Fly low or high ?

[JG3]

I do quite a few long x-country flights in my Savannah, usually below 3000 QNH because I enjoy the perspective and the action I can see on the ground from that height. But often it gets fairly bumpy at that altitude by midday, and sometimes really rough in later afternoon.  Going up to 6500 or 7500 ft usually finds smoother air, but I don’t enjoy it so much up there because the progress over the ground feels really slow and tedious, and the perspective flattens the terrain and I can’t sticky-beak the same detail on the ground.
 

Of course indicated airspeed goes way down and that is expected, but I had thought that TAS made up for that and maybe some extra speed due to thinner air, etc. But measurements have shown that isn’t the case at 6500 ft, and actually lost 3-4 kts running at the same RPM by going higher.
 

First results surprised me, so I repeated the tests five times on four different days, and the results repeated consistently. It’s very satisfying when such testing is so repeatable; shows that the procedure and controls are valid.

Testing 4-way GPS at 2000 and 6500 QNH, several times.
From many 4-way GPS runs, I know my ASI is spot on at 2000, so the change in altitude from that base to 6500 is 4500.
Calculated TAS = IAS + (IAS x 2% x altitude in thousands). In this case IAS is 80 kts and difference in altitude is 4500, so the correction in brackets would be 7.2, rounded to 7 kts.

With prop pitched to 5800 rpm WOT at 2000 ft, found the same WOT of 5800 rpm at 6500 ft.

RPM      IAS        kts Calculated TAS           kts                    4-way GPS             kts Fuel burn          L/hr    2000 / 6500              6500               +7  2000 / 6500 2000 / 6500 5500 90   -10    80              87  90   -4   86  20       19 5200 84    -9    75              82  84   -3    81  17        17 5000 79    -8    72            79  80   -3    77  15        15

Observations from the results:
- IAS reads 8–10 kts less than at 2000 ft. To be expected.
- Calculated TAS is 1 kt higher than actual speed measured. (Maybe the correction factor should be 1.8% instead of 2%.)
- Speed at 6500 ft measured 3–4 kts less than at 2000 ft, when running at the same RPM.
- Fuel burn was essentially the same at both altitudes for the same RPMs….

The results for actual airspeed are consistently 3-4 kts slower at 6500 as compared to 2000. So to fly at the same RPM at 6500 actually lose 3-4 kts…..

We know that the engine produces less power in thinner air with increasing altitude, so that would account for the reduction in speed when running at the same RPM.
But that reduction in power means that 5500 is no longer max continuous, so there’s still a good margin to safely run continuously at 5500 rpm when at 6500 ft.

Running at 5500 rpm at 6500 ft gives an actual speed of 86 kts at 19 L/hr fuel burn.
Running at 5300 rpm at 2000 ft also gives a speed of 86 kts but at 18L/hr fuel burn.
So, higher fuel burn for the same speed at 6500 ft.
Would that higher fuel burn for the same airspeed be due to the basic carburetors not being fully compensated for altitude and thus running richer??

Comparing fuel efficiencies (mileage):

Flying at 86 kts at 6500 ft and 19 L/hr = mileage of 4.5 nm/L.
Flying at 86 kts at 2000 ft and 18 L/hr = mileage of 4.8 nm/L.
That works out to 18 nm more range from a 60 L tank of fuel to fly same speed but lower.

Flying at 5500 rpm at 6500 ft gives 86 kts at 19 L/hr = 4.5 nm/L
Flying at 5200 rpm at 2000 ft gives 84 kts at 17 L/hr, = 4.9 nm/L.
That works out to 24 nm more range from a 60 L tank of fuel to fly slower and lower.
The difference of 2 kts in speed makes for a time saving of only 1.4 minutes per hour of flight time…. Is it worth it??
 

So it looks like no speed or fuel advantage to going high, in fact advantages to stay low.
 

I usually cruise between 1500–2500 ft AGL  at 5200 rpm, making 84 kts, so I guess I’ll continue doing the same unless seeking a more favourable wind, or of course a smoother ride…..

[RossK]

Interesting numbers JG, I'll pay more attention next time I'm changing altitude to see if there is a ground speed difference.

Typically I'm picking an altitude base on airspace limits, clouds, and comfort. I generally prefer to be at least 2-3000ft AGL for the what if factor.

But agree, lower is more interesting and the sensation of speed is more noticable.

I always enjoy the run from YLIL to the western suburbs of Melb, using the coastal VFR route, 2500ft across the suburbs and then 2000 across the front of the city. But options for engine outs are very limited - freeways and golf courses basically.

At 7500ft AGL, everything looks pretty flat and moves very slowly.

