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High altitudes-X/C (another engine vs. engine thread)

Duncannon

Active Member
There are multitudes of threads regarding engine comparisons (160 vs. 180 vs. 200 horsepower, etc.). And having browsed through many of them, I’ve learned that while the bigger engines give you considerably better climb performance, there’s only a small difference in cruise speed (unless you’re formation flying with RVs with bigger engines than yours; THEN it’s a big difference :D ).

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 you’re that high?
 
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=Thus%2C%20if%20drag%20is%20proportional,so%20much%20faster%20than%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)...
 
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!
 
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.
 
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. There’s 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).
 
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
 
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!
 
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? :cool:

I routinely fly 6 and 7 hour legs direct nonstop, because I can.
 

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

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 you’re that high?

Indeterminate question with no possible answer except rules of thumbs. First if you are talking (I)O320 160HP, (I)O360 180HP (parallel valve), IO360 (I)(angle valve) you are talking different weight engines and fuel burn. Prop is as or more important. A constant speed prop is going to give better overall performance and efficiency than fixed pitch, with the trade off of cost and weight.

In general higher HP will get you what you expect, better t/o distance, climb, cruise, top speed and service ceiling. True more HP does not give quantum jumps in speed as drag is a square root. Look at van's data for different models with different HP. What does help and is almost free lunch is LOWER DRAG... So your HP question is kind of moot. Some RV's are built light, very good fit and finish, rigged well, better engine cooling (less drag) and will fly better than an RV with less HP that is heavy, rough and has less efficient engine cooling.


To your mission of cruise at 9K to 13K feet any RV with any HP can do it. Keep in mind MAX power at 8,000 feet for any engine is 75%. 8,000 ft by the way assuming zero wind at all altitudes is typically best speed and efficiency. If you climb to 12,500 ft (highest alt w/o oxygen and VFR cruising altitude). You will be down to about 55%. You will be going about 20 mph slower but saving gas. Favorable tail winds I would climb to the mid teens with O2. More HP would allow you to climb to higher altitudes (easier) and maintain higher airspeed, especially if temps are above standard and you are heavy.

ALL RV's have a service ceiling above 22 or 23 thousand feet except the RV9a which is 18,000 ft. I don't think the RV-9 is the U2.

I built and flew a 150HP O320 RV4 with constant speed Hartzell and could easily fly (solo no cargo) to the mid teens and cruise at 50-55% power over 175 mph true on 6 to 6.5 GPH.
 
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? :cool:

I routinely fly 6 and 7 hour legs direct nonstop, because I can.
WOW. If you got it flaunt it. Ha ha. What facts do you have to say it is so superior that are quantifiable data? Superior how and by how much? RV-9 wing looks long and skinny. and is indeed 3-5 ft longer than other 2 place RV's. The higher aspect ratio which suggests lower drag. However wing is not tapered so it's not likely to realize all of the advantage of being longer, at least at altitude, but more wing area suggest lower stall which was Van's goal for the RV-9, which is 7mph slower. High altitude performance was not a design goal.

To be clear the comment about the superior wing of the RV9, is misleading. First all your range is not due to the wing but bigger tanks and LOP operations, right. Of all the RV's with recommended engine the RV-9a has the lowest service ceiling, 18,500 ft. All others are in the 19,500 to 23,500 range and faster cruise speeds. Most RV's have service ceilings of 21,000 to 23,500 feet and are faster in cruise (albeit some of that is engine size).

Let's compare a 160HP RV4 (same wing on all RV's) to a 160HP RV-9. The RV -4 has 1000' higher service ceiling and 7mph cruise speed advantage. If the wing was so much better why does the RV-4 do better? OK may be it's the narrow fuselage, still I am not seeing a big or superior advantage here.

The RV-9 was made according to Van's to have a more docile lower speed handling character, not to "emphasize speed or aerobatics". This is out of Van's brochure. Indeed the RV-9 has slower stall.

Almost RV's starting at the RV-3 on use the excellent NACA 23013.5 airfoil. The reason the RV's garnered a great reputation over 4 decades is not based on the reputation of the RV9 wing but the NACA 23013.5 wing used on most of the other RV's. To be sure the NACA airfoil, wing area, wing loading, aspect ratio is a golden ratio, one of the many reasons RV handling and performance is outstanding. NACA 23000 series airfoil has been used in some of the greatest planes ever made well beyond just the Van's RV series.

My RV4 with 150HP and Hartzell could do what your RV-9 does except for range, because I had standard tanks. OK, as you were...
 
