Non-turbo high(er) altitude flying?

CJones

CJones

Final Approach
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While punching holes in the sky in the RV today, dad and I got into a discussion about going 'high' in the RV for cross-country flights. With the climb performance, we can realistically get to higher altitudes a lot faster than when we were flying the Cardinal or Cherokee Six.

My question is this: Is it worthwhile to go 'high' in a non-turbo'd/non-pressurized aircraft like that? We're running a 200hp IO-360, so we have a little extra HP to carry along the way. We're thinking about investigating a relatively inexpensive O2 system (a la Dr. Bruce) so we can get up to 12-14k fairly easily. Our thinking is that we have a larger window to find favorable winds as well as decrease overall fuel burn (of course that's at the price of lowered engine performance). Where does the decrease in engine performance outweigh the thinner air/better winds of high altitude occur for non-turbo'd a/c?

What say ye more edumicated individuals on this topic?
 
it seems like most normally aspirated engines are best in the true airspeed department at about 8000 feet.

Basically, find out the maximum altitude you can put out 75% (or whatever you cruise at) power and fly at or around that altitude. This will give you the highest TAS.

However with the astounding climb characteristics of your RV, it could be worth it, especially on a west to east trip, to climb well above that altitude, into the low teens, take a slight hit on TAS but pick up major winds. Having an oxygen setup will at least allow this option if the opportunity presents itself.
 
CJones said:
Where does the decrease in engine performance outweigh the thinner air/better winds of high altitude occur for non-turbo'd a/c?
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I've always heard in the 7,000 or 8,000 ft range is ideal. Of course you really need a trip of length in many planes to justify climbing that high but that's probably not an issue in a rocket like your RV. I tend to go to higher cruise altitudes in the Diamond too because getting up there is easy.

I just spent a bunch of time going through the Diamond POH to try to answer this. But I'm not a big fan of these charts. Actually they suck bad because they don't show TAS on the same chart as engine performance.

So now I'm looking at the 172N POH which I like much better and it looks like the sweet spot as far as speed versus fuel burn would be between 7,000 to 8,000 feet pressure altitude.
 
I would say that the 8-9-10,000 ft range is pretty much accurate for us as well. If I remember correctly, that's about where we get (on standard-ish day) with throttle full forward and still get ~23.5"MP. I know we'll definitely loose power (which leads to loss of TAS), but I'm thinking it might be worth the loss to get up there with the nearly 50kt or so winds aloft.
 
Chirs:

I often flew the A-55 Baron at 12,000 to 13,000 and still trued out at about 175 to 180; that made it worthwhile. I have a friend that's taken his RV up that high to get over weather and to pick up good winds. Give it a try a couple times and see what you think. That can give you some added flexibility. As Tony said, if that 7 to 8,000 is available, that would probably be best, but there are times the extra altitude could be a benefit.

Best,

Dave
 
A lot depends on how much power your plane has. The Comanche 400 was introduced so it could get to turbo altitudes without a turbo. Too bad they could not work out the cylinder cooling issues. It is a great bird.

Also, it depends on the winds. It does not bother me at all to climb to 12 to 13K to catch favorible winds, even though I am only at 65% power or so. Saves gas and goes fast. OTOH, if the winds are against you, which they will be most of the time, staying in the 8K range is about the best you can get.

Flight today, flew west at 6K and returned east at 9K. 110 knots outbound and 190 knots returning. Only 160 NM's, so it did not make sense to climb higher.
 
I routinely fly cross-country flights at >7000. When I fly to Alabama (from Iowa), I usually end up around 9-10k so I can get above anything that might cause course correction enroute. I'm just curious about going higher still - say around 12-13k. It sounds like it is viable, so maybe we'll look at getting some sort of O2 system set up so we can go up there safely.
 
I have a turbocharged plane, so this is filtered with that mentality.

I fly high - routinely at 15K-17K eastbound. I choose wesbound altitude for most favorable wind situation.

Pros:
It's smoother up there.
It gets you above a fair amount of the clouds and weather.
It offers more alternatives if you have an engine problem.
There is a lot less traffic. Most of the FLIBs are below 10K.
You can see a lot farther.
Better radar and radio coverage.
Able to go over Class C and Class B (in many cases - Houston will let you over at 16,000 and 17,000 and sometimes 15,000 - DFW generally won't), making routings more direct.
Cooler in summer.
Higher tailwinds - faster to destination.

Cons:
The need for O2 raises expense a bit and the canulas/masks get uncomfortable after a while.
Climb rates generally make it inefficient for short trips.
On a non-TC engine, less power out of the engine.
Need good management of engine temp due to climb rate and thinner air

I like being higher. But it's not for anybody.
 
Flying to Florida from Ohio in an 0-320 RV8 we always go high, turn on the 02, lean her out and pick up optimum winds. 14.5-17.5 thousand regularly.
 
Go high... The less traffic up there is pretty darn nice.

o2 is cheap too. :)
 
For pure TAS reasons, your plane is fastest at whatever altitude the engine produces what you consider max crulse power. If efficiency is the concern, the "optimum" altitude depends on how fast you want to go, or more specifically, how much faster than max range airspeed you want to fly. At the max range speed (usually around your "best glide" IAS) altitude has no effect on efficiency but it does affect your TAS since IAS is constant and TAS/IAS goes up with altitude. RPM is another consideration. With a C/S prop the faster you turn the engine, the higher you can go while producing the same power, but higher RPM generally means less prop efficiency and more noise.

In my plane I normally run 60-70% power and can get 175-180 KTAS with that power from 5000 through 11000 MSL. By cranking up the RPM I can get 170 KTAS as high as 14-15k MSL and the TAS drops off a few knots per thousand above that.

Oh, and if you are going to cruise at oxygen alitudes, you really need a way to monitor your O2Sat and use it frequently to insure you don't get hypoxic.
 
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wsuffa said:
The need for O2 raises expense a bit and the canulas/masks get uncomfortable after a while.

What Canulas? We all don't use canulas!!

I like being higher. But it's not for anybody.
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Wow! Who'd of thought you had something in common with Timothy Leary!! :D

Best,

Dave
 
wsuffa said:
Someday my D-Jet will leave your P-Baron in the dust.
;)

Just make sure the trip's not too long! Right now, my P-Baron will still beat the Eclipse to San Diego (it has to stop for fuel and my P-Baron doesn't unless winds get crazy) :P


Whew. It coulda been worse.... I see Hanoi Jane is in the news again today... :(
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Yea! The first time this happened, I was in a foreign land complements of Uncle Sam and she had my best interests at heart. Luckily, mine's still beating after all he help!

Best,

Dave

 
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