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Ed Cesnalis

WOT above 8,000' ?

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207WF posted about needing WOT when flying at 12,000' density altitude, he felt as though he should get 5,200 RPM with 70% throttle.

I have a lot of trouble seeing the logic here.

As you climb your performance increases due to the less dense air and less drag until you hit ~7,500' at that point the reduction in O2 has robbed you of ~25% of your available power so as you climb higher performance decreases, available hp decreases and fuel burn decreases.

With the lower fuel burn, lower available power and lower speed why pull back on the throttle? Say your cruising at 10,000' isn't 65% or 65hp already being kind to your engine and your wallet wihout pulling back?

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This is coming from the LOP world of normally-aspirated, fuel-injected engines, but...

 

...among Cirrus aircraft, much is made of operating WOT, and controlling power with mixture. The point is made that anything less than WOT means voluntarily putting a restriction in the free flow of air to the engine (the throttle plate) and is compared to the effect of operating with a dirty air filter. IOW, for best performance why would you want to restrict air getting to your engine?

 

Obviously, we don't have mixture controls, so the only way we can affect power produced is via the throttle (and altitude). So, for more economy/range at lower altitudes we must clamp down on that air flow to the engine. But you're right that altitude, buy itself, eats into maximum power. In most planes, the higher you go the faster you go on any given fuel flow/power setting.

 

Here's a graphic example of that:

 

Cruise%20Performance%20Chart%20(True%20Airspeed),%20Langley%20Flying%20School.gif

 

BTW, I still haven't seen a good performance chart for the ROTAX 912 that shows fuel flow/%power at different altitudes. The one for my Sky Arrow only goes up to 6,000', and the fuel flow and range numbers have always seemed suspect to me, fuel flow going up with altitude and range going down, neither of which makes much sense to me.

 

web.jpg?ver=12994992170001

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BTW, while one aspect of the posted chart is specific to a particular Cherokee (the speeds), others are simple physics and universal:

 

Cruise%20Performance%20Chart%20(True%20Airspeed),%20Langley%20Flying%20School.gif

 

IOW, 100% power can only be achieved at sea level, declining as soon as you start climbing*.

 

75% power can only be maintained until about 7,600', at which point it begins to decline.

 

65% power can only be maintained until about 10,300', at which point it begins to decline.

 

55% power can only be maintained until about 12,000', at which point it begins to decline.

 

All this assumes standard day conditions and normally aspirated, of course.

 

Note the slant of the "Rated Power" lines. That's where you can see that for any given power setting and fuel flow, the higher you go, the faster you go. So the optimum altitudes for both speed and range for each power setting would be 7,600', 10,300' and 12,000', respectively.

 

 

*This is a trick question I'll use on a checkout or BFR and to which I rarely get the right answer. IOW, I'll ask: "What's the HP of the engine in your CT?" Answer: 100hp. Most will get that right.

Next question: "Up to what altitude can you get that 100hp?" Most pilots will choose some altitude (apparently at random). The correct answer is "Well, that 100hp is only available at SL on a standard day and starts dropping as soon as you leave the ground".

 

Don't say you weren't warned! B)

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..."Up to what altitude can you get that 100hp?" Most pilots will choose some altitude (apparently at random). The correct answer is "Well, that 100hp is only available at SL on a standard day and starts dropping as soon as you leave the ground".

 

 

my answer is that you can realize 100hp at any altitude agl as long as da is 0' or less. if only 0' da then you need WOT and 5,800 RPM as well

 

same is true with your other examples:

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I miss the performance charts of the Part 23 aircraft. Even with a constant speed prop, you'd see various perf tables/charts for MP settings. I know Rotax can't put out a universal chart given the different props but you'd think the manufacturer's could come up with a specific reference chart for a specific setup. For my plane, I could envision one of the Piper-like charts with the following legend: Gross Weight: 1320lbs, Woodcomp SR-200 Ground Adjustable propeller, 21 degrees pitch.

 

Just dreamin'. :-)

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Afternoon All: I couldn't pass this one up. I had a Cherokee 140 and it wasn't until I added aaal the speed goodies and did the 160 hp mod, that I approached Piper'ws wildly optomistic charts. With the Rotax and attached CTLS I find I really don't have wo worry about such mundanities as "at what altitujde do I have 100hp" to answer is as someone stated: when you have a standard day and are at sealevel. Consider, I flew from Albuquerque to Phoenix at 10,500 and susubsequently due to weather and terrain I have exceded 10000' on I imagine less than 100 horses straining to take me up.