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RV9 service cieling

I am thinking the RV9 service ceiling is lower because it wasn't tested up higher.
In reality it doesnt matter because nobody cruises up that high anyways due to O2 limitations in an emergency. And nobody should be making decisions on published service ceilings.

Just curious what the real service cieling is of an o320 RV9. (I will have my own data shortly)
 
I don’t think so…

8,000 ft by the way assuming zero wind at all altitudes is typically best speed and efficiency. If you climb to 12,500 ft (highest alt w/o oxygen and VFR cruising altitude). You will be down to about 55%. You will be going about 20 mph slower but saving gas.
.

1. 8,000’ is typically best speed - true, if power is limited to max recommended 75%. But best efficiency - not at all.
2. 12,500’ is max vfr cruising altitude w/o O2? Sometimes true, sometimes not true.
3. At 12,500’ you’ll be down to about 65% power, and slightly slower (but not 20 mph). True, you’ll be saving gas, which is why 12,500’ is more efficient than 8000’.
 
I agree with Bob. Better efficiency is found going higher than 8000 feet. We've done a fair amount of testing up to 20,000 feet on the RV-10. Does some amazing numbers up high if you have O2.
 
Just to throw my .02 in -

With my -14A I've recently made several 3.5+ hr trips at various altitudes (12k-14k) & maintained ~160kt - 165kt @ 2,400 RPM. Fuel burn verified by fill up and FF meter ranged from 8.5 -9.5 GPH running peak EGT.

I think these numbers are pretty close to what Scott reported a few years back after the first few long x-countries in the factory plane.
 
ALL RV's have a service ceiling above 22 or 23 thousand feet except the RV9a which is 18,000 ft. I don't think the RV-9 is the U2.

You're painting with a pretty wide brush again, George. I've had my 9A up to FL210 and was still climbing at almost 300 fpm there. Realistically there is no reason to go up there except to clear weather or some testing and curiosity. I have done extensive long XC efficiency curve mapping and my best efficiency, in my airplane as I built it, is very flat between 14,000 and 17,000 and then starts to decline. The day I was at FL210 I was coming home from Reno and needed FL190 to top some weather, and that put me right in the very tops of the cloud layer and I was concerned about ice. I requested, was given, and climbed to FL210 without trouble. Attached is a screenshot of me back at FL190 for the remainder of the trip after clearing the weather.

I'm not going to banter with you point for point while you pontificate, I'm going flying. See my post #8, this thread https://vansairforce.net/community/showthread.php?p=1547812#post1547812
 

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my RV9 has been to 21K but I almost passed out. I was 90 kts indicated so I could have gone higher. the handling was mushier but fully acceptable.
 
I say the -10 also performs best in that 14-17K range as Greg found with the -9.

The bigger engine can always be run at lower rpm with a C/S prop and you get lower frictional losses and better BSFC doing that. Always nice to have more power when needed to depart high DA airports and climb out over high terrain.

The bigger engine is a win win in my book, giving you more choices at no penalty in fuel burn with a C/S prop.
 
Always nice to have more power when needed to depart high DA airports and climb out over high terrain.

Departing out of Denver heading west, directly toward the mountains and climbing over them without turning, is a real hoot! :D
 
1. 8,000’ is typically best speed - true, if power is limited to max recommended 75%. But best efficiency - not at all.
2. 12,500’ is max vfr cruising altitude w/o O2? Sometimes true, sometimes not true.
3. At 12,500’ you’ll be down to about 65% power, and slightly slower (but not 20 mph). True, you’ll be saving gas, which is why 12,500’ is more efficient than 8000’.
I agree with Bob. Better efficiency is found going higher than 8000 feet. We've done a fair amount of testing up to 20,000 feet on the RV-10. Does some amazing numbers up high if you have O2.
I agree with both you Gent's you are 110% right. No one said going higher than 12,500 msl can't add efficiency. However a 160HP O320, 160HP at gross weight, hot day not getting to FL200. Sorry. If it got there it would not be efficient nor fast. Seems to be a contest to see how high we can fly. Let me say Steve Bohannon in his modified RV-4 went to 47,500 feet, missing the absolute record by 300 feet set by a B29. There is also Dave Anders, Cafe' Foundation Triaviathon winning RV-4. Most RV'ers are not IFR O2 sucking with high HP high compression/stroked Lycs. We are talking flying near the corner of the performance envelope. However average RV's and pilots can fly well into the teens, but there are times that is a very inefficient things to do or not practical.