[FlyBoy1960]

you also have to remember that for most aircraft powered by a Rotax 6500 feet is the ideal altitude for performance. Once you go above this than the power from the engine drops off and the propeller loses some efficiency due to the  thin air. There was an EAA seminar online about exactly this. I will see if I can find the details

[facthunter]

ALL normally aspirated Pistons lose power with higher altitude. MP Manifold pressure is LOWER and if the mixture is right the fuel flow will be lower also. Prop drag at the same revs is lower too because the air is less dense. and the IAS will be below TAS by  a bigger margin. The cruising pitch attitude will be more nose up the higher you go.  Somewhere in there will be the best L/D figure for your plane for range.  ie where you get best Ground NM/ LITRE  of fuel. Wind velocity will come into that also. Nev

[Thruster88]

This performance charts shows how increasing altitude affects,  propeller rpm required for a given % of power, range at a given power setting,  true airspeed, fuel flow required for a given power setting. 

 

This aircraft runs about half the rpm of the geared 912 but the % changes would be very similar because they fly in the same air. Fixed pitch prop.

 

There is a slight gain in efficiency and maximum range with altitude. Speed has the greatest effect.  

 

Edited February 3, 2022 by Thruster88

[kgwilson]

I tend to fly high when going on a long trip & low when doing a local. I cruise at about 2750 - 2800 rpm (Jab 3300A) when I am low at 1500 - 2500 feet. Fuel burn is 18-19 lph. IAS is about 105 knots, TAS about 109 knots

 

On a long trip I fly pretty high (7500 or 8500 hemispherical) with rpm about 2850 - 2950. Fuel burn is about 24-26 lph. IAS is about 110 knots TAS about 126 knots.

 

Getting high and in the smooth air is much less stressful & I can see a long way ahead. I don't have auto pilot & don't intend to ever fit it. I like to fly my aircraft not be a passenger.

 

3 hours at low altitude & getting thrown everywhere takes too much out of me (& stresses the airframe more) so I arrive knackered. 3 hours high & I arrive ready to party.

 

Also if there is a tailwind it is often faster at high altitude and smooth so GS can get right up there. I have had over 170 knots GS on occasions.

Edited February 3, 2022 by kgwilson

[Flightrite]

Everything in aviation is a trade off, there’s no such thing as a free lunch😉

There’s so many variables that it’s not worth sweating the small stuff. Different story for us jet jockey’s, a 160 kt H/W hurts!

Under a 100 kts the numbers aren’t significant, just fly the plane, enjoy the experience and remember the fun & challenge is just getting there👍🙂

Edited February 3, 2022 by Flightrite

[Thruster88]

On 03/02/2022 at 9:55 AM, JG3 said:

With prop pitched to 5800 rpm WOT at 2000 ft, found the same WOT of 5800 rpm at 6500 ft.

 

Thruster88. This is the same for static RPM, static RPM is always the same regardless of field elevation, pressure or temperature. It is a sure way to know the health of an engine fitted with a fixed pitch propeller. 

 

 

RPM      IAS        kts Calculated TAS           kts                    4-way GPS             kts Fuel burn          L/hr    2000 / 6500              6500               +7  2000 / 6500 2000 / 6500 5500 90   -10    80              87  90   -4   86  20       19 5200 84    -9    75              82  84   -3    81  17        17 5000 79    -8    72            79  80   -3    77  15        15

Running at 5500 rpm at 6500 ft gives an actual speed of 86 kts at 19 L/hr fuel burn.
Running at 5300 rpm at 2000 ft also gives a speed of 86 kts but at 18L/hr fuel burn.
So, higher fuel burn for the same speed at 6500 ft.
Would that higher fuel burn for the same airspeed be due to the basic carburetors not being fully compensated for altitude and thus running richer??
 

Thruster 88. If we said 5500 rpm @ 2000' was 75% power then 5500rpm @ 6500 would be 67% power. Fuel flow of 20lph @2000 should reduce to 18.4lph @ 6500. I agree the carburetor is running richer with altitude negating the normal small gain in efficiency that a mixture control aircraft will achieve. 

 

I love flying low but will always climb to 7500-9500 on trips of 100nm or more to get into smooth cool air especially in summer.  

 

Edited February 4, 2022 by Thruster88

[facthunter]

There's a thing called "inversion level" that goes higher as the day progresses. In Central Australia it's commonly above 10,000 feet in summer in the afternoons and it's smoother above it. Nev

[Flightrite]

You often see an inversion layer around the cesspool cities, goes to show the crap the people in ghettos breath in! Typically 5-10K’

[skippydiesel]

I nearly always fly at the level recommended by OzRunways, modified according to AT control steps, turbulence (stay above ) and good terrain clearance. Seems to work for me.

 

I fly for the  shear pleasure of it. Don't get me wrong I am interested in performance but mostly in how fuel efficient my baby is - So speed is important (trip time), as is whole of trip fuel consumption per hour and, fuel flow @ given rpm. To this end I keep a fuel log, with trip time and comments on relevant flight conditions, engine settings etc. Probably boring stuff for most but ticks the right boxes for me.

[Old Koreelah]

36 minutes ago, skippydiesel said:

I nearly always fly at the level recommended by OzRunways…

 Missed that feature; where does this recommended level appear?

[MattP]

If you get the winds in the flight plan page it pops in it’s guess of most optimum cruise level hemispherically based on those for your plan. Usually works well if the winds are right. Always best to check it though like anything you plug in electronically. Garbage in and garbage out.

