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FL250

Traash

Well Known Member
In case you want to see the numbers of a RV-10 @ FL250.

Last week I needed to fly from KCXO to KOSH, a distance of 880 NM. Normally I would stop enroute but the winds at altitude beckoned. I filled for 17,000'. The tailwinds were stronger, the fuel flow lower and the mpg was higher the higher I climbed. I found myself asking Center for higher and higher and ended up at FL250 which made it possible to fly the route non-stop on 47 gallons of fuel. The picture shows the cruise numbers. I didn't start out to go that high so I didn't record any other data as a true test flight would. It just happened that conditions were perfect to see what she would do.

Conditions:
Lightweight (solo, no cargo, 35g fuel @ TOC), Cold (ISA +3*), Strong tailwinds, No clouds or turbulence, Full Oxygen and mask available, Destination VFR, Traveling from high to low pressure (slight descent in the flight levels), No traffic.

Equipment:
Thunderbolt IO-540, 9.1 pistons, 2 Slick mags, Airflow Performance fuel servo, 2 blade Hartzell prop, standard fuel tanks (60g), Mountain High EDS-4ip Oxygen.

Performance:
Final climb rate: 150fpm @ 90kias (140ktas)
Leaned to Max power (75* ROP)
Maintained 160ktas thru FL200

If you want to test your aircraft to it's service ceiling remember that the canula breathing method is only good to 18,000'. You need a mask above that to 25,000'. Above that you need pressure breathing.

As you can see, a stock RV-10 is a great x-country aircraft. I can't thank Van enough for designing such a capable machine and making it possible for me to build it.

Note: Had to stop on the way home. Luckily the winds had shifted Westerly and we're merely 70k crosswinds.
 

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That’s awesome! Less than 10 inches of MP lol! How does the mic work through the mask?
 
So if an RV-10 and an F1 Rocket leave Houston at the same time, headed for Oshkosh, and the RV-10 flies at FL250, who gets to Mahoney’s first? ;)
 
So if an RV-10 and an F1 Rocket leave Houston at the same time, headed for Oshkosh, and the RV-10 flies at FL250, who gets to Mahoney’s first? ;)

On this particular flight to OSH, the F-1 (my neighbor) flew commercial and my best guess is that by the time he arrived Traash was trashed and Mahoney's was closing!
 
Got there early enough to drive to Fisk for a refreshing S. Cow at Wehrmann's on the tracks. Yes... The tracks North of Ripon.
 

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2.4° AOA in cruise... is that a normal value? Flaps reflexed?

Normal. Flaps up (reflexed). Note the -3* indication.

The extended level horizon is the white line.
The simulated green terrain is -2.5* due to the altitude (curvature of the Earth).
The green segmented circle is the Flight Path Vector (FPV). Where the airplane is traveling. (Offset right due to left crosswind component).
The tip of the two yellow chevron's is the nose. Where the airplane is pointed.
AOA is represented by the angular difference between the FPV and the nose. Not that you would do this in flight but if you measure the picture you'll calculate 2* AOA. Quite normal for cruise. Stall occurs just before 15* AOA.
 
Q: What would be the effects on regular Slick mags if subjected to sustained or regular operation at these heights?

I used to do atmospheric testing for NOAA in a Cessna 337 Skymaster. We collected air samples from 25,000 down to 1000 feet every month. The engines were turbocharged but the mags were not pressurized. We would get a few engine stumbles during the climb at the higher altitudes, generally above 23,000 as the mag distributor arced across terminals. Other than the occasional engine stumble there were no operational issues and no apparent damage to the mags after doing this scenario for several years.
 
Interesting, I would have expected a lower fuel flow. I haven't yet been above 15K in my 10 (limited to 18K due to basic med). At 14 or 15K, I have been getting 165 KTAS at around 9.4 GPH. I would have expected another 10K would have made it better. Not a fair comparison, as I have EI and pilot variable timing. The advance is a good bit less at 15K than it is at 8K, as I need to be closer to peak than 80 LOP.

Curious if you had cooling issues in the climb up there. I have found the climb temps get pretty high after 12K and now do LOP climbs after 10K to keep the temps under control.

Larry
 
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Interesting, I would have expected a lower fuel flow. I haven't yet been above 15K in my 10 (limited to 18K due to basic med). At 14 or 15K, I have been getting 165 KTAS at around 9.4 GPH. I would have expected another 10K would have made it better.

Larry

I'm not sure of the setup here, but my FF (red cube) starts becoming progressively less accurate after 11-12k ft. On a recent trip at 16k, after fill up, I calculated my FF to be off by ~1 GPH (9.5 indicated vs 8.5).
 
I'm not sure of the setup here, but my FF (red cube) starts becoming progressively less accurate after 11-12k ft. On a recent trip at 16k, after fill up, I calculated my FF to be off by ~1 GPH (9.5 indicated vs 8.5).

I have posted about a similar (and very repeatable) problem on an RV-14A. For his plane the indicated fuel flow went above actual starting at 7K’ or so.

