Turbo Cirrus Blog By Braly - All Comments

re: New Induction Air Temp probe in the Cirrus Perspective

George Braly — Fri, 16 Oct 2009 05:28:02 GMT

Marcus,

Sorry for the inexcusable delay in seeing your post.

The Cirus/GARMIN implementation of our algorithm (the intellectual property for this belongs to TAT) for estimating the correct target fuel flow is pretty well done. So typically, if the blue line is indicating 16.9 gph when the MAP is 29.3,4,5,6+ and the RPM is near 25xx - - that will mean it is a rather hot day and the mass air flow from the turbo is decreased due to the temperature (ie, you will have a higher density altitude than your pressure altitude by several thousand feet.)

(As an aside, the only reason this "blue line" target fuel flow indicator was not implemented on the AVIDYNE was a general perception at the time that the original Avidyne CPU was cycle challenged. R9 is very much more capable and I would expect to see some nice upgrades in future software releases from Avidyne for the R 9 systems.)

The bottom line - - - if the blue line is indicating 16.9 and you try to ignore that by more than small amounts - - I would predict that you will end up with CHTs rising past the 380dF working target.

It is useful to take a look at the actual language in the AFMS for the turbo leaning procedures to see how those instructions nicely "integrate" with the concepts in this response.

So, if you put a conforming G3 with Avidyne in formation with a conforming G3 with G1000 I would expect, when the G3 Avidyne person finally finished pushing the fuel flow as far up as he could and still stay under the 380dF CHT target - - that the fuel flow on that airplane would likely be reasonably close ( +- 0.2 gph) to the fuel flow on the formation aircraft G3 G1000 blue line.

Cut me some slack on the numbers because there are a lot of calibration errors and tolerance build ups that will make that exercise very difficult to re-produce in the real world.

Does that answer your question ?

Regards, George

re: INTERCOOLERS - - ARE WONDERFUL DEVICES!

George Braly — Sat, 25 Apr 2009 23:29:14 GMT

Brian,

See the solid red line in the last graph in my response to Mike Busch's question in the "Compressor IAT, CDT vs Alttiude" blog.

It depicts the performance of a belt/gear driven compressor with a limted pressure ratio of around 1.4:1, and a compressor efficiency of around 0.60. I think that graph is relevant to the numbers you are trying to generate in your spread sheet.

Your model makes the assumption that the compressor is only putting out around ~ 30" compressor discharge air at sea level.

For a belt/gear driven compressor that has its speed derived from the crankshaft, that is impossible unless the inlet to the compressor is choked down. Even then, the compressor just continues to do its work and raise the compressor inlet ambient pressure by is rated pressure ratio (at the fixed RPM) . That means there is, as I understand it, a lot more work done on the incoming airstream and therefore the sea level standard (or hot day) day compressor discharge temperature will be much higher than I believe you are using in your calculations. If you had a variable speed compressor - - you could arrange for the numbers to work out to much lower induction air temperature values as you are depicting.

Thus, if the pressure ratio is around 1.4 at rated crankshaft speed, then, at sea level, standard atmospheric pressure, any belt/gear driven compressor is ejecting air at a compressor discharge pressure that is about ~1.4 X 29.92" ~ = 41.9" HG. So the heating value into that airstream is pretty large.

You might get some of that back, when you choke down the airsupply, upstream of the throttle, with a butterfly plate controller, but that process is also very inefficient.

This is all very different from the free shaft turbo compressor which can spin at a large variety of different RPMs, depending on the mass flow of air that is required. At sea level, the typical TN turbo compressor discharge pressure is around 31", not 41.9" like the calculations indicate for a belt/gear driven compressor with a 1.4:1 pressure ratio.

Given the routine 160dF to 175+dF temperatures that I can calculate from the thermodyamic equations - - the reduction in detonation margins is substantial.

One of the ways to avoid detonation is to require the engine to be operated at very high fuel flows while operating at higher manifold pressure settings.

I can, on the test stand, run the induction air temperature on a normally aspirated IO-550 at sea level up into the 160 to 170d F range and, with other parameters operating under hot day conditions, I can, at normal fuel flows, get it to detonate rather easily, even on 100LL that exceeds the minimum 100LL fuel octane specifications by a significant amount (ie, 104MON verses 99MON minimum spec fuel).

