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The amazingly efficient wing on the -9

airguy

Unrepentant fanboy
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It's been a while since I did a really good cross country, I've made a few that qualified as XC since Sun 'n Fun, but nothing really good. Today everything lined up and I decided to go fly a bit. It's absolutely amazing what $100 of fuel can do.

I'm based in west Texas on a private strip, and decided to continue some efficiency testing that I've had in progress for a while, to get good numbers for transoceanic hops on a RTW trip. With that in mind, I topped off my tanks (67 gallons in the wing) and did a round robin tour from home, around the White Sands Missile Range, and back to home.

https://flightaware.com/live/flight/N16GN/history/20210622/1844Z

I wish there was a way to embed photos with descriptions in the body of a post without a file sharing site, but you'll just have to look at the attachments I suppose.

My route was 742 nautical miles, home-MAF-INK-SFL-ELP-KLRU-LAYEN intersection - TCS-ONM-CNX-CME-HOB-MAF-home. That kept me comfortably outside the restricted areas and MOA's.

In the interest of true efficiency testing, I took off and immediately reduced power to 2500 rpm, 21" map, and went lean of peak and added 4 degrees of timing to my ignition. Then I set my climb rate to 500fpm and activated the flight plan. As I climbed I added more throttle to keep 21", and I played with the mixture to keep my hottest cylinder between 390-400 - always lean, but rich enough to produce climb power. Around 12,000' I was out of throttle and dropped the climb rate to 400 fpm, and at about 15,000' I had to drop down to 300fpm for the rest of the climb - but keep in mind that at takeoff I was right at 1750 pounds and I was climbing at low power lean of peak. I settled in at 16,500 for the west bound leg and did a few mixture sweeps and adjusted my cylinder fuel flow trims (SDS injection) and get them all lined up happy, and then just proceeded to enjoy watching the scenery roll by. At 16,500' I was burning 6.0gph (about 40-50 LOP, Air-Fuel ratio in the low 16's) and showing consistently 143-144 ktas, with a DA of 18,568.

After making it west past El Paso and turning north, just before Truth or Consequences NM at LAYEN intersection I turned into the first half of the compass rose and climbed up to 17,500' for the remainder of the flight, at 300fpm. Temperature there was 28F and DA was 19,665. I was burning 5.8gph and making 143-144 ktas consistently. I turned east at Socorro and then southeast at Corona, and back home.

Total trip was 5 hours 8 minutes from crank to shutdown, including taxi. I covered 742 nautical air miles on less than 30.7 gallons of fuel - and that was burning automotive 93E10 from Walmart, all blood guts and feathers. Less than $100 actual fuel cost. That an average of 24.5 nautical miles per gallon, significantly better than my pickup. On my screenshots, the standard inboard tanks are bottom left and my outboard tanks (15.5 gallons each) are bottom right.

I took one screenshot at top-of-descent, and it shows range remaining in my tanks of 952 nautical miles, after having flown 5 hours and 742 nm. Another shot was taken of the screen at shutdown to show the flight timer and fuel remaining. It just absolutely blows my mind what this wing airfoil can do and how much it loves the high altitude.

Thanks again, VAN!
 

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Greg, That Is Impressive!

RTW? I’ve missed that intention in your posts. That will be fun to follow.
 
this is a great post! thank you for sharing.

i wonder if you could further increase your gas milage with lower RPM. Don't know if you had 2500 RPM also at cruise. I experimented a bit with my 7 and lower RPM makes LOP easier and you come closer to carsons speed with the engine still being in the efficient power setting range.
 
Agreed on the efficiency, but the ride in turbulence... not so wonderful. One very knowledgeable source says that the RV-9 "rides like a buckboard."
 
Agreed on the efficiency, but the ride in turbulence... not so wonderful. One very knowledgeable source says that the RV-9 "rides like a buckboard."

I know sacrilege, but if you slow down the ride to spam can speed it is so much smoother then that of a Piper Warrior or 172 IMO. My copilot requests that flight profile when the puffies are building fast down here.
 
Greg,

You might want to get data when not so deep LOP. At altitude I find just 10–20 degrees LOP yields a nice operating point. Here I suspect power is dropping faster than fuel burn as you go further LOP.

Carl
 
Having the extra fuel onboard makes a huge difference. I wish I could carry more. When we had our 9A I didn't have extra fuel and was always afraid to run a tank to empty (or even close to it). The result is I could not have made that flight non-stop.

