Slow down to speed up

[n5lp]

Our speedy, high performance airplanes give us capabilities that we don't always use. Sometimes we pick a favorite cross-country altitude without really thinking about it.

I wanted to have lunch in Brenham, Texas, yesterday (4/19/09), for Louise House's birthday. That leg is 424 nautical miles and there was a real good tailwind, increasing with altitude. In this situation, a lot of people would choose to go high, an option we have. I did so, and averaged 185 knots (212 MPH) block to block with a really low fuel burn. That was a really quick and easy leg.





This is the first significant water I have seen in a loong time. Probably more water in this picture than my whole county has received this year.

When I was ready to return home, it was from an airport 500 nautical miles from home, with at least a 30 knot headwind that didn't decrease too much with altitude. There were now decisions to be made. At times I have actually been able to get tailwinds both ways by using radically different altitudes but that wasn't going to work today, and with the wind it wasn't certain I could make it home without a fuel stop.

Leaving Polly Ranch to refuel (Louise Hose photo)

With a turboprop or turbocharged airplane it is often worth it to go ahead and climb up high. The TAS increases enough to maybe make it worth it anyway. With a normally aspirated piston engine it is normally better to stay as low as possible with a strong headwind, because the wind is almost always less at lower altitudes. So for getting home quickest I should fly low at a high power setting to get a real good speed? No, because the increased fuel burn at low altitude and high power setting would mean I would have to stop for fuel and that takes a really long time.

My decision for this trip was to fly at about 1,000 AGL and power way back. When you do this, you shouldn't look at the GS on the GPS. It is too depressing to see 130 to 135 knots, but it can work out.

The drawbacks are the low speed and bumping around in low level turbulence; the upsides are you get home faster, in this case, and you get a much different and interesting view of the scenery.


If you live on the Edwards Plateau, there is a high likelihood you have a sinkhole in your yard.

I had never had a close look at the Texas Hill Country before and it was fascinating to see all the flooding, hundreds of game ranches and thousands of sink holes. On this day it seemed that much of central Texas was either under water or on fire. You also get a great geography lesson as the trees and green suddenly disappear and sand and creosote appear.

Who knew there were rock hills like this in central Texas?

This trip worked out just right. The lower fuel consumption allowed me to make it home with about 8 gallons left, which is right around my minimum. If I had flown faster I would have had to stop and the trip would have taken much longer.


Aaah, now this is more like it. I'm starting to feel more at home without all that water and humidity

My average speed coming home was still about 139 knots (160 MPH) as the headwind abated near my destination. That may not sound all that slow to folks that aren't privileged to fly such flexible airplanes.

Another drawback of flying low into a headwind is that you have fewer landing options in case of a problem. This is a real trade off, but I felt much safer, even in the hill country, at this altitude, than I did bumping around at low level over flooded country and around 2,000 foot towers, under the Class B at Houston.

[Ironflight]

Nice write-up, and very good reasoning Larry!

I think those "big rocks" look like enchanted Rock State Park - believe it or not, there is a cave in that big one - well, a collapsed slab cave....

Paul

[frankh]

I play that game as much as I can as well.

A question..Why only 13.5K?..You need O2 above 12.5 after 30 mins (from fading memory) so if TWinds increased with altitude why not go to 17.5 VFR?

SAdly My Wife often can't stand high altitude without getting sinus problems..Normally she is limited to about 13.5 but I got her up to 15.5 at SB for a couple of hours at a time to get over hight clouds.

of course the higher you go the more O2 you use too so there is another variable.

Cheers

Frank

[n5lp]

Originally Posted by frankh

I play that game as much as I can as well.

A question..Why only 13.5K?..You need O2 above 12.5 after 30 mins (from fading memory) so if TWinds increased with altitude why not go to 17.5 VFR?

...

A few times, I have found it worth it to go up around 18,000 but not often. This day I was kicking myself on the way down for going so high. The wind gradient with altitude just wasn't enough to make it worthwhile to go to 13,500. I should have been at 9,500 or so.

[n5lp]

Originally Posted by Ironflight

...I think those "big rocks" look like enchanted Rock State Park - believe it or not, there is a cave in that big one - well, a collapsed slab cave....

Paul

Thanks Paul. Looks like that might be an interesting place to visit sometime.