Let me know if I have not answered your question, and I will try, again.

George

re: INTERCOOLERS - - ARE WONDERFUL DEVICES!

Brian Goldsmith — Sat, 25 Apr 2009 19:09:52 GMT

George,

Disregard the comment above. Reviewing your blog, I see that I initially assumed ideal (100%) compressor efficiency. Incorporating 0.6 efficiency yields the following:

[View:www.cirruspilots.org/.../0511.ISA-and-ISA_2B00_30-SC-Power-and-IAT-Estimation_5F00_v5.xls:550:0]

The power result: on an ISA day, power drifts down from 98 to 92% as you climb from SL to 6000 feet, and then the SC engine performs like a NA flying at an altitude 3000 feet lower, with a ceiling (based on NA comparable power) of 20 to 21,000 feet. On a very hot ISA +30 day, power drifts down from 89 to 83% as you climb from SL to 6000 feet, and then the SC engine performs like a NA flying at an altitude 3000 feet lower, with a ceiling (based on NA comparable power) of 18 to 19,000 feet.

The IAT result: on an ISA day, climbing from SL to 21,000, IAT peaks at 32C/90F at 6000 feet, decreases to 29C/84F on the way up to 16,000 feet, and then climbs again to 38C/100F at 21,000 feet. On a very hot ISA+30 day, IAT peaks at 66C/151F at 7000 feet, decreases to 63C/145F on the way up to 14,000 feet, and then climbs again to 69C/156F at 19,000 feet. The ISA+30 peak IAT detonation-free HP margin would decrease by 55HP (based on George's detonation-free margin vs. IAT graph).

May I ask you to review the model and comment on need for corrections or refinements? If the model looks acceptable as a estimate of power and IAT, what would you expect to be practical implications of a loss of detonation-free margins between 55 HP? Would an intercooler be beneficial or necessary for detonation-free flight in such conditions, or are there other engine management techniques that the pilot could exploit to stay detonation-free?

Brian

re: INTERCOOLERS - - ARE WONDERFUL DEVICES!

Brian Goldsmith — Sat, 25 Apr 2009 15:52:16 GMT

George,

I'm estimating that the FAT supercharger will increase the SR22's IAT to a peak of 52F/126F when climbing to 6000 MSL from a SL airport on a ISA+30 day (45C/113F), and a peak of 45C/113F on a ISA+23 day (38C/100F). Based on your graph, I interpret the loss of detonation-free margin to be about 28 and 15 HP, for the 113 and 100F SL temps respectively. The fact that your graph's data was collected at 5000 MSL is fortuitous, because that elevation is very close to the FAT SC's critical altitude and hence the region of peak IAT warming. I estimate that at peak IAT on an ISA+30 day, the SC SR22 will make 268HP WOT Best Power (86% Power).

I've based these IAT estimations on an excel model (ISA and ISA+30 SC Power and IAT Estimation_v4.xls) attached in "FAT Supercharger gets FAA approval" forum discussion.

May I ask you to review the model and comment on need for corrections or refinements? If the model looks acceptable as a estimate of power and IAT, what would you expect to be practical implications of a loss of detonation-free margins between 15 and 28 HP? Would an intercooler be beneficial or necessary for detonation-free flight in such conditions, or are there other engine management techniques that the pilot could exploit to stay detonation-free?

Brian

re: New Induction Air Temp probe in the Cirrus Perspective

Marcus Adolfsson — Sat, 28 Feb 2009 22:02:18 GMT

George,

Let's say the blueline shows an optimum fuel flow of 16.9gph for the given conditions, which results in a power less than 85%. If you increase the mixture to 17.6gph to gain some power, is that detrimental to the engine? (How does that compare the pre-perspective turbo's that are usually operated at 17.6gph when LOP)

re: Compressor IAT , CDT vs Altitude - - Mike Busch's Request

George Braly — Wed, 18 Feb 2009 05:09:41 GMT

>>why didn't you/Cirrus simply increase the allowable airflow into the upper section of the cowling? i.e. why not have slightly bigger holes behind the prop for air to enter - thus providing sufficient airflow for both cylinder AND intercooler to use? I am presuming it has something to do with drag (or perhaps aesthetics :)) but am not certain.<<

Aesthetics? Maybe Elegance. We didn't have to increase the air inlet size on the Bonanzas to get them cool. I was pretty sure we could do it without changing the Cirrus cowling.

