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  #1  
Old 08-22-2021, 02:31 PM
Duncannon Duncannon is offline
 
Join Date: Jun 2021
Location: Georgia
Posts: 32
Default High altitudes-X/C (another engine vs. engine thread)

There are multitudes of threads regarding engine comparisons (160 vs. 180 vs. 200 horsepower, etc.). And having browsed through many of them, Iíve learned that while the bigger engines give you considerably better climb performance, thereís only a small difference in cruise speed (unless youíre formation flying with RVs with bigger engines than yours; THEN itís a big difference ).

However, if you spend a lot of time around 9-13k feet on lengthy cross countries, would the more powerful engines have more of an effect on cruise at those altitudes? Also, can you lean so as not to burn all that much fuel if youíre that high?
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  #2  
Old 08-22-2021, 03:21 PM
blaplante blaplante is offline
 
Join Date: Jan 2010
Location: Southern California
Posts: 451
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So, I have 160HP (injected) in a 6A.
With the injector flow balanced, I can run lean of peak, which will slow you down maybe 10%, but cuts fuel flow by maybe 20%, so for economy cruise at less than 75% power this is the way to go, particularly if you aren't fighting a headwind.

But after trying LOP a couple times, I'm not sure that I'm a fan. Not that I'm concerned about the engine, but rather when flying x-country I've got a destination, and it isn't close. Getting there 10% sooner might be the priority. And flying at 10,000+ and often at 13,000+ density altitude well, the fuel burn rate is already down substantially.

What you are fighting is explained here: https://physics.info/drag/#:~:text=T...an%20bicycles.

That to go 2x faster you'd need 8x more power. Now you see why a 200 HP RV isn't much faster than a 160 HP one.

Then, a 200HP angle valve weighs more, costs more to buy (usually)...
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  #3  
Old 08-22-2021, 03:56 PM
BobTurner BobTurner is online now
 
Join Date: Dec 2011
Location: Livermore, CA
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Quote:
Originally Posted by blaplante View Post
But after trying LOP a couple times, I'm not sure that I'm a fan. Not that I'm concerned about the engine, but rather when flying x-country I've got a destination, and it isn't close. Getting there 10% sooner might be the priority.
.
Curiously, I seem to have a fair number of x-country flights where flying LOP gets me to the destination sooner - because I can skip a fuel stop!
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  #4  
Old 08-22-2021, 06:56 PM
David Z David Z is offline
 
Join Date: Aug 2010
Location: Thunder Bay Ontario
Posts: 932
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You can always reduce the power on a bigger engine. Can't go more than full on a smaller engine. By reduce the power, the options are RPM, LOP or throttle and in whichever order and flavour is permitted/preferred.

2 posts up, blaplante mentions that LOP means he goes 10% slower as compared to ROP. If a bigger engine could maintain that original TAS while LOP, wouldn't that save fuel?

I'll say it another way. He's up at 11500' making 90hp and needs to be ROP to do so. A bigger engine flying through the same air, making the same 90hp and going the same speed might be able to do that while running LOP and burning less fuel.

I too am quite curious about the speed difference of the different engines at much higher altitudes. The Van's numbers are all at 8000'. How do the engine sizes compare at 13000'? Also 3000' would be interesting.
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  #5  
Old 08-22-2021, 07:05 PM
Duncannon Duncannon is offline
 
Join Date: Jun 2021
Location: Georgia
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Quote:
Originally Posted by David Z View Post
You can always reduce the power on a bigger engine. Can't go more than full on a smaller engine. By reduce the power, the options are RPM, LOP or throttle and in whichever order and flavour is permitted/preferred.

2 posts up, blaplante mentions that LOP means he goes 10% slower as compared to ROP. If a bigger engine could maintain that original TAS while LOP, wouldn't that save fuel?

I'll say it another way. He's up at 11500' making 90hp and needs to be ROP to do so. A bigger engine flying through the same air, making the same 90hp and going the same speed might be able to do that while running LOP and burning less fuel.

I too am quite curious about the speed difference of the different engines at much higher altitudes. The Van's numbers are all at 8000'. How do the engine sizes compare at 13000'? Also 3000' would be interesting.
Same. Thereís less drag while the bigger engine has a bit more power to work with, so that extra power would make more of a difference (not unlike how a turbocharged aircraft at high altitudes).
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  #6  
Old 08-22-2021, 07:13 PM
chubsterini chubsterini is offline
 
Join Date: Jun 2021
Location: California
Posts: 108
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Quote:
Originally Posted by David Z View Post
You can always reduce the power on a bigger engine. Can't go more than full on a smaller engine. By reduce the power, the options are RPM, LOP or throttle and in whichever order and flavour is permitted/preferred.

