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  #1  
Old 03-15-2015, 09:51 PM
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jenatepilot jenatepilot is offline
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Default Best Cruise Altitude

So Ive got a whoppin 14 hrs on my bird and noticed something weird. At 3000Ft, my TAS is awesome. At 8000ft, it's pretty good, and at 12,500 it's not that good. Is there an "optimum" altitude for cruising a long ez based on things like IAS or something? I want to have a prop built based on what altitude the airframe is fastest. Anyways, I may be way out in left field on this one but would like your input. Thanks!
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Old 03-15-2015, 10:58 PM
longezdave longezdave is offline
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Default Re: Best Cruise Altitude

It depends on if you are going for the best altitude for speed or economy/range. If you just are looking for speed, then the lower the better. If you want to cruise at 75% power, that usually makes around 8K the best for speed.

If you want economy/range, then up in the teens is what you want. If I'm going on a long trip and the weather is suitable, I go up to 16 to 17K. Up there I can lean it out and get the fuel flow below 4 GPH. You can go a Long way up there if head winds aren't an issue.

You're making me wonder about the mixture control on your engine. Does it lean automatically up high, or do you manually lean it? If I didn't manually lean my engine up in the teens, I wouldn't get nearly as much speed or economy.
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Old 03-15-2015, 11:11 PM
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jenatepilot jenatepilot is offline
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Default Re: Best Cruise Altitude

Dave. That's exactly what I was looking for. UL claims that their engines automatically lean at altitude but give no way for the user to control it. Wonder how "lean" it really is.

I've also got a poor setup for induction. Small 2" naca on top of the turtle deck. That's gonna change.

The engine is very fuel efficient so I'm most concerned about "getting there quick"
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Old 03-16-2015, 12:48 AM
Marc Zeitlin Marc Zeitlin is offline
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Default Re: Best Cruise Altitude

Quote:
Originally Posted by jenatepilot View Post
So Ive got a whoppin 14 hrs on my bird and noticed something weird. At 3000Ft, my TAS is awesome. At 8000ft, it's pretty good, and at 12,500 it's not that good. Is there an "optimum" altitude for cruising a long ez based on things like IAS or something? I want to have a prop built based on what altitude the airframe is fastest. Anyways, I may be way out in left field on this one but would like your input. Thanks!
Dave A. gave you a lot of good info. But in general, you want to cruise at the highest altitude at which you can produce the amount of power that you want to use - IOW, you're at WOT with a fixed pitch prop (and with a Lycoming/Continental, leaned out - not in your case).

So if you want to use 65% power, go to about 10K - 11K ft. For 75%, about 8K ft. With most Lyc/Conti's, you never want to cruise for long periods of time above 75% power, just because you can't lean it past LOP and so you use a LOT of fuel.

TAS will ALWAYS be highest at SL, since that's the only place that a normally aspirated plane can produce 100% power. I can get about 190 KTAS at SL, wide open (burning 17 gph), but only about 175 KTAS at 12.5K ft. On the other hand, I'm burning maybe 9 gph up there.

Normally, I cruise at 170 KTAS, since that happens at 2600 RPM basically any altitude between 7500 - 15500 ft. in my COZY MKIV. FF between 9.4 gph down low (7500) and 6 gph up high (15500).

I have no idea what makes sense for an engine that you can't lean manually, or what they say about maximum power output. Maybe if they control leaning themselves, they don't give a crap what your power output (and hence fuel burn) are as long as the temps are acceptable.

But as Dave says, generally higher is more efficient, if not faster.
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Old 03-16-2015, 12:52 AM
ZG4Me ZG4Me is online now
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Default Re: Best Cruise Altitude

Wot Dave sed, and it's complex.

I'm flying rental Barf Rockets for now, have done LMO-TWF and back twice. I'd get fuel mid route (RKS), as running out of fuel in the west can be...

My last trip back the winds aloft were 30-40 in my favor. Took me time to get up there (OK, a LONG time), and TAS was slower, but I didn't *have* to stop for fuel mid-way, and arrived at LMO much sooner than planned.

So.... depends on TAS/GS, time, fuel burn, and length of trip. IAS is important, but kind of useless for time/distance/fuel calcs.

The latest Foreflight (standard) has an interesting feature. During setup, you enter airspeed during a climb (and decent), as well as TAS cruise. Foreflight will calculate climb time, winds aloft, cruise, decent time, and give you a pretty accurate time of flight. And it will tell you there's no sense climbing to 15,500' for a 50kt trip even though winds up there are 200kts.

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Old 03-16-2015, 11:18 AM
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jenatepilot jenatepilot is offline
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Default Re: Best Cruise Altitude

More fantastic info gentlemen. Great
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