The root cause was never clearly identified by the experts I contacted. The problem was eliminated however when we moved the red cube away from the per plans location on the engine to the engine mount, and strictly followed the red cube installation instructions on no 90 degree fittings, up hill runs and such.

Carl
 
SpO2?

I’m curious as to how well the Mountain High system worked for you. Did you happen to check your oxygen saturation level when at FL250? I have the same system on our RV-10 so your results are of interest to me.
 
I’m curious as to how well the Mountain High system worked for you. Did you happen to check your oxygen saturation level when at FL250? I have the same system on our RV-10 so your results are of interest to me.

Worked well but canula (nose breathing) is different than mask (mouth breathing). At 250 needed to be a bit more deliberate with breaths. I wouldn't trust it any higher and in fact because it is not pressure breathing you shouldn't.
 
Worked well but canula (nose breathing) is different than mask (mouth breathing). At 250 needed to be a bit more deliberate with breaths. I wouldn't trust it any higher and in fact because it is not pressure breathing you shouldn't.

Take care up there. A small mistake can result in a very bad day, as you know. Lots of things can go wrong, and single pilot in the O2 levels has a lot of risk.

http://www.kathrynsreport.com/2021/10/beech-b36tc-bonanza-n75rm-fatal.html
 
Next time

Next time:
Wear a pulse oxymeter and report results.
Try flaps in trail. Steve (airfoil designer) has suggested you may see a few extra knots at this altitude with flaps in trail.
I don’t think pressure breathing is necessary at FL250, but a non-leaking mask might be advised.
 
AOA/Nose Position Question

The tip of the two yellow chevron's is the nose. Where the airplane is pointed. AOA is represented by the angular difference between the FPV and the nose. Not that you would do this in flight but if you measure the picture you'll calculate 2* AOA. [/QUOTE said:
Joe,

The above post infers that you aligned the depicted nose position with the chord line of the wing rather than the level airframe ( top of the longerons). Am wondering how you did this and what is the angular difference between the chord line and the top of the longerons.

Thanks.
 
Next time:
Wear a pulse oxymeter and report results.
Try flaps in trail. Steve (airfoil designer) has suggested you may see a few extra knots at this altitude with flaps in trail.
I don’t think pressure breathing is necessary at FL250, but a non-leaking mask might be advised.

I see a 1-3 knot gain at all altitudes above 5k through all loading conditions with the flaps in trial. Occasionally its 4 knots.
 
I think flying solo at 25k feet in an unpressurized aircraft even with an oxygen mask is very risky.

Interesting though.
 
Next time:
Wear a pulse oxymeter and report results.
Try flaps in trail. Steve (airfoil designer) has suggested you may see a few extra knots at this altitude with flaps in trail.
I don’t think pressure breathing is necessary at FL250, but a non-leaking mask might be advised.

Bob is correct on both counts.

1) I think at that altitude, you might find a slightly better performance from zero flap than -3. Someone please try it and report back.

2) I recall from my chamber ride that pressure breathing is required to maintain sea level O2 partial pressure above 33,000 ft. Below that, 100% O2 at ambient pressure exceeds sea level O2 partial pressure. I've been up to 30,000+ a couple of times in gliders, with a Scott mask and old standard A-8 O2 regulator.
 
Bob is correct on both counts.

1) I think at that altitude, you might find a slightly better performance from zero flap than -3. Someone please try it and report back.

I see an airspeed gain at 0 degrees well below 25k regardless of where in the cg envelope my -10 is. Generally 2-3 knots but can be as little as 1 knot and seldom a 4 knot gain.
 
So it's somehow less risky with more impaired people on board? The logic here escapes me.

-Marc

Peoples lungs are different and absorb O2 at different rates. I would expect 2 people to begin Symptoms at different times. There is a 50% chance that the pilot succumbs first and therefore the co-pilot can take over. Not necessarily the best redundancy strategy, but better than nothing.
 
otto

Peoples lungs are different and absorb O2 at different rates. I would expect 2 people to begin Symptoms at different times. There is a 50% chance that the pilot succumbs first and therefore the co-pilot can take over. Not necessarily the best redundancy strategy, but better than nothing.

Or just program the autopilot to fly this maneuver with the next fix at a lower altitude. I personally would let Otto fly.
 
Peoples lungs are different and absorb O2 at different rates. I would expect 2 people to begin Symptoms at different times. There is a 50% chance that the pilot succumbs first and therefore the co-pilot can take over. Not necessarily the best redundancy strategy, but better than nothing.

Yep. I’ve spent time in an altitude chamber with a group of people, and it’s interesting how obvious impaired behavior is to those not (yet) impaired, while the hypoxic individual thinks everything is okay. Sometimes it’s equipment (more leaky mask), other times it’s individual physiology. Sometimes it’s deep breathers vs shallow breathers.
 
Oxygen

Tolerance for high altitude has many variables. Younger non smokers who have acclimated at a lower altitude can handle very high altitudes.
Mount Everest, at 29,029' has been climbed without oxygen. A young healthy non smoker who lives at 10,000' for extended periods would have no problem with 25,000'.
 
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