2 posts up, blaplante mentions that LOP means he goes 10% slower as compared to ROP. If a bigger engine could maintain that original TAS while LOP, wouldn't that save fuel?

I'll say it another way. He's up at 11500' making 90hp and needs to be ROP to do so. A bigger engine flying through the same air, making the same 90hp and going the same speed might be able to do that while running LOP and burning less fuel.

I too am quite curious about the speed difference of the different engines at much higher altitudes. The Van's numbers are all at 8000'. How do the engine sizes compare at 13000'? Also 3000' would be interesting.
It really sounds like you're looking for reasons to justify getting a bigger engine. Just go for it.

I'm building a 6a and also leaning towards a 360. Given my weight (and that of my likely passengers) and mission, it just seems the plane would be easier to load (cg-wise) with a 360 engine and cs prop up front.

-c
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  #7  
Old 08-22-2021, 07:27 PM
BobTurner BobTurner is online now
 
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Quote:
Originally Posted by David Z View Post
I too am quite curious about the speed difference of the different engines at much higher altitudes. The Van's numbers are all at 8000'. How do the engine sizes compare at 13000'? Also 3000' would be interesting.
The "3% speed gain for 10% power increase" rule depends on the approximation that induced drag can be neglected in comparison to parasitic drag. At 13,000' that's probably still a pretty good approximation. But still higher, as you approach the service ceiling, the approximation fails, and the higher power engine starts to go noticeably faster. Obviously, high enough, and only the higher power engine can get there!
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  #8  
Old 08-22-2021, 08:07 PM
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airguy airguy is offline
 
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Location: Garden City, Tx
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Some of the Vans wings are better at it than others - the 9 has a superior high altitude wing and the larger engine lets you take advantage of it. Yes, you have to pay attention to the speeds on the way down, but the long range cruise is fantastic. And yes, Scott McDaniels is correct in admonishing most people from doing this - it's not for everyone and you must remain vigilant to keep the airplane in the envelope. Nevertheless it can be done, and safely.

In my case, I also installed long range tanks giving me excellent range. This was my trip from home near Midland, TX to Sun and Fun, going out over the gulf to stay south of a pile of weather. The screen shows I've been airborne for 3 hours and still have over 1400nm of range left in the tanks. I'm showing 162 knots true at 6.3 gals/hr. Those numbers are hard to argue with, regardless of which side of the ROP/LOP debate you fall into. The screen indicates ROP, but I was actually running just on the lean side of stoichiometric at an AFR of 15.0.

Oh - did I neglect to mention that I'm doing this on Wal-Mart grade 91 octane auto fuel, with ethanol?

I routinely fly 6 and 7 hour legs direct nonstop, because I can.
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Last edited by airguy : 08-22-2021 at 08:32 PM.
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  #9  
Old 08-22-2021, 08:42 PM
blaplante blaplante is offline
 
Join Date: Jan 2010
Location: Southern California
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Quote:
Originally Posted by chubsterini View Post
It really sounds like you're looking for reasons to justify getting a bigger engine. Just go for it.

I'm building a 6a and also leaning towards a 360. Given my weight (and that of my likely passengers) and mission, it just seems the plane would be easier to load (cg-wise) with a 360 engine and cs prop up front.

-c
Sounds to me like you'll be over gross. In my o-320 plane, Catto prop, full tanks, full baggage and two 200 lb folks on board, I'd be over gross. Putting a heavy prop up front will help CG, but not help with gross weight.

And no, I don't have a bunch of vacuum gyros weighing me down.
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  #10  
Old 08-22-2021, 09:26 PM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by airguy View Post
And yes, Scott McDaniels is correct in admonishing most people from doing this - it's not for everyone and you must remain vigilant to keep the airplane in the envelope.

Nevertheless it can be done, and safely.
I have never dis-agreed with that point.
If anyone wants to look back through old posts when I have commented on the subject, I am pretty sure you will find that it (admonition) is always in the context of some people promoting the use of larger engines, while stating that they are always careful to never exceed _____ (fill in the blank) performance perimeter, when what they mentioned wasn't the primary design perimeter concern.

Doing it safely requires being proactive about controlling the usage of the extra power (speed), and having a full understanding of what the actual design limits are.
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