How to find out where to cruise at high level

[Pikey]

It's not often I ever needed to go high, but I am struggling to find out where the Spirtfires 'happy altitude' is. I know that altitudes depend on pressure, high level winds and such but generally higher is better for jets. Props are a tad more complex, so where is the Spitfire mk9's cruising best performing altitude?

I read that the LF.IX was 404 m.p.h. at 21,000 ft. and this was the altitude at which it's top speed was at that day. Does this mean 21,000ft is also a good goal for cruise? Just to be clear, the purpose would be distance and speed rather than fuel performance but if someone could provide one answer for distance and loiter/fuel it would help me also to understand.

[Pikey]

I found http://www.spitfireperformance.com/spitfire-IX.html which probably gives the answers but doesn't help me understand the physics and the why.

[razo+r]

Had a brief glimpse through this, maybe it helps a bit? https://commons.erau.edu/cgi/viewcon...&context=jaaer

 

In the manual, there is a table with the cruise power settings and the maximum altitude with it. Maybe that helps too?

[DD_Fenrir]

Short answer: Yes. 21,000ft is the full throttle height for the LF.IX; ergo it is the point at which the drag curve and power curve intersect. Go higher and the drag is less but the lack of oxygen prevents the engine from generating +18lb boost, even with the supercharger in full song. Go lower and you'll get the oxygen the engine needs but the drag on the airframe of the increased air density means you won't reach 404mph.

 

Simply put if you fly at a lower power setting (advised as you have only 5 minutes at WEP - that is 3,000 RPM @ +18lb) then you are still at the best height to balance Speed and Power. It theoretically could change (perhaps going higher till the engine is only able to produce the power you desire) but as a starting point it makes sense.

 

Bear in mind that you'll need RPMs to be quite high to maintain thrust at those higher altitudes and this will negatively affect your fuel consumption, but your TAS (and ergo GS) will be higher.

[grafspee]

In spitfire's pilots notes you should have table with cruise settings and alt for different scenarios, max range, max duration etc.

There is one trick which will bring your fuel consumption low, you can block supercharger to switch to high speed, and then you can climb higher with Low supercharger speed, this will bring your fuel consumption very low.

Very useful when you realize that you should RTB long time ago and you need to RTB on fumes :)

If it come to optimal alt i would say that you should cruising at full throttle height. this mean that what ever you rpm is, is it 2500 or 2600 or 2400 you look for alt where you have desired boost at throttle wide open.

This will give you fastest cruise speed, but not best economy.