Besides, Dale and the Cirrus folks were already pretty skeptical about the whole turbo concept back in February of 2006 when we first met and talked about it. Everybody said we couldn't keep it cool - - - without major changes to the cowling. So we hunted around on the computer and put up on the screen the climb cooling test results from the G1 prototype with the standard cowling. It cooled better than the normally aspirated Sr 22.

We promised to finish the STC without changing the cowl inlet design ---- didn't want it to look like a Columbia with soccer ball sized inlets on each side of the spinner!

George

re: Compressor IAT , CDT vs Altitude - - Mike Busch's Request

Kevin G Boles — Wed, 18 Feb 2009 01:55:42 GMT

Actually it was a wonderful explanation! And it actually makes good sense. Thank you very much! One hopefully short follow-up question: why didn't you/Cirrus simply increase the allowable airflow into the upper section of the cowling? i.e. why not have slightly bigger holes behind the prop for air to enter - thus providing sufficient airflow for both cylinder AND intercooler to use? I am presuming it has something to do with drag (or perhaps aesthetics :)) but am not certain.

re: Compressor IAT , CDT vs Altitude - - Mike Busch's Request

George Braly — Tue, 17 Feb 2009 23:57:17 GMT

>>>> the efficient turbo's in the TN SR 22 coupled with the non-standard design of the intercoolers (rather different than the TCM approach to their configuration)

George, can you please comment on that a bit further? Thanks in advance! <<

The turbo's on the TN SR 22 are among the two most efficient models (both nearly identical) ever put in aircraft engines. Further, the design point for the engine results in their operation in the typical climb and cruise at a mass airflow and pressure ratios that keep them very near the center of the sweet spot in the compressor maps. We spent a lot of time and effort to ensure that as part of the preliminary design trade offs and early flight testing.

The intercoolers - - are another story. Take a look at a Columbia (TSIO-550C ) some time or at a Lancair IVP (same engine).

Look at the intercoolers. They are roughly the same length as the TN SR 22. But they are WIDER and they are SHORTER (vertically) than the TN intercoolers.

The result is that they have much larger cross section for cooling air to flow through them - - and the cooling air has a much lower restriction for air flow. The result is that those intercoolers "enjoy" a much larger air flow of cooling air than do the intercoolers on the TN SR 22.

At this point, I probably have you shaking your head, right ? Your thoughts are, so lets see, if intercoolers are so wonderful, wouldn't it be "more wonderful" if you gave them more and more cooling air flow ?

And the answer is "yes" - - but only if the extra cooling air flow is "free" - - like it would be in, for example, a liquid cooled automotive application.

In the airplane, any of that extra airflow inside the upper side of the cowling is airflow that liberally and easily goes through the intercoolers - - and NOT .... where ?

Answer: NOT through the cylinder heads.

We had a fairly extensive "discussion" with the people that do intercooler core designs about our desire to NOT design these the way they had been traditionally designed - - which was a design which had no constraint on the amount of cooling air that was available.

So we optimized the intercooler physical dimensions to force a smaller amount of cooling air flow to "interact" with the compressor discharge hot airstream for a "longer" period of time.

By using this configuration, we actually need LESS cooling mass air flow than is used by the TSIO-550C in order to get equal or better reduction in the compressor discharge temperature before the air hits the throttle plate.

We pay a very small price for that in the weight of the intercooler "core" - - but we more than made up for that by engineering much lighter weight "tanks" that are welded on to each end of the intercooler to connect up the plumbing.

The result is a "system" that has a larger fraction of the total cooling airflow that is available to cool the cylinder heads, than is the case with the typical TSIO-550 installation.

Does that answer your question?