 

As I have learned over the years, density altitude is the single most important factor in talking about performance. Again,(based soley on my experience) one can take a MGWT CTLS from sealevel where the R/WY tempature exceeds 100F and climb to 10,500' without leveling off inbetween; certainly the rate of climb above 8000' is sluggish to say the very least. The CTLS, as a point of reference, has better performance than my ole Cherokee; I fly it conservatively and don't really try to press obvious limits.

 

I guess if I were looking to a performance wish list, I would love to have Rotax introduce a modern direct fuel injection system and some "cleaning up" of the exhaust system. These would add modest performance inprovements, particularly at higher density altitudes.

 

Nancy and I are planning to go to the Page Fly-in, via Phoneix, Sedona, then to San Diego, and back to Pensacola; I will concern myself with performance, but I really do not want to see early snow. My basic point is that within the range of "normal" flying we CTers do, less than 100 hp takes us safely where we want to go, without fretting about the prepherial issues.

 

Enjoy, Ken Nolde, 840KN 400 hours of fun.

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certainly the rate of climb above 8000' is sluggish to say the very least.

 

When the climb rate starts getting sluggish, go to 0 degree flaps. Your airspeed will drop, but the climb rate kicks back in.

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I know one or two CT pilots that have been to 18K. Rotax says 16K is ok for their engine. Weight and porp setting are everything at those altitudes.

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You guys are all wrong. B) I would guess that none of us can get 100HP from our little Rotax. The only way is to pitch the prop for 5800 RPM at sea level on a standard day.

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You guys are all wrong. B) I would guess that none of us can get 100HP from our little Rotax. The only way is to pitch the prop for 5800 RPM at sea level on a standard day.

 

we are not all wrong: from above "my answer is that you can realize 100hp at any altitude agl as long as da is 0' or less. if only 0' da then you need WOT and 5,800 RPM as well"

 

 

 

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we are not all wrong: from above "my answer is that you can realize 100hp at any altitude agl as long as da is 0' or less. if only 0' da then you need WOT and 5,800 RPM as well"

 

I'm sorry I missed that. I didn't recall anyone mentioning the 5800 RPM. It was late when I typed that. I was waiting on the timer for the runway lights to go back to low. I think we have had some lightning damage. One runway and the taxiway does not come on with the PCL, and I couldn't get the taxiway on at all.

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You guys are all wrong. B) I would guess that none of us can get 100HP from our little Rotax. The only way is to pitch the prop for 5800 RPM at sea level on a standard day.

 

The question was just meant as a springboard for discussion.

 

Certainly if someone responded, "Well, the little ROTAX is rated at 100 hp, but that's at sea level on a standard day, so as soon as you start climbing the hp starts dropping. But that's also only available at 5,800 rpm, and the props on our planes usually don't even permit that, so..."

 

I'd stop him there and consider the pilot understands the effect of altitude (and rpm) on available hp and move on.

 

But I think you'd be surprised how many pilots will say the 100hp (or whatever) is available up to some altitude, usually 6,000' to 10,000'.

 

Try it!

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Roger, do you know how Rotax does their dyno test? Lycoming does testing with straight stacks, so when the engine gets installed in the airplane the exhaust system can rob power output. In a Cherokee with the 150hp engine dyno output is only about 130hp because of the installation. Power Flow used to claim bringing the power back up to almost 150hp with their tuned exhaust. Just wondering how much power loss we have in our aircraft installation. Tom

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Hi Tom,

 

I don't know how they do their testing. Jeremy has been back to the factory maybe if he reads this he might be able to comment. All engine Mfg's do their test under ideal conditions which we will never achieve in the field. I never get too deep into this because other than adjusting my prop where I want it I can't really do much about it. It's kind of, "It is what it is" and we have to live with it. Same with all the other engines out there. We can have some effect with a few tweaks, but we're limited.

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Roger's attachment shows how bad the rotax performance data are. I wish FD would make us something like fast Eddie's Cherokee 140 chart! WF

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