low sound they actually lose aerodynamic efficiency, which is really something u dont want to happen.
Cavitation may occurr if some of the given specs (dive speed, RPM settings etc..) are not respected. A prop cavitating isnt usually a good sign, expecially if in a normal flying attitude.

Alex

Er Alex – its the prop……

Actually given the posts above am more concerned about what other engines we should consider fitting to our T-6 and your Harvard! 😮

TT

Yup.

It takes twice as long to walk 62.8 ft as it does to walk 31.4 ft

I don’t think I need an explanation of that.

However what I did find was that to make the times identical it was necessary on the bigger circle to run at double my walking pace.

Ergo, the tips of the longer prop move faster. QED

Moggy

lol did u actually go out and try? 😉 😀

Alex

Ok go in the garden tonight with a stick and a bit of string, make two circles one 20ft dia the other 10ft. walk the 20ft circle at your normal walking pace and time it, then the 10ft, when you have done this twice come back with your f ndings and I will explain why and what happend to you.

Yup.

It takes twice as long to walk 62.8 ft as it does to walk 31.4 ft

I don’t think I need an explanation of that.

However what I did find was that to make the times identical it was necessary on the bigger circle to run at double my walking pace.

Ergo, the tips of the longer prop move faster. QED

Moggy

Give up Moggy and BELIVE ! it will help.

Ok go in the garden tonight with a stick and a bit of string, make two circles one 20ft dia the other 10ft. walk the 20ft circle at your normal walking pace and time it, then the 10ft, when you have done this twice come back with your f ndings and I will explain why and what happend to you.

er… angular velocity is kinda clear mate… if u r on the tip, u need to cover a longer distance to keep up with the center, hence u have to go faster.. tips are faster mate..

Alex

ok, let’s try to explain myself…

the sound of the T-6 is just like a trade mark, and it’s due to the exhaust and prop (just like for any other plane). I already explained what cavitation is, and it might occurr to any prop plane, expecially when overrevving or if the prop governor goes bonkers (it can happen to fixed pitch props too).
To sum it up every plane can make a louder and lower sound (I heard microlights cavitating), and it happens almost always when overrevving, but it’s not a constant thing, it might last a bunch of seconds.
The T-6 is just a loud machine because of the short exhaust, that’s it. Every plane has a sound of its own: the merlin purrs, the griffon growls, the DB whistles..

Alex

Alex I’m sure its the prop :p

I agree…..havent really noticed the difference in the past between a short exhausted (?) T-6G and a long equipped Harvard IIB or 4.

Whatever it is, its a lovely sound!!

Martin

repeat with me… cavitation :rolleyes: 😉

Alex

Alex I’m sure its the prop :p

NOPE it’s the prop :rolleyes:

repeat with me… cavitation :rolleyes: 😉

Alex