George

re: Compressor IAT , CDT vs Altitude - - Mike Busch's Request

Kevin G Boles — Tue, 17 Feb 2009 15:02:44 GMT

>> the efficient turbo's in the TN SR 22 coupled with the non-standard design of the intercoolers (rather different than the TCM approach to their configuration)

George, can you please comment on that a bit further? Thanks in advance!

re: Compressor IAT , CDT vs Altitude - - Mike Busch's Request

George Braly — Mon, 16 Feb 2009 04:49:57 GMT

MIke,

I've got to confess that I'm glad you teased me into taking the time to do those graphs.

I had forgotten how nearly "perfect" the IAT curve is for the TN SR 22 as one goes to altitude. [I checked the mathmatical model in the graph against the certification data and they agree pretty closely.]

The pressure ratio in the TN SR 22 at 25,000 feet is nearly 3:1. As you know, the turbo's in the Cessna T 3xx and T4xx series aircraft tend to be pressure ratio limited above 16,000 feet (about 1.9 : 1 on the pressure ratio.)

If the pressure ratio is not increasing with further altitude, then one can start to mitigate the rise in IAT (as demonstrated in the 1.4:1 pressure ratio limited example in the second graph.)

But doing that when the pressure ratio is continuing to rise in proportion to further altitude changes above 16,000 feet - - is another trick, entirely.

With that in mind, it is all the more "nice" that the efficient turbo's in the TN SR 22 coupled with the non-standard design of the intercoolers (rather different than the TCM approach to their configuration) results in the IAT that essentially remains constant in the face of the rapidly rising pressure ratio and the corresponding rapid rise in CDT. This is largely due to the efficiency of the intercoolers that make good use of the steadily dropping OAT as one goes to altitude.

Regards, George

re: Compressor IAT , CDT vs Altitude - - Mike Busch's Request

Michael D. Busch — Sun, 15 Feb 2009 22:36:33 GMT

That's extremely interesting data, George. Thanks for developing it. As you know, I fly a Cessna T310R powered by a pair of TCM TSIO-520-BB engines. These are 285-hp, very modestly boosted 7.5-to-1 compression-ratio engines (32" MP redline up to 16,000', then pressure-ratio limited above that) and non-intercooled. Although I do not have instrumentation that lets me see CDT=IAT, I do know that keeping CHTs well-controlled at Flight Level altitudes is difficult (despite cold OATs) and often requires operating profoundly LOP. I suspect that the situation would be very different if these engines were intercooled. Unfortunately, nobody offers an intercooler STC for the T310R, and engineering one would be challenging because the geometry of the induction system on the Cessna T310 and 402 mount the throttle body physically right at the turbocompressor discharge port.

I've looked at a lot of turbocharged engine installations in a lot of airplane makes and models, and I've never seen one that even comes close in efficiency and maintainability as the TATI TN in the Cirrus SR22. Ever since I saw the prototype TN SR22 with its cowling off in Ada years ago, I've been suffering from a severe case of turbo-envy!

re: INTERCOOLERS - - ARE WONDERFUL DEVICES!

Dave Brown — Sun, 15 Feb 2009 16:26:30 GMT

As a mechanial engineer flying a Cirrus I love this discourse between two smart guys. COPA doesn't get any better than this. I'm hanging by my thumbs waiting for the rest of this exchange. Thanks to you both.

re: INTERCOOLERS - - ARE WONDERFUL DEVICES!

George Braly — Sun, 15 Feb 2009 05:59:05 GMT

Mike,

I can do that.

But it is a fair amount of work to dig it out of the files ( or model it up ).

Can you give me a hint as to where this is going ? I might be able to help you with the end point easier than all the intermediate stuff ?

George

re: INTERCOOLERS - - ARE WONDERFUL DEVICES!

Michael D. Busch — Sun, 15 Feb 2009 05:28:40 GMT

George, could you post a graph that shows the relationships between OAT, CDT, and IAT as altitude varies from SL to FL250. In other words, how much temperature increase is produced by the turbocompressor and how much temperature decrease is produced by the intercooler as the aircraft climbs up to the Flight Levels?

INTERCOOLERS - - ARE WONDERFUL DEVICES!

Turbo Cirrus Blog By Braly — Sat, 14 Feb 2009 16:25:50 GMT

INTERCOOLERS - - ARE WONDERFUL DEVICES! As is indicated in the previous blog on Turbochargers vs Turbonormalizers