Andy Ross

> [Starting a new thread.  The reply nesting level in my 
> mozilla window  was getting freaky.]
> Vivian Meazza wrote:
> > The engine I'm trying to specify developed 1140 HP at engine 
> > revolutions of 2850 rpm at a boost pressure of 9 psi. It was fitted 
> > with 1:0.477 reduction gearing, which I think means that 
> the propeller 
> > turned at 1360 rpm.
> Hrm, 1360 RPM is very slow for a cruise value, just over idle 
> speed for a smaller plane.  Likewise, 2850 RPM really isn't 
> that fast for a piston engine.  It's at the top end of 
> ungeared engines like a Lycoming O-360 or whatnot, but not 
> really very fast for four stroke engines as a whole (my 
> Saturn redlines at 6000, for example).
> Is it possible that the 2850 number is a *propeller* RPM at 
> max power? Then you'd get a max power engine speed of 5975, 
> which seems plausible to me and avoids the problems with 
> solving for a propeller which "cruises" at a pitch where 
> normal props would be windmilling.
> Does anyone have good info on whether the cockpit engine 
> speed gauge in a Spitfire (which is presumably what most 
> sources will quote for
> "RPM") reads engine or propeller speed?
> Andy

I pondered that question for quite a while before I decided to use that
data. And I agree that the max engine rpm sounds low when compared to modern
engines, particularly modern automotive engines. Propeller rpm seems
impossibly low, and I wondered if I am misinterpreted the meaning of the
published gear ratio of 1:0.477.

All documents that I have seen quote the max engine of the Merlin as 3000
(2850 is the max cruise). Similarly, all the POH (Hurricane/Spitfire/p51d)
quote the cockpit instrument as "engine rpm"

Compare the 2 engines

Bore 5.4 in, Stroke 6 in, Displacement 1,649 cu in (27 litres). Max rpm 3000
Bore 3.38 in, Stroke 3.46 in, Displacement 180.75 cu in (2.962 litres) max
rpm 6000

This is a rough formula derived for automotive applications. A piston speed
of 3500 fpm is usually quoted as an estimate for non-high performance modern

 RPM limit = (Piston speed (fpm) * 6) / stroke (in)

If we take the Saturn data, and re-arranging, we get:

        Piston Speed = 6000*3.4/6 = 3460 fps

We can see that the Saturn complies with this paradigm.

Now taking the Merlin data:

        RPM Limit = 3500 * 6/6 = 3500 rpm

We can say that it is highly unlikely that the Merlin engine would have been
capable of achieving the 6290 rpm required if the max rpm were quoted as
propeller rpm. This would call for a piston speed of:

        Piston Speed = 6290 * 6/6 = 6290 fps

4000 is usually quoted as the maximum for high performance engines, although
the modern F1 engine exceeds this, 4000 would be a reasonable limit for a
1930's engine

I think it is safe to assume that the rpm quoted for the engine rpm for the
Merlin is indeed the engine rpm.
I believe the low propeller rpm was to do with tip speeds approaching or
exceeding Mach 1 at high aircraft speeds. I will research that next.



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