Climb vs cruise

[PilotjohnS]

I have a RV9a with io320 that i can run lean of peak.

So the question is climb or cruise?

Do i spend 45 min and climb to 14500 and cruise up there, or do I climb to 9500 and cruise there?

It seems my climb takes most of my gas, so cruise is very low.

How to do I figure this out to optimize for a trip? What is the math?

I dont seem to be getting the range numbers than Vans publishes.

 

[airguy]

How rich are you running for climb? I keep mine just fat enough to keep the cylinders below 400 on the hottest one, and once I level at altitude then go LOP. How fast are you climbing? I usually adjust the climb rate to keep my indicated airspeed between 90 and 100 knots.

Lately I've also been playing with LOP climb, using ROP to about 6000 and then go LOP and reduce the climb rate, for a "cruise-climb", and again I keep the mixture (on the lean side now) rich enough to make good power but lean enough to keep it below 400, turns out to be an AFR around 15.3-15.5 or so. You'll drop your climb rate by maybe 30% but save a lot of fuel in the climb. You've got to carefully monitor the engine, not just absolute temperatures but also trends on cylinder temps.

 

[Bavafa]

How high to climb has some to do with the distance/time that you will be in cruise mode.
If my destination is only an hour away, I would not spend 30 minutes to climb that high and 5 minutes later start my descent. Also, just as Greg, I lean during my climbs to just about 50 ROP but I have seen some folks that they don't lean until they get to their cruise mode which is wasting some fuel.

 

[PilotjohnS]

I think i will try peak egt climb if below 65% and 400F at 300fpm then lop at cruise.

 

[Taltruda]

Fltplan.com has the best fuel burn/flight planning software.. 1000 times better than foreflight. There’s even an advanced option where you can fill in a matrix of climb speeds, climb rate, fuel burn at every thousand feet, customized for your aircraft. Then when you plan a flight, it factors winds aloft along the route and tells you the optimal altitude. You can even see where the better winds are and step climb or descend to make it even more efficient.

 

[gasman]

Fltplan.com has the best fuel burn/flight planning software.. 1000 times better than foreflight. There’s even an advanced option where you can fill in a matrix of climb speeds, climb rate, fuel burn at every thousand feet, customized for your aircraft. Then when you plan a flight, it factors winds aloft along the route and tells you the optimal altitude. You can even see where the better winds are and step climb or descend to make it even more efficient.

I fly has the same thing...maybe better. try it free.... https://iflygps.com/

 

[howardtlcc]

Foreflight also has this feature

You can enter your aircraft performance and it will show total time for the trip for the different altitudes.

 

[DanH]

I have a RV9a with io320 that i can run lean of peak.

So the question is climb or cruise?

Do i spend 45 min and climb to 14500 and cruise up there, or do I climb to 9500 and cruise there?

The optimum cruise altitude is more or less fixed based on the airframe design. Find it by experiment, or just accept most of the time you'll pick based on comfort, winds, and terrain.

Climb at best power mixture (100~150 ROP) or some lean of peak value? Heck, I'll take a shot at it. Climbing LOP is probably false economy. Climb rate is proportional to excess thrust HP. LOP means low HP, so low climb rate, thus more time and distance to reach any given altitude, so less time and distance at a higher cruise speed.

If climbing requires LOP to keep CHT away from redline, well, to be blunt, it's a crippled setup.

Here, a quickie spreadsheet, not perfect. Download and insert values you think appropriate for your RV-9:

https://www.danhorton.net/VAF/Climb vs Cruise/Climb ROP or LOP.xlsx

 

[Mconner7]

General rule was climb 1/3, cruise 1/3, descend. 1/3. You will always want to cruise at the optimum altitude for winds. I fly a 45 minute flight across Florida to work and frequently fly westbound at 9500 or 11,500 and eastbound at 4500 or. 6500 for winds. YMMV

 

[rzbill]

For climb, I use takeoff EGT values. With GRT lean function turned on before takeoff, the instrument will peak at TO values and then it is easy to see the EGTs drop into negative values (colder than peak due to overrichness) during the climb so a twist of the knob is easily done to restore the TO values.

 

[rocketman1988]

General rule was climb 1/3, cruise 1/3, descend. 1/3. You will always want to cruise at the optimum altitude for winds. I fly a 45 minute flight across Florida to work and frequently fly westbound at 9500 or 11,500 and eastbound at 4500 or. 6500 for winds. YMMV

I really hope you transposed those numbers...

