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
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