TB21s have the same tanks as TB20 no? If they fly at higher power settings at altitude thanks to turbocharging then less range would make sense.
Yes, same tanks. The TAS gain is higher at altitude but possibly not enough… We had a thread on this a long time ago, about the theory that power required is proportional to TAS. I can’t get my head around this stuff very well. I certainly get more MPG at a higher altitude (same TAS for a lower fuel flow).
I suppose this is correct for non-turbo engines, but for turbo engines? A friend of mine just bought a Turbo Commander and we are both trying to find out how to operate it. The POH is very specific about never letting the EGT get higher than 1725 degrees, and sure enough the EGT gauge shows a red line there.
First of all, if your turbo aircraft is dear to you, get CHT + EGT (all cylinders) + TIT on a digital display, otherwise it’s just dangerous guesswork. The limit is not actually an EGT limit but a TIT limit (turbine inlet temperature). You can be at the limit, that is quite normal but don’t go over the limit for more than a few seconds. The turbo gives you the great advantage of being able to choose among different MP/RPM combinations for a specific power setting and by varying this, you can find the best one for you. I generally try to go at the lowest RPM possible and choose a higher MP. My 65% cruise is usually 23"/2150 which works at all altitudes. I always fly Peak EGT, sometimes LOP (to the extent possible), never ROP beyond the initial climbout.
TB21s have the same tanks as TB20 no? If they fly at higher power settings at altitude thanks to turbocharging then less range would make sense.
The SFC of the TB21 is a tad worse than the TB20 due to back pressure and low compression cylinders. Also the aircraft makes you go at 75% which on a TB20 is not an option unless you are down low.
Would be good to see a comparison of the power setting charts. If a TB21 runs 23inches MP versus say your 17 at altitude then more air = more fuel flow :)
Yes – forgot about the compression ratio.
SFC is proportional (roughly) to the square root of the compression ratio so if e.g. you increase CR by 10% you get 5% better SFC (or MPG, if you like).
If a TB21 runs 23inches MP versus say your 17 at altitude then more air = more fuel flow :)
Definitely yes, but you are also going faster.
But going faster means less MPG, because the drag is mostly parasitic.
It would be interesting to see a TB21 and a TB20 side by side, same MP, same fuel flow, and see what speeds they do. The 21 should be slower, due to the lower CR.
You can see some of the comparison at http://1drv.ms/1qqMVwF, in particular the last two worksheets.
The paper is indeed well written, and easy to understand, and the references are trustworthy (MIT professors of mechanical engineering departments). However, I have a few questions/critiques:
I am not sure how the survey was conducted. Am I right that engine and cylinder overhaulers usually never see or hear anything about the pilot that flew the engine? For them its just another anonymous engine or cylinder. So how do they know how the engine was operated, unless they have talked directly with the pilot?
It’s surprising that so much heat gets shed through the little contact surface between valve seat face and seat insert, and the two surfaces aren’t even in full contact all the time. Yet, CHT is directly correlated how well heat is transferred through this contact surface and the valve stem. As the paper states: “CHT have a direct affect on the temperatures of the exhaust valves”. The way I understand this, is that CHT is a direct proxy on how hot the exhaust valve is. It matters less what causes the high exhaust valve temperatures. In this sense I don’t understand why to look at the sum of EGT + CHT. The paper does not say which one of the two factors is more dominant. If I would have the choice between 425 + 1400, or 375 + 1450, I would choose the later setting.
It is quite in contradiction with Mike Bush’s claim that absolute EGT values are meaningless, and on top of that highly inaccurate.
Indeed. He (and also John Deakin, George Braly, etc) always said that you can’t “burn your valves” through high EGTs, but there does seem to be at least a slight bit of truth to it.
OTOH, the difference between peak EGT and “other” mixture settings is often no more than 50 F, so we are talking say 1400 vs. 1450 degrees. Can’t really make a difference, no?