I completely agree with the comments regarding the ride in rough air. There were a couple flights we made in the 9A that were rough at altitude, but on the edge of dangerous when trying to land. How ironic that the plane that was so easy to fly became harder (for me) to land than my Lancair in gusty WS conditions.
 
It would be interesting to experiment with different climb profiles too. Intentionally throttling the engine to 21" down low is resulting in more time spent at the lower, less efficient altitudes.

My guess would be to climb at a low drag airspeed, throttled only enough to go LOP, and use RPM to control power as needed. Using RPM reduces the internal friction losses within the engine.

I also wonder about prop RPM at high altitude. What is the speed of sound, and the prop tip mach number? Extra RPM and fuel might be going to creating useless shock waves. I'm not sure the mach number that will even create shock waves on the front of the propeller blade tips. Something else to do some math on and experiment with.

This is why it's called experimental aviation!
 
Agreed on the efficiency, but the ride in turbulence... not so wonderful. One very knowledgeable source says that the RV-9 "rides like a buckboard."
Most of my RV experience is in my 9A. I would be interested to hear from others that have experience in the 9 AND other RV models comparing the ride in turbulence. I would have thought the ride between a 7 and a 9 would be similar given the similar wing area (121 sq ft verses 124)?

Fin
9A
 
Most of my RV experience is in my 9A. I would be interested to hear from others that have experience in the 9 AND other RV models comparing the ride in turbulence. I would have thought the ride between a 7 and a 9 would be similar given the similar wing area (121 sq ft verses 124)?

Fin
9A

My first airplane was an RV-4. Flying in loose formation with a Johnson Rocket, he hit a bump that registered 2 G and the RV-4 registered 5 G. Not necessarily the same bump, but...

I took a CFI friend flying in the back of my RV-8A when I had it. After 45 minutes in very light turbulence, he'd had enough of being jostled side to side aft of the c.g. Up front, I'd hardly noticed a thing.

As I understand (?) it, there are two main factors that affect bumps (vertical motion, also known as heave). One is wing loading, the other is how much lift changes with angle of attack.
 
Extra fuel can be nice, but imagine the performance gains from not hauling an extra 30ish gallons around. I realize the stock 9 has 5 gallons less than my 7 (41) but it is more than enough to explore the USA when operated LOP and using altitude to your advantage.
It does allow an amazing round trip range to fill up on the same cheap MoGas though.
 
Extra fuel can be nice, but imagine the performance gains from not hauling an extra 30ish gallons around. I realize the stock 9 has 5 gallons less than my 7 (41) but it is more than enough to explore the USA when operated LOP and using altitude to your advantage.
It does allow an amazing round trip range to fill up on the same cheap MoGas though.

I dont think there are any performance losses having the larger tanks. Only when there is fuel in those tanks - same as baggage or a bike. If you need the fuel you are willing to have some loss early in your flight the same as if you want your bike or camping gear or passenger.

Greg has plans for long trips at high altitudes so it is perfect for him. Cool screen shots Greg. My phase 1 is half finished! Finally!
 
I'm not so sure the performance gains will be significant leaving weight behind. Yes, climb will be slower, but so long as the aircraft is still reaching the desired cruise altitude, there's maybe 1 kt lost of TAS vs a much lower weight. Leaving 200lbs of gas in the truck might make that difference, but that's a lot of fuel to leave behind.

We're not flying big transoceanic jets here. If we carried 1000lbs of fuel and were limited to 12000' by weight and could step climb up to 17000 as fuel weight burned off, then I'd agree.
 
Very inspirational, Greg. A similarly equipped -9 is my next project, once I get some irons out of the fire. The option of alternate fuels, SDS, and the larger engine makes for a nice combination. My GF might like a BRS installed, but that's another topic.

Didn't we meet once at Terlingua Ranch? You in a Cessna 177 maybe. Me in my maroon and white Aggie -8.
 
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A few answers here to a few of the questions raised....

this is a great post! thank you for sharing.

i wonder if you could further increase your gas milage with lower RPM. Don't know if you had 2500 RPM also at cruise. I experimented a bit with my 7 and lower RPM makes LOP easier and you come closer to carsons speed with the engine still being in the efficient power setting range.

It certainly could, in cruise. As I stated in my post this is efficiency testing, and I have data already for 2200, 2300, and 2400 rpm in long-range cruise. This flight was to complete the 2500 rpm data. I also have data for various density altitudes from 9000 to 21000. The idea is to explore the envelope and figure out not only what is "best" but what also is "pretty close to best" for changing flight conditions.