[Bob Axsom]

In the 1996 Great Cross Country Flying Race from Denver to Oshkosh I flew as fast as I could In my 1981 Piper Archer II and made one fuel stop in Cherokee, Iowa. Art Mattson flew an older but highly modified for drag reduction Cherokee in the Corinthian Certified Division (Production airplanes with 360 cu. in. engines). We both had a passenger but his passenger sat in the rear seat to minimize level flight angle of attack induced drag. He flew non stop. We got to use the restroom and still beat him by 41 minutes and 3 seconds (6 hours 24 minutes vs. 7 hours 5 minutes 3 seconds) and averaged 122.66 mph compared to his 116.81 mph.

In last year's AirVenture Cup race from Mitchell, South Dakota to Oshkosh I made up a Excel matrix for our RV-6A to predict race speeds for altitudes in 1,000 ft increments to 12000 ft including time to climb at 500 fpm 100 kts and descend at 500 fpm 10 kts over cruise with cruise ground speeds from 150 to 240 kts cruise. It was fairly straight forward to add or subtract the winds aloft forecast to get the predicted race speed for each altitude. The exact altitude of preference on that day was 9,000 ft. At one point in the race weather forced descent and/or deviation to remain race legal VFR. The ones that deviated were really hurt bacause the minumum distance to complete the race was increased but the ones that only descended had to make a decision to stay at the new altitude or climb back to the prefered altitude when the clouds were passed. The right decision on that day was to climb back to 9,000 ft. I did and I finished 3rd at 223.89 mph just beating Alan Carroll in his faster RV-8 at 222.97 mph. On that day the winds were such that a higher or lower climb would have resulted in a lower speed. Rules of thumb are good but not always correct.

Bob Axsom

[Phil]

Definitely Enchanted Rock near T82....

Should have stopped in for a burger there too!

Good write up.

Phil

[kentb]

I don't have a lot of cross country experience and have a question about flying performance and the winds.

I have read that when the wind is with you, you should slow down and enjoy the free ride, but if you are fighting a head wind that you should fly as fast into it as you can.

So did I misunderstand what I read?

Always wanting to learn more, Kent.

[n5lp]

Originally Posted by kentb

...I have read that when the wind is with you, you should slow down and enjoy the free ride, but if you are fighting a head wind that you should fly as fast into it as you can.

So did I misunderstand what I read?

Always wanting to learn more, Kent.

A lot depends on what the individual's goal is for each flight.

The rule of thumb about slowing down when the wind is with you would assume that you don't want to make maximum speed and that economy is important on that trip. It can be fun to see the MPG over the ground figures on those kind of trips, but if you are in a hurry you would fly at the maximum power you are comfortable with, no matter what the wind, oh but range may be a factor also and then there is the altitude question. Usually winds increase with altitude but available power decreases (normally aspirated) so you have to look at the variables for the particular trip you are making. It hasn't happened to me that often, but sometimes it is worth it to climb up around 18,000 feet to get good speeds at a very low fuel burn. This usually takes a long leg to be worth it and it also means dealing with oxygen.

I believe I was getting about 40 statute miles per gallon (at 237 MPH) when the above photo was taken.

Flying into a strong headwind, if the goal is best speed, is simple as long as you have the range for the leg. You fly as low as possible (assuming normal wind gradients) at full power. As I pointed out there are drawbacks to this plan also.

Really the point is that they are infinite number of possibilities and it is probably a good idea to try to think about them for each flight leg. What do we want to accomplish and what is the best way to do that? Sometimes we get locked into flying at 6,000 to 8,000 feet at 65% power and that may not always be appropriate.

[Alan Carroll]

Originally Posted by Bob Axsom

At one point in the race weather forced descent and/or deviation to remain race legal VFR. The ones that deviated were really hurt bacause the minumum distance to complete the race was increased but the ones that only descended had to make a decision to stay at the new altitude or climb back to the prefered altitude when the clouds were passed. The right decision on that day was to climb back to 9,000 ft. I did and I finished 3rd at 223.89 mph just beating Alan Carroll in his faster RV-8 at 222.97 mph. On that day the winds were such that a higher or lower climb would have resulted in a lower speed. Rules of thumb are good but not always correct.

Bob Axsom

Bob - I started at 9500 and had to descend for the cloud deck, but made the wrong decision and didn't climb back up once past it. This is part of the fun of these races; its not just about who is faster. Sometimes it's about who is smarter!

Somewhat surprisingly, in the same race Lee Behel in his Legacy beat the Nemesis NXT by 16 mph. The Nemesis went very low dodging weather as I recall, and the Legacy went up to 17,500.

Regarding rules of thumb, my rough number is that max TAS decreases by about 1 knot per 1000'. How does this compare with your spreadsheet?