Eastbound altitudes are Odd+500

Westbound are Even+500

Would not want to meet you at those altitudes!

 

[Taltruda]

You can enter your aircraft performance and it will show total time for the trip for the different altitudes.

Yeah but it’s not as good.. for example, it doesn’t account for the reduced fuel burn as you climb. You are only able to enter one climb rate, and associated fuel burn in the climb. Same with cruise, you can only enter one cruise fuel burn value in foreflight.. it’s very primitive. The Fltplan.com advanced matrix allows you to enter performance at every 1000 foot intervals.

 

[Mconner7]

I really hope you transposed those numbers...

Eastbound altitudes are Odd+500

Westbound are Even+500

Would not want to meet you at those altitudes!

Guilty but not the numbers, I seem to directionally dyslexic. Going to work is Eastbound high, home is westbound low.

Thanks for straitening me out….

 

[DanH]

For climb, I use takeoff EGT values. With GRT lean function turned on before takeoff, the instrument will peak at TO values and then it is easy to see the EGTs drop into negative values (colder than peak due to overrichness) during the climb so a twist of the knob is easily done to restore the TO values.

That's clever. I have the same system and had not thought of using it to peg target EGT for best power for climb leaning. Gonna try it!

 

[Toddsanderson]

My -14 is fastest/most efficient around 9k WOT/2500 RPM LOP about -20F. For a normally aspirated engine you should always climb at WOT/Full RPM unless the engine manual has a full power time limitation; however, that limitation is not usually a factor because you will be below 80% power within most 5-minute limitation due to thinner air. You should climb with your hottest EGT around 1300F and monitor CHTs to be ideally be below 400F in the climb and 420 as an absolute maximum. On a 85F day at 1000MSL mine climbs with CHTs ranging from 350-380 targeting 1300 EGT. It will usually give me around 2000 FPM and loses about 100 FPM per 1000 ft of altitude gain. Again, it is best for your engine to run at it's maximum power setting in the climb. The APS course gives very good reasons for this. The old wives tails of oversquare, shock cooling, etc were debunked a long time ago. LOP climb in a NA engine is not beneficial.

As for cruise - again, find the altitude that your plane is most efficient and take into consideration the length of the trip, the wind, and the weather. If I am going 50 miles I usually will stay below 6k since it makes little sense to climb high for a short trip. If I am doing a long cross county I will fly at 9k unless there is a good tailwind up high or if I want to be above the weather and bumps. I am a huge fan of LOP with cylinder temps in the 275-320 range and great fuel savings with electronic ignition that can really take advantage of the lean mixture by advancing timing. I have had my plane at nearly -100F LOP and still running fairly smooth, but the speed loss was nearly 10 knots and the savings was only about .2 GPH vs a -20F LOP cruise that only shaves off about 3 knots.

I can't say enough good about P-Mags. You run superior quality automotive plugs that are $5 each instead of $40 along with less expensive harness. You also get the advantage of advancing the timing and they are self powered.

 

[DanH]

For a normally aspirated engine you should always climb at WOT/Full RPM....

There is no mechanical reason to maintain max RPM for the entire climb with an NA Lycoming, but all things being equal it does generate max climb rate. For many constant speed operators, a slight RPM reduction after a few thousand feet can result in lower oil temperature.

You should climb with your hottest EGT around 1300F...

1300 may reflect best power mixture on one airplane, but for another it may be a different numerical value, for example 1275 or 1350. And the reference cylinder is the nearest to peak, which may or may not be the hottest.

I have had my plane at nearly -100F LOP and still running fairly smooth, but the speed loss was nearly 10 knots and the savings was only about .2 GPH vs a -20F LOP cruise that only shaves off about 3 knots.

Same here, on 23 degree fixed timing. Ever run a 390 with without the advance? The difference is quite small, unlike the parallel valve applications.

 

[MarkW]

I don't always fly for best Economy flight performance.
Very often in Florida it is about the Comfort in Dan's comment.

you'll pick based on comfort, winds, and terrain.

Get high quick for comfort.
Had a friend tell me while I was building, that a RV doesn't need A/C as we can fly to comfort quickly.

 

[Toddsanderson]

There is no mechanical reason to maintain max RPM for the entire climb with an NA Lycoming, but all things being equal it does generate max climb rate. For many constant speed operators, a slight RPM reduction after a few thousand feet can result in lower oil temperature.



