Climbing-Boost and RPM's?

[francesco.doenz]

I am suddenly confused, while climbing with the Mark one and the de Havilland 20 airscrew, When the RPM's are too high, around 3000, is it wiser to reduce the the boost or to use the airscrew control to get down to around 2600 RPM?
I guess the answer can be extended to general flying conditions, if RPM's too high, how should one reduce them, by reducing boost or airscrew control?

[shortspecialbus]

I am suddenly confused, while climbing with the Mark one and the de Havilland 20 airscrew, When the RPM's are too high, around 3000, is it wiser to reduce the the boost or to use the airscrew control to get down to around 2600 RPM?
I guess the answer can be extended to general flying conditions, if RPM's too high, how should one reduce them, by reducing boost or airscrew control?

I usually try climb in the Mk.Ia 2-pitch with about 4-6lb boost and I fanagle about 2600RPM. For low altitude cruising I'll use about 0-2lb boost and about 2400RPM. I use autorich mixture for both.

High altitude cruise (25k or so) I tend to have throttle full with low RPM, around 1800-2000 on autolean. At that RPM, I get about -3 to -4 boost or so at that altitude.

I may be wrong on some of this, but it's worked successfully for me so far.

-stefan

edit: to answer your question about using RPM or boost, it's really kind of both in the 2-pitch. Whatever the situation dictates, I guess - I don't know if there's a hard and fast rule.

[Killratio]

Low revs high boost will get you safely home to roost.

Always reduce rpm in preference to boost, it is how the engine is set up to work most efficiently, fuel and maintenance wise.

(note that reducing RPM will automatically reduce the maximum boost available in any case other than at very low altitude and high speeds.

[francesco.doenz]

Thank you Killratio for your input as always! Is the drop in boost when you reduce the RPM simulated as in real life? I find the drop in boost rather abrupt when I reduce the RPM's!?

[Killratio]

Always a pleasure Francesco,

I believe the modelling is spot on. On more than one occasion in testing something seemed "off" or a figure wasn't matched and Scott would come back with "oh, I found x" and suddenly a whole RANGE of figures would match up perfectly, not just the one that was tested. This is, IMO, testament to who well Scott's model runs like a real engine driving a real propellor. An example I remember well was a particular Watts Airscrew figure at 13,000ft that wasn't "clicking" with me. After only a few minutes Scott sent the next "update" and all of a sudden a pile of Watts, DeH and Rotol figures jumped into line with the Boscombe Downs testing figures. But I digress...

The layman's explanation (and I am most certainly a layman when it comes to superchargers) is that normally a rapid reduction in RPM would increase boost as the system "backed up" under pressure (Cliffs of Never's Spitfire and Hurricane are modelled , incorrectly, like this largely,reading the back and forth at the time, because the Rev Heads in that community tried to transfer their motor vehicle experience to aircraft) but with the Merlin the supercharger is direct drive off the engine and therefore decreased RPM results instantly in less boost being available..the supercharger is physically incapable of providing it as it is no longer spinning as fast...sort of "engine braking" for blowers!

Sorry I don't have time to go into it more deeply but I am sure someone will correct any bad assumptions I have made.

regards

Darryl

[whiic]

Killratio's explanation is rather good.

Direct-drive centrifugal superchargers are highly rpm variable in their charging capability. They produce better boost at high rpm.

Direct or belt-driven roots supercharger would produce a rather flat additional boost regardless of rpm (and with increased losses the boost might even drop at higher rpm)

Turbo-changers would have turbo-delay both increasing and decreasing their rpm whine neither of centrifugal and roots have any delay since they are connected to the crankshaft either directly, via a transmission, or via a belt. Turbo-chargers also have varying boost depending on amount of exhaust gases. It doesn't even care about the engine rpm (provided manifold pressure is adjusted to provide equal amount of exhaust gases to the turbine).

____

The abruptness of boost changes with changes in rpm is thus because:
- centrifugal type superchargers being highly rpm dependent for effectiveness
- direct drive of the supercharger.

The abruptness is most noticeable at high altitude.

Also one big factor why it appears to be very abrupt is that MkI's boost gauge is limited to -4...+5 psi (i.e the most usable boost range for continued operation). For comparison, a full vacuum would be around -14.5 psi, and the full WEP is +12 psi. Even with the throttle plate fully shut (and wind driving the propeller) you can't of course reach a full outer space comparable vacuum but around -13 psi is realistic. So out of -13...+12 psi, you only see -4...+5 psi on the gauge.

A2A P-51D for example has a range of 10-100 inHg on the boost gauge. That corresponds to -10...+34.5 psi. On a scale of -4...+5 every movement of the needle is quite a bit faster than on a scale of -10...+34.5. Your EXTREMES on scale of -4...+5 psi makes the needle on P-51D barely even twitch! (That range corresponds to 22...40 inches on P-51D's boost gauge that uses absolute pressure in units of "inHg". Where as Spitfire uses relative to normal pressure at sea-level as reference "0".)

So I'd say the abruptness of boost changes is highly psychological. The needle moves faster but only because the boost gauge has a VERY limited range of measurement.

___

2600 and throttle firewalled (with boost cut-out on, i.e limited to 6 psi max) is a maximum sustainable power setting for climb. I'd prefer to keep it within gauged area at all times, i.e +5 psi max just so you can verify the boost is within parameters and not blindly trusting the cut-out.

I do prefer using lower power settings for climb, like 2400rpm +4 psi auto-rich or less. Usually I climb with 2000...2200rpm 0...+2 psi auto-lean, using a shallow climb angle. I'm not in a hurry and I hope to get at least some four digit figure out of the engine without overhauls. So far only fresh oil and filters.

[n421nj]

So boost in the spit is like manifold pressure?