I never meant to imply 8,500 to 12,500 are the ONLY altitudes thou shall fly cross country. I said 8,500 is a good compromise in no wind conditions which produces a balance of efficiency and speed. That is my story and I am sticking to it. :D Part the reason (in general) is you get 75% Pwr with WOT, which is goodness for non turbocharged Lycoming's in wear and tear, temps, and SFC (specific fuel consumption). Although Lycoming recommends 65% power as optimal long life. Having full wide OPEN throttle, the throttle plate is wide open, reducing engine "pumping losses". 75% is a happy place in the SFC range while scooting along. Of course 55% is better for lower fuel burn, so you fly higher (do not reduce throttle). There is no magic about 55%, could be 50% or 59%. 12,500 I suggest is just because it's max altitude for no O2.

However for the average RV driver, A/C weight, HP, Prop, atmospheric conditions, winds, terrain, length of the flight segment will determine best XC altitude. If you have O2 and IFR capability for flight over 12,500' or even FL180 may be a good option, but it's not within reach for all RV's due to weight and HP. That is why 8500-12500 ft rule of thumb, practical. There are diminishing returns on how high a SE normal asperated airplane can or should go, regardless of HP, wing, etc. and size of your O2 Bottle or size .

My old RV4 with O320 150HP Hartzell I'd fly in the mid teens fairly often solo. Going from Seattle to Phoenix in the winter. When a high WX system was off the CA coast flying of Sierra Nevada mountains I saw ground speeds over 210kts sometimes. Also I needed altitude for terrain. I love that trip.
However on the flip side, another trip flying from WA State to Oshkosh, summer, same 150HP RV-4, Max gross, max aft CG on a very hot day. I struggled to get to 12,500 MSL, and had toasty CHT engine temps. I went back down to 7,500'. An "average" RV can fly into the upper teens, but just because you can does not mean you should. We don't want to mislead folks.

Back in the day, I flew at FL210 to FL250 (Ted Smith unpressurized twin turbocharged Aerostar) sucking O2 from a mask for many hours. It was fun and topping weather is great. HOWEVER if the weather builds above your service ceiling (even in a jet) and you fly into icing, turbulence near your stall and/or Vne/Vmo/Mmo not fun. You can't climb and a high altitude "upset" is possible. You can get down drafts. SE planes near service ceiling need to be careful topping weather.

Walking around Oshkosh this year I did not see a lot of O2 systems or masks. (May be they were hiding it?). it may be best to stay on the deck at 25 square (if there are howling headwinds aloft). In one case I had tail winds down low and head winds aloft. Sometimes I fly 22 square 2000 ft AGL and enjoy the countryside just cuz. Obstacles and clearance is important.
 
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12,500 I suggest is just because it's max altitude for no O2.

If you go look up the actual FAR you’ll see that the above statement is only true about half the time. Whenever the altimeter setting is less than 29.92, flight at an indicated altitude of 12,500’ requires O2 use (after 30 minutes), because the O2 rules are written using pressure altitude.
 
I have plenty of customers flying at 14-17K with cannula O2. Pretty comfortable. If you want to do a long leg and catch a good tailwind, that can be a good option. The bigger engine gets you up there faster and you can go faster. Those are good reasons for more power, especially it you fly in hot and high places.

Big engine in a light airframe makes for good times. I have a friend with a 360 in a Kitfox with our EFI. The climb angle is crazy and over 3000 fpm. Fun whether you are 20 or 75...:)
 
You're painting with a pretty wide brush again, George. I've had my 9A up to FL210 and was still climbing at almost 300 fpm there. Realistically there is no reason to go up there except to clear weather or some testing and curiosity. I have done extensive long XC efficiency curve mapping and my best efficiency, in my airplane as I built it, is very flat between 14,000 and 17,000 and then starts to decline. The day I was at FL210 I was coming home from Reno and needed FL190 to top some weather, and that put me right in the very tops of the cloud layer and I was concerned about ice. I requested, was given, and climbed to FL210 without trouble. Attached is a screenshot of me back at FL190 for the remainder of the trip after clearing the weather. I'm not going to banter with you point for point while you pontificate, I'm going flying. See my post #8, this thread
Yes I am generalizing, Greg. :D This video says it all.
https://www.youtube.com/watch?v=lj60OAh7O5U.

Greg, I am not "pontificating" (express one's opinions in a way considered annoyingly pompous and dogmatic). That hurts my delicate feelings. I am a CFI after all. Did you tell your flight instructors stop pontificating? Ha ha. :D I won't take it personal. I am on your team.

I can see you are sensitive about your RV-9 IO-360 with "too much power and too much fuel". You might be projecting arguments you had with others onto me? I only suggested that much of your performance of your RV might be due to the big engine and big tanks and not the wing? Guessing you are a slim trim handsome man and fly solo at lighter gross weights. I am sure you built a really clean plane. Your determination to fly at high altitudes is key. RV-9 Roncz airfoil 28' span wing is "superior" I am willing to concede. it is more efficient and appreciate your efforts on high flying efficiency. Can we be friends now. Fly safe.