1300 may reflect best power mixture on one airplane, but for another it may be a different numerical value, for example 1275 or 1350. And the reference cylinder is the nearest to peak, which may or may not be the hottest.



Same here, on 23 degree fixed timing. Ever run a 390 with without the advance? The difference is quite small, unlike the parallel valve applications.

Agreed 100%. If you have an oil temp problem then reduce the RPM and that will help, but my goal is to get to smooth and cool air as fast as possible and that is max RPM. With that said, you get slightly better volumetric efficiency by reducing RPM with a C/S prop. 1300F was merely a suggestion that has worked with all 60 airplanes I have owned from the 380HP turbo Beech Duke down to this little RV-14A. I basically take off full rich (we are flatlanders here) and usually start leaning around 4000 ft.

 

[Toddsanderson]

I don't always fly for best Economy flight performance.
Very often in Florida it is about the Comfort in Dan's comment.

you'll pick based on comfort, winds, and terrain.

Get high quick for comfort.
Had a friend tell me while I was building, that a RV doesn't need A/C as we can fly to comfort quickly.

I consider it necessary when IFR in FL. I have seen temps in the summer(and crazy humidity) be 80F at 5k ft where ATC parks you from Tampa to Fort Myers. Very uncomfortable in a high wing Cessna that blocks a lot of the sun and unbearable in an RV with the greenhouse glass. Also it is very useful on the ground, especially when waiting for a clearance to take off in Fort Myers. Now that I have it in the RV I wouldn't have another one without it.

 

[gmcjetpilot]

Depends.
Want Econ or Speed?
What are the winds?
Density altitude?
Distance of leg?
Gross weight?

Other factors
Want to suck O2?
Weather, terrain?

Rule of thumb (normally asperated GA plane) 8500' or higher for cross country efficiency, nominally. 12,500' max as supplemental O2 needed and trade-off of lower TAS speed to reduced FF is not great, assuming wind not a factor.

 

[MikeyDale]

Lately I've been cruise climbing. After I reach a safe altitude, unless its hot and bumpy, I lean out like I'm cruising, set VS on 200, and let the autopilot climb till I get as high as I want. I dont have any figures to see how good the economy is but I can judge the headwind /tailwind situation better and my ears can adjust to the altitude better. (I have very sensitive ears!) Same way coming down.

 

[ChiefPilot]

Rule of thumb (normally asperated GA plane) 8500' or higher for cross country efficiency, nominally. 12,500' max as supplemental O2 needed and trade-off of lower TAS speed to reduced FF is not great, assuming wind not a factor.

As you said, it depends on speed. I can cruise at 179KTAS on 9.6gph at 9600' DA, or I can cruise at 165KTAS on 5.8-6.3gph (weight dependent) at 17,000 DA. I disagree that the tradeoff in speed going above 12,500 is not great. On the contrary, as described above, the relatively small drop in speed is far outweighed by the increase in range and on longer legs the elimination of a fuel stop on a long trip makes it faster overall as well.

Going higher up is possible; I've collected data on cruise speed / power settings / fuel flow at various weights from 7000' to FL210 in 2000' increments at 150, 155, 160, 165, and 170ktas values. It was a lot of flight testing, but I have data to support flight planning for trips of various lengths with pretty good accuracy. And there are definitely times where going higher is both faster and more efficient than lower.

 

[rocketman1988]

As you said, it depends on speed. I can cruise at 179KTAS on 9.6gph, or I can cruise at 165KTAS on 5.8-6.3gph (weight dependent) at 17,000. I disagree that the tradeoff in speed going above 12,500 is not great. On the contrary, as described above, the relatively small drop in speed is far outweighed by the increase in range and on longer legs the elimination of a fuel stop on a long trip makes it faster overall as well.

Going higher up is possible; I've collected data on cruise speed / power settings / fuel flow at various weights from 7000' to FL210 in 2000' increments at 150, 155, 160, 165, and 170ktas values. It was a lot of flight testing, but I have data to support flight planning for trips of various lengths with pretty good accuracy. And there are definitely times where going higher is both faster and more efficient than lower.

…and there are times when it is not…

That is what flight planning is about.

 

[ChiefPilot]

…and there are times when it is not…

That is what flight planning is about.

Well, obviously. Flight planning should be based on data, not speculative guesses. There seems to be a lot more of the latter, and not much of the former.