O2 and your nose
MSFT-1 said:
I have an RV-8 with the O-360AIA and a C/S prop.
I live on the east coast where the highest obstacles are generally <4K.
I often fly at 6K to 8K but have been thinking about the advantages of flying higher. My throttle is at the firewall and my MP is at about 21" by the time I get much higher than 8K but people tell me there are lots of advantages to going higher.
The guys from Van's say they often fly at 16K (with O2 of course).
So my question is does it make much sense to buy a small O2 bottle and climb up to 12-16K for longer cross country flights in an RV?
thanks, Bruce N297nw
If you read Van's Oshkosh report, in the last RVator he flew at 9.5 to 11.5 to Oshkosh. The RV-10 made it in less than 10 hours from Oregon. They went over some big hills with no O2. He made the comment that neither he or Ken where interested in making it a record flight or minimizing the stops. I suppose they could have used O2 and extended their range, reducing the number of stops and trip time, but they could not be bothered.
As far as flying at 16k, sure that depends on aircraft weight, HP and of course the winds. I like to fly 8.5 to 12.5, depending on weight, temp and winds. I have gone down on the deck if needed due to winds. Other times flying lower is just more fun with better view.
I have cruised up to 17,500 feet w/ a nose bag in my RV. It's kind of fun to see how high you can fly and little fuel you can use. However when your nose is dry later that night on the ground, its not so fun. When you get that high, you're going to get there but way slower. Your speed is dropping down not withstanding any tail wind. If you plan on flying high, plan on sitting for a long time, like close to 6 hours. You can turn a 4 hour flight into a 6 hour flight easy, unless there are good tail winds to be had high up. Also forget sight seeing, but the reason for flying high is fuel, range or weather. How much can you save? 3 gal/hour or 150-200 miles more on the tanks? That's not bad.
Its real simple math, the higher you go, the less power your ATMO engine makes, the slower you go and the less fuel is burned. The plus of flying higher is less drag, so 50% power at WOT and 17,500 feet is better than 50% power with partially closed throttle at 1,000 ft. They both save fuel but you'll have better TAS at 17,500. Flying slower puts you closer to max or long range cruise speed. Also winds tend to be stronger with altitude and change direction (remember the Coriolis effect?). Having the option to fly well above 12,500 ft can be a big benifit, but how much would you really take advantage of it?
The difference between 50% and 75% power on a O360 Lyc is almost 3 gal/hr! At 17,500 feet with WOT, 2,500 rpm is about 50% power. If equipped w/ a constant speed prop the slower you set your RPM the better. However at real high altitudes you may need to spin faster to just keep the airspeed up.
Now the down side sucking O2 thru the tubes in the nose. You will have to spend $500 to $800 to buy a 9 to 22 cu in tank system. It'll take some time to recoup that cost. If you fly long X-C all the time it may save money on fuel. Also O2 dries your nose out, you have to fill the bottle up which cost money and it's a hassle. Also there's the weight and clutter of bottle, regulator, hose and mask.
At 15,000 feet, a 9 cu-in tank gives about 9.5 man-hr; a 22 cu-in tank has 14.8 man hours duration. You see different numbers from various vendors. Some claim their special super flow metering systems are better. A 22 cu-in tank is fairly large and heavy, about 11 lbs when full. Plus the other components add a few more pounds.
You can see with a small 9 cu-in bottle solo or 22 cu-in with two people you could be filling the tank every other or third flight. With a passenger you use twice the O2 and the plane will not climb as high. Two up with a 22 cu-in bottle is good for about 7 hours. I can tell you 7 hours on a bottle in one day will kill your nasal passages.
RV's don't always fly well above 12,500 ft, especially on a hot day, gross weight and lower HP engine. I tried it once with a low HP RV-4, fully loaded on a hot day; it mushed along. Look at Vans service ceiling altitudes. Service ceiling means you can barely climb and almost behind the power curve, mushing along. I take Van's MAX service ceiling and subtract 5k or 6k for a max practical high cruise altitude. That is about 12,500 ft for most gross weight RV's with a 160 HP engine. They will go higher but its not comfortable or effecient unless there is wind. Yes you may save some fuel but the time you add to the trip is not a good tradeoff.
I say go for it and experiment. Buy a used rig. Sell it if it's not your cup of tea. I do love the fuel savings of flying high, but I also like to sight see on a X-C and not get a dry nose. It's easier to say, "Oh that airport looks cool lets land I'm hungry", at 9,000 than 16,000 feet.
To each his own, but you can save lots of fuel high up. Also it may allow you to make a trip in less legs. The cool thing is hooking up to a 40 kt tail wind. I have a small 9 cu-in if needed, weighs about 8 lbs total, but overall 12,500 feet is plenty high. Borrow a system and try it out on your next X-C trip before buying? Cheers
