In the various Spit Speed tests in the Nat archives I actually came across one report that actually achieved better speed at 2800RPM than 3000RPM. It was so unusual that it was cause for comment in the report. I will try and hunt it down.

Edit Found a ref to this phenomenon in Hurricane I ROTOL trials. In the Hurricane test they specifically tested for for this phenomenon as it had occurred on Spit I ROTOL trials on aircraft N3171 in AAE/692. This reference in the Hurricane report AVIA 16/681 Hurricane I ROTOL Trials.

Here is the relevant page in the Hurricane tests that references it:

http://i40.photobucket.com/albums/e2...rurpmspeed.jpg

EDIT 2
Here is the text from the N3171 report:

4.2 Controlling R.P.M. Speed tests were made at three heights below full throttle height with the boost pressure maintained constant at 6¼ lb. per sq.inch whilst the R.P.M. was varied over a range from 2600 to 3000.

The results show that the maximum level speed is reached with the airscrew controlling at 2800 engine r.p.m. On increasing the r.p.m. to 3000 the speed was reduced, on the average by 4 m.p.h.

For the particular engine fitted there is a reduction of 17 b.h.p. at constant boost (+6¼ lb) when the r.p.m. are increased from 2800 to 3000. The loss of speed is therefore probably due to the loss of power accompanied by a slight decrease in airscrew efficiency. The matter is being further investigated by Messrs.Rolls-Royce and Messrs.Rotols. It will be noted that reducing the R.P.M. from 3000 to 2800 lowers the full throttle height by 2000 feet.

Conclusions.

1.This aeroplane has a much better take-off and climbs faster than other Spitfires fitted with wooden fixed pitch or metal two pitch airscrews.

2.There is a drop of 13 m.p.h in maximum level speed compared with the 2-pitch airscrew aeroplane but of this, 8 m.p.h. can be attributed to sources other than the airscrew.

3. Below full throttle height an increase in speed of about 4 m.p.h. can be attained by controlling the engine R.P.M. at 2800 instead of 3000.

4.The limiting diving speed can be reached much more rapidly with this aeroplane than with Spitfires fitted with fixed pitch wooden and 2-pitch metal airscrews.

This text cut and paste coming from here:

http://www.spitfireperformance.com/n3171.html

I'd just like to add that RPM does affect boost in reality. The greater the RPM, the greater the suction in the intake leading to lower intake pressure. With lower RPMs the suction reduces and the pressure rises slightly.

However, if the boost is high enough for the boost cutout mechanism to be in operation then no change in boost should be apparent with a change in RPM since the mechanism will automatically compensate to keep boost at it's maximum permissable value. I believe this is what you are referring to. Just don't want people thinking the whole link between RPM and boost is a quirk in CloD.

Hi Sutts,

I agree with you but would state it slightly differently:

Although manifold pressure ("boost") and rpm are linked in a physical sense, the Merlin boost controller manipulates the throttle valve to maintain a boost pressure setpoint. It will compensate for rpm and height changes within it's mechanical limits, and thus keep boost pressure constant.

IvanK:

Very interesting report. It does make sense that the airscrew efficiency curve could cause a reduction in thrust with increasing rpm. But I think you are right that this is an unusual finding in a specific case that requires investigation. There is not much point giving 3000 rpm availability on the CSP control generally (as was ubiquitous on Hurris and Spits) if this represents a thrust reduction due going past an optimum on a prop efficiency curve.

Sorry for the stupid question, but summaryze the report, what is the maximum achieved speed on Hurry Rotol and Spit Rotol?

Spitfire IA (beta patch): 248 mph @ 0m

That's a 'tad' bit too slow. I assume this is for combat rating, 6.25 / 3000..?

All out +6.25 and 3,000 RPM, however change from 2,600 RPM to 3,000 RPM didn't make a notable difference. Radiator open/close also didn't change anything

So still that radiator bug ...

My fear is that there are so many issues and considering that they are currently working on the big issues right now and how long it takes, it will take another 3 years to fix this sort of issues ... *sigh*

Should be 284mph (305 :(

My friend tells me he can't get Spit II to more than 270mph @ 500feet, should be ~290mph :(
He also finds max Hurricane speed at SL is 220mph. Not even what they published for the patch (272mph) or RL data 262mph.

EDIT: I found Hurri gives 234mph

Well, my beta patch gives me CTDs within 5 mins of taking off in ATAG MP. 50% of people chatting seem to be in the same boat, whereas the other 50% are totally CTD cured for hours on end. Go figure :)

So what else to do but go single player, turn off the cockpit and get SL speeds for the patch?

Technique: skim waves, centre ball, settle as close as possible to 0fpm. As noted above the tool tip in cockpit view does not agree with the cockpit guage (possibly it rounds down to nearest 10mph?), all these speeds are based on the guage in no cockpit view.

My SL speeds and observations:

Spit II
275-280mph 3000rpm full throttle (~6.25psi boost)
290-295mph 3000rpm full throttle red tab (~9psi boost)

rpm boost quirk (drop in rpm increases boost) is gone, boost stays constant with rpm. Dropping rpm to 2600rpm does not lead to speed increase any more

Spit Ia

255-260 mph 3000rpm full throttle (~6.25psi boost)

As per Spit II, rpm boost quirk is gone (boost constant with rpm), and dropping rpm now does not increase speed. Boost cut out still does nothing.

Rotol Hurri

240-245mph 3000rpm full throttle (~5.25psi boost)

Boost quirk is still there for the Hurri, i.e boost increases to approx correct value (+6.25 psi) only if rpm dropped to 2600rpm. However unlike before, this does not increase speed.

109E4
with prop auto at 2300rpm only
450-460kmh (280-286mph) at 1.35ata
460-470kmh (286-292mph) at 1.45ata


On the bright side, E4/Spit II matchup may be rather "balanced" with max SL speeds almost identical. Both Spit Ia and Rotol Hurri are now "I like a challenge" only planes :)

camber