PS in defense of the inferior winged RV's, Steve Bohannon and Dave Anders set records with standard crummy RV winged RV's. Steve Bohannon, set time and to climb records in a RV4 derivative Rocket, to 47,530 feet, 380 feet lower than the all-time U.S. piston altitude record of 47,910 set in 1946 by a U.S. Air Force B-29. That is NOT a typo!!! Also he has 2 time to climb records 3,000 m (9,843 ft) under 150 seconds, and 6,000 m (19,685 ft) in approx 390 seconds. https://en.wikipedia.org/wiki/Bohannon_B-1

Then there is Dave Anders RV-4 - CAFE Foundation Triaviathon Champion. I recall 10 yrs ago he stalled at 39 knots, hit a top speed of 218 knots and climb at more than 3,300 fpm. exceptional performance. I think he has improved on it. https://www.kitplanes.com/putting-the-experiment-in-experimental/

These are exceptional planes with the NACA 23013.5 short Hershey Bar wing.

However many average lower HP RV's especially at gross, hot day are not going up that high regardless of the wing. The RV-9 was made for 160HP and has less spectacular performance than your "off plans" RV-9. You have convinced me your RV-9 is superior high flyer. :D
 
Steve Bohannon, set time and to climb records in a RV4 derivative Rocket, to 47,530 feet, 380 feet lower than the all-time U.S. piston altitude record of 47,910 set in 1946 by a U.S. Air Force B-29. [/url]

For the record, the owner and pilot of the Exxon Flying Tiger is BRUCE Bohannon.

Skylor
 
I have plenty of customers flying at 14-17K with cannula O2. Pretty comfortable. If you want to do a long leg and catch a good tailwind, that can be a good option. The bigger engine gets you up there faster and you can go faster. Those are good reasons for more power, especially it you fly in hot and high places.

Big engine in a light airframe makes for good times. I have a friend with a 360 in a Kitfox with our EFI. The climb angle is crazy and over 3000 fpm. Fun whether you are 20 or 75...:)

In the 10, I tend to fly long trips at 14 or 15K. I find that the fuel burn is 1 to 1.5 GPH less than the same speed at 9 or 10K. For me, it is an efficiency thing. I can say that with the 10 wing at altitudes up to 15K, it is more efficient as it goes higher (drag drops faster than engine HP). I have built in 02 and often carry two tanks to support that altitude in both directions of the trip. Based upon this experience, I would not say that the optimal efficiency of the 10 is between 8-12K, as GMCpilot proposes. The 10 is definitely over powered at the cruise speeds I use (165 KTAS) and that may be part of the equation.

Larry
 
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RV-4 O360 data point

Just recently did a flight in my O360 fixed pitch prop -4. 17,500 feet doing 166 to 173 knots at 7gph. I only pulled the throttle back just until the enrichment valve closed as noted by an increase in EGT, it only lost about 0.5 mp. Was able to retain 15 inches of mp and with the metal Sensenich fixed pitch prop, rpm was 2650. After 2 hours, I landed and fuel burn was 16.3 gallons, or 8.15 gph average with the climb and descent. On my way home this weekend, I’m going to do 16,500 and throttle back a little and see if I can get more efficient.. perhaps 5 or 6 gph and I’ll see where the speed and range falls.
 
My RV 6A with an IO320 and Hartzell prop does it’s best work around 10,000 plus. I get decent speed, use less fuel, love it. Autopilot on and trucking along between 160-164 knots depending on pressure. We have taken it coast to coast twice in the last couple of years :)
 

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For the record, the owner and pilot of the Exxon Flying Tiger is BRUCE Bohannon.

Skylor
Gosh darn it, I've got a mind like a steel trap - rusty and illegal in 37 states. :D Thanks

If you go look up the actual FAR you’ll see that the above statement is only true about half the time. Whenever the altimeter setting is less than 29.92, flight at an indicated altitude of 12,500’ requires O2 use (after 30 minutes), because the O2 rules are written using pressure altitude.
Bruce I don't think so. FAR says 12,500 MSL not density or pressure altitude. If I am wrong please give me ref.

14 CFR § 91.211 - Supplemental oxygen.
(a) General. No person may operate a civil aircraft of U.S. registry -

(1) At cabin pressure altitudes above 12,500 feet (MSL) up to and including 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration;

(2) At cabin pressure altitudes above 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen during the entire flight time at those altitudes; and

(3) At cabin pressure altitudes above 15,000 feet (MSL) unless each occupant of the aircraft is provided with supplemental oxygen.