Agreed on the efficiency, but the ride in turbulence... not so wonderful. One very knowledgeable source says that the RV-9 "rides like a buckboard."

No turbulence up high - let the wing do it's thing. It was 90+ degrees on the ground at takeoff, and I did experience thermals up to about 8k', but nothing above that. Even in the hottest summertime thermal conditions of west Texas I have never run out of climb performance before I ran out of turbulence, though admittedly sometimes that happens above oxygen altitude. I love to cruise in the mid-teens for that very reason - it's cool, smooth, your radios reach out 100 miles, and you can see forever.

Greg,

You might want to get data when not so deep LOP. At altitude I find just 10–20 degrees LOP yields a nice operating point. Here I suspect power is dropping faster than fuel burn as you go further LOP.

Carl

Agreed - as noted above this was a datapoint to join with others. I already have data for rich of peak, stoichiometric mix, and this flight was for ideal lean of peak to plug in a datapoint. My data shows that stoiciometric is the best power point above roughly 12,000 density altitude, but I was looking for lower speed and higher mpg with 40-50 lean.

It would be interesting to experiment with different climb profiles too. Intentionally throttling the engine to 21" down low is resulting in more time spent at the lower, less efficient altitudes.

Agreed - I normally climb at full power ROP but not long ago I did a 24" LOP climb, and this flight was for 21" LOP climb - again to fill in datapoints on the total performance envelope. We don't know what we don't know, until we start asking all the questions.

As noted I am planning for an around-the-world flight, and I want some knowledge about what is "best" to do in various changing weather conditions. If I am cruising at 17,000 and run into a line of thunderstorms over the ITCZ that forces me under them at a thousand feet above the sea for a couple hundred miles, I need to know how best to conserve my fuel for that portion of the flight before it happens. What about changing wind conditions - same question, same answer.
 
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ROP 46%

How is it that it shows lean of peak on the EGT's , but rich of peak under the MAP gauge (46% is horsepower)?
 
Agreed - I normally climb at full power ROP but not long ago I did a 24" LOP climb, and this flight was for 21" LOP climb - again to fill in datapoints on the total performance envelope. We don't know what we don't know, until we start asking all the questions.

Once you crunch the numbers, are you able to share the data? I'm quite curious to see how the different climb profiles compare.
 
403 Forbidden

I also got the access error with the above link, but was able to easily find the report by going to the cafe foundation website and navigating through research to the APRs i.e. aircraft performance reports.
 
How is it that it shows lean of peak on the EGT's , but rich of peak under the MAP gauge (46% is horsepower)?

Good question - one for Dynon. I don't know how the logic works in the HDX to determine that, but I can say with confidence that I was indeed lean of peak since I have an oxygen sensor in the exhaust stream. My Air-Fuel Ratio at that point was mid-16's.
 
It is the aspect ratio that makes it efficient and work well at high altitude, not the airfoil.
I appreciate that point. But then why did RV change the airfoil? I would have thought it contributed fairly significantly.
 
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Total noob question here, but looking at the website it lists fuel capacity for the RV-9/A at 36 gallons. Is that per wing, or do you have optional larger capacity tanks? I kicked around the website to try and find optional larger tanks but didn’t see any.
 
Total noob question here, but looking at the website it lists fuel capacity for the RV-9/A at 36 gallons. Is that per wing, or do you have optional larger capacity tanks? I kicked around the website to try and find optional larger tanks but didn’t see any.

Standard per-plans wing tanks are 18 gallons each side, the inboard half of the leading edge from each wing. I converted the outboard half of each leading edge to additional tankage, at 15.5 gallons each. This is not a Vans-offered conversion.
 
Thanks! I’m in the formative stages of getting my ducks in a row to begin my own build. Not sure I’d ever need more than “as-designed” capacity but I was intrigued about the idea of having more. Like they say, the only time you can have too much fuel is if you’re on fire… if you’re planning a RTW trip, I hope you’ll take us along for the ride!
 
Fowler

Except i dont think the rv9 wing will do well with fowler flaps; the wing has a slight concave surface aft of the main spar.
 
Coming back from Osh. OSH-MKE-TOL-MRB.
648 miles on 22.7 gallons. Had a good tailwind at 15.5k
Made it home in 3 hours 11 min.
My first trip with O2. Really got to see how well the 9 likes it up high.
Can’t wait for more long cross country flights.
 

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Happiness at 14.5k' sucking on O2.

i-2Ls37ks-M.jpg
 
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