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Ram air TIO540

I am surprised at the difference that ram air makes to the power of my TIO540s.

For example, yesterday I was flying in mild mountain wave (at FL120) with my TAS varying between 185 and 197 kts. As my speed increased, so did MP, by maybe +/- 2" either way. Both engines were identical.

So, if I was doing 193 kts/34" on average, it was 197/36" to 185/33".

Similarly, if I pitch forward to descend, without reducing power, the MP increases quite quickly with airspeed (not slowly, as the air gets thicker.)

So two possible explanations; this is normal, demonstrated by the fact that both engines behave the same, or they are both set up equally wrong.

Any thoughts? (Really looking forward to APS course!)

EGKB Biggin Hill

That is not immediately explainable by straight application of physics. The difference in dynamic air pressure between 185 KTAS and 197 KTAS at FL 120 is around 700 Pa or around one fifth "Hg according to my book.

I remember the old Mooney Executive I use to fly, that had a “Russian turbo lever” that would bypass the air filter and expose the intake to the full ram pressure. It gave an extra 1" (and 5 more knots), which is quite close to the actual dynamic pressure of air at the typical 130 KIAS.

huv
EKRK, Denmark

I wonder if Timothy’s wastegate mechanism is being affected by the air pressure, more than intended?

The difference in dynamic air pressure between 185 KTAS and 197 KTAS at FL 120 is around 700 Pa or around one fifth "Hg according to my book.

Doesn’t that depend on the geometry of the air duct? What I am getting at is that a tapered air duct can create more pressure. Can’t it? Is that a thermodynamic impossibility? How do ram jets work?

Somewhere in this thread is an assertion that the 10 degC temp rise which has been measured cannot be explained by the ram air compression, too. Not entirely unrelated.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

I wonder if Timothy’s wastegate mechanism is being affected

That is also what I would suspect. Maybe the mountain wave induced a bit of bootstrapping in the turbo controller?

Last Edited by huv at 17 Apr 07:31
huv
EKRK, Denmark

That both engines work the same, the entire turbo charging system having been overhauled by different people at different times, seems to imply expected behaviour as opposed to maladjustment, no?

EGKB Biggin Hill

Timothy wrote:

I am surprised at the difference that ram air makes to the power of my TIO540s.

What plane is this ? Does it have a “fixed” wastegate system or does it have a controller ?

It’s highly unlikely that the ram air has a direct effect on the MP.

It’s most probable a result of something like a change in the static reference line to the MP gauge or dropping the induction air temperature from added cooling.

Last Edited by Michael at 17 Apr 09:28
FAA A&P/IA
LFPN

1973 PA31-310

EGKB Biggin Hill

First of all, I agree with the statement above; I don’t think this is due to ram alone, the physics are quite out of proportion.

Secondly, I realise there are many different types of turbos, but at least in a wastegate controller setup and as long as you’re not at full throttle, the controller should adjust to maintain the MP you set. If you have a controller, it may be worth checking if it’s been properly lubricated (with mousemilk). I do that at every oil change (25h), and I get bootstrapping if I don’t (or even loss of MP at altitude). Many mechanics won’t do it if not asked.

EGTF, LFTF

One is very recently overhauled (40 hours ago), the other 400 hours ago.

The thing is, they are identical in behaviour.

EGKB Biggin Hill

I know zilch about twins, so this is a very stupid question; but someone on another thread mentioned that one of their engines was “drawing more MP than the other”, is there any kind of equalisation/crossover mechanism that would mean that an issue on one of your turbos could affect the other?

EGTF, LFTF
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