Again cannuala good only to FL180. FL180 is a pressure altitude 29.92). May be you are thinking of that.

Lecture alert coming:
At FL180 above you need a rebreather mask up to FL250. Above that you need a diluter demand mask that can provide 100% O2. As you climb your "hemoglobin" can not absorb O2 (actually O2 can not attach) as easily and needs to be forced. Blood hemoglobin transports oxygen, is chemically unable to bind oxygen molecules. CO poisoning keeps you from absorbing O2 causing hypoxia. This is why it is so dangerous and causes subtle incapacitation like hypoxia.

Also a Pulse Oximeter should be standard equip for any pilot desiring to fly above 10,000 ft and a MUST for flight at O2 altitudes. I have a smart watch that measures BPM, O2 saturation and even blood pressure I know my normal. I don't know how accurate it is, seems to match my finger Pulse Oximeter. As far as Blood pressure I think it is close but not perfect, but it's a datum.
 
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For the record, the owner and pilot of the Exxon Flying Tiger is BRUCE Bohannon.

Skylor
Gosh darn it, I've got a mind like a steel trap - rusty and illegal in 37 states. :D Thanks

If you go look up the actual FAR you’ll see that the above statement is only true about half the time. Whenever the altimeter setting is less than 29.92, flight at an indicated altitude of 12,500’ requires O2 use (after 30 minutes), because the O2 rules are written using pressure altitude.
Bob I don't think so. FAR says 12,500 MSL not density or pressure altitude. If I am wrong please give me Ref. I read MSL it means to me altitude adjusted for local Baro. or QNH.

14 CFR § 91.211 - Supplemental oxygen.
(a) General. No person may operate a civil aircraft of U.S. registry -

(1) At cabin pressure altitudes above 12,500 feet (MSL) up to and including 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration;

(2) At cabin pressure altitudes above 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen during the entire flight time at those altitudes; and

(3) At cabin pressure altitudes above 15,000 feet (MSL) unless each occupant of the aircraft is provided with supplemental oxygen.

Again cannuala "mask" good only to FL180. FL180 is a pressure altitude (29.92 in-hg) or QNE. May be you are thinking of that.

Lecture alert coming:
At FL180 above you need a rebreather mask up to FL250. Above that you need a diluter demand mask that can provide 100% O2. As you climb your "hemoglobin" can not absorb O2 (actually O2 can not attach) as easily and needs to be forced. Blood hemoglobin transports oxygen, is chemically unable to bind oxygen molecules. CO poisoning keeps you from absorbing O2 causing hypoxia. This is why it is so dangerous and causes subtle incapacitation like hypoxia.

Also a Pulse Oximeter should be standard equip for any pilot desiring to fly above 10,000 ft and a MUST for flight at O2 altitudes. I have a smart watch that measures BPM, O2 saturation and even blood pressure I know my normal. I don't know how accurate it is, seems to match my finger Pulse Oximeter. As far as Blood pressure I think it is close but not perfect, but it's a datum.
 
Gosh darn it, I've got a mind like a steel trap - rusty and illegal in 37 states. :D Thanks

Bob I don't think so. FAR says 12,500 MSL not density or pressure altitude. If I am wrong please give me Ref. I read MSL it means to me altitude adjusted for local Baro. or QNH.

14 CFR § 91.211 - Supplemental oxygen.
(a) General. No person may operate a civil aircraft of U.S. registry -

(1) At cabin pressure altitudes above 12,500 feet (MSL) up to and including 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration;

(2) At cabin pressure altitudes above 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen during the entire flight time at those altitudes; and

(3) At cabin pressure altitudes above 15,000 feet (MSL) unless each occupant of the aircraft is provided with supplemental oxygen.

Just read the reference you posted! It clearly says "...pressure altitudes...".
Pressure altitude is the physically correct parameter, as it takes a minimum partial pressure of O2 to push the oxygen across the lung's membranes. This is the limiting factor in getting oxygen into the blood.
The (MSL) simply means the instrument reads as feet above mean sea level. They could have published numbers in psi.
 
One thing I know...

The effective service ceiling of my -9A is "below 18,000 feet," simply because I fly on BasicMed.

Also, it gets freaking cold up there. :)

I am thinking the RV9 service ceiling is lower because it wasn't tested up higher.
In reality it doesnt matter because nobody cruises up that high anyways due to O2 limitations in an emergency. And nobody should be making decisions on published service ceilings.

Just curious what the real service cieling is of an o320 RV9. (I will have my own data shortly)
 
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