 

 

[kgwilson]

If it relies on what you get from NAIPS it is more often wrong than right so I wouldn't bother. Windy is always more accurate but don't believe anything is right. You can work it all out as you go with your GPS ground speed compared to your IAS heading drift, and your own comfort requirements. On a long trip I prefer to find the most comfortable altitude rather than the best ground speed altitude. Taking an extra 10 or 20 minutes is neither here nor there unless you have failed to plan for weather or daylight.

[Old Koreelah]

8 hours ago, MattP said:

If you get the winds in the flight plan page it pops in it’s guess of most optimum cruise level hemispherically based on those for your plan..

Still cannot see a “recommended cruise level”. Do you men the “Optimize altitudes” list which pops up when you select those 4 vertical sliders next to the windsock?

[MattP]

Yep those.

 

To kgwilsons point your relying on the input to be right and should always plan what works for the situation, and be prepared. Just hitting the button and going isn't a good habit to get into.  

 

Case in point was recently when taking a friend who can be a bit nervous for a trip. Warm day with a slight northerly in the morning on the ground and low and westerly higher up above 7000. I knew it was going to be rough low as we went so took a 20kt headwind at 8k to make sure he was comfortable. Only added 20 min to our fight time but much better spent and more comfortable. Trip home later that day was fun looking at the gs 😀

Edited February 4, 2022 by MattP

[skippydiesel]

20 hours ago, Old Koreelah said:

 Missed that feature; where does this recommended level appear?

Fair cop - I put in my minimum safe altitude, weather and I get options  (for each" leg")- I choose the one that gives best projected ground speed that is above turbulence.

[Old Koreelah]

We flew to Barraba for breakfast this morning and choosing a cruising level was topical.

 

We had a strong SE wind so on the trip north my ground speed was impressive- but the plurry mechanical turbulence was not.

Coming home, we all chose whatever altitude we were comfortable with. Two stayed low with their large wings and coped with the bumps, hoping to reduce their fuel burn in lesser headwinds. I climbed into smooth air at 7, and found the headwind less than down low. 

[JG3]

<< I'll pay more attention next time I'm changing altitude to see if there is a ground speed difference. >>

Be sure to do a 4-way GPS at each altitude to get a true comparison allowing for changes in wind.

 

I had heard many theories that the speed would be faster in the thinner air at higher altitude. I'm quite skeptical of theories without solid evidence, especially amateur theories that have been repeated for generations. So I go and do testing to find the evidence. I've done that in the past with props comparisons and VGs, with revealing evidence in each case. The evidence in this altitude comparison testing was surprising, but satisfying because I now know the real world results. I have no doubts about the consistent results. The real airspeed is not faster up higher but in fact 3-4 kts less. So no speed advantage unless more favourable wind effect.

 

On x-country flights I seldom fly a straight line direct course. I'm forever watching the ground, and if I spot something interesting off-course I head over to have a closer look. I don't fly my aircraft just to get up in the air or just to get point to point; I use it more as a 'high clearance' vehicle for exploring the countryside. Of course this is more effective at lower altitudes. My Savannah handles turbulence pretty well, mostly just lively 'whoop-de-do' which I call 'rock and roll'. Not violent enough to cause structural overload in this well-braced high wing machine. A low wing cantilever design flying faster and thus hitting the bumps more abruptly is a whole different case. Yes the rock and roll is tiring, but I don't suffer motion sickness so just carry on, slowing down if need be. When it gets too annoying I either go high or call it a day and land at the next convenient airfield or off-field if suitable. Shouldn't plan on  still being airborne that time of day anyhow..... Each to their own, but I've really enjoyed flying like this for 3000 hrs and seen so much interesting countryside and activities all over the place......

 

Windy.com is a real boon for chasing wind at different levels. I have found it very reliable and useful so many times. 

[howe]

The best part of flying is the take off and landing, the rest is well so so.😊

 

[skippydiesel]

10 hours ago, howe said:

The best part of flying is the take off and landing, the rest is well so so.😊

 

I agree, up to a point. True, the challenge of integrating all the varied, competing aspects that go into making a "professional"  landing/take-off at an unfamiliar field is very satisfying.

 

However that hard to describe feeling of calm satisfaction but not quite relaxation, a little after establishing my aircraft straight & level, on track, is worth every penny of my overly expensive hobby.

[pmccarthy]

The best part for me is arriving somewhere hundreds of kilometres from home and thinking I have been in a time machine.

[facthunter]

Many judge the flight by the "smoothness" of the landing, but the good management of the rest of the flight is what gets you where you want to Be with minimum avoidable risk or uncertainty. Nev

[Flightrite]

Arriving safe is the most satisfying part, the rest you can talk about at the bar😉 Over 200 kts today @ 9.5K ISA+17c, living’ ‘da dream👍

[ClintonB]

22 hours ago, howe said:

The best part of flying is the take off and landing, the rest is well so so.😊

 

I agree, Doing stuff is a lot more fun and interesting, the cessna is a lazy mans plane, feet barely required.

 

raaus planes take a lot more effort.