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What is more efficient - to ride the wave or keep an accurate altitude?

I was flying over the channel again at 5,000 ft a few days ago. I have a “wings leveler” Autopilot which works well. On some days, after the Aircraft is trimmed correctly, it can sit for ages at a given altitude. Perhaps 10-15 minutes within 100 feet and no intervention.

On other days the plane will “ride a wave” where if you hold the yoke steady (or just let it go) the Aircraft will slowly deviate + or – 50 feet (or perhaps 100 feet) above and below a mean altitude. This seems to happen quite a bit over the sea at circa 5,000 feet.

So what is more efficient. Is it best to “let it go” – after all, to hold it steady you are constantly pushing and pulling on the yoke a small amount so I’d imagine there is more friction as the stabilator/ elevator has to drag itself through the air and produce a moment of force. Perhaps not correcting keeps the control surfaces cleaner.

I can also see the argument maintaining an exact altitude also.

Any thoughts – what uses less fuel?

United Kingdom

Is that an autopilot without altitude hold ? Just heading ?

EDLN, Germany

Archer-181 wrote:

So what is more efficient

Can you elaborate what do you mean by efficiency? will make short time/distance? consumes less fuel?
The answer may also depends if density altitude (DA) is the same and the type of propeller you have…

For solid/solid motion, here is something along the lines, the correct answer is 2: the ball on the dip will always have a speed that exceeds the one on the flat track

But that conclusion will not be easy to translate to flying in fluids as 1/ the flat track is actually when the aircraft “hunts for constant speed” (oscillate in altitude but keeps constant speed), 2/ you have both dips & humps, 3/ you also need the two track in the same DA

For sure, “flying physics” changes with height much more than “gravity physics”

Maybe try it yourself next time and tell us what you get? tough a very difficult to experiment to set-up
- With +/-50ft oscillation, I doubt you notice any differences: as will need to fly long distances, so things get scrambled by changes in DA due to conditions along the way
- With +/-1000ft oscillation, you can notice the differences in short flying distance but then DA changes due to height…

EGSX, United Kingdom

A very interesting question.

I think one would need to do a proper calculation, to see which factors are the greater cost – the extra drag as the control surface AoA is changed, or the extra drag of allowing the greater airspeed to build up if you maintain the baro altitude while flying into an updraught. Where is @DavidS when he’s needed?

My feeling is that the latter is the greater cost – parasitic drag increases as the square law (?) of the IAS.

That would mean allowing the plane to ride up and down is more efficient than holding the baro altitude.

It is a bit like this problem to which nobody posting knew the answer…

Most of the website in the above post doesn’t work for me… but it sure looks fascinating!

Shoreham EGKA, United Kingdom

Peter wrote:

That would mean allowing the plane to ride up and down is more efficient than holding the baro altitude.

I tend to agree with that if we are talking about +/-50ft, if you are interested in min fuel efficiency then then you should fly at min drag
The drag curve is convex with speed (square law on one higher speeds and blow up on Vstall), so:
- If you want max fuel efficiency, you should fly exactly at V_min_drag and not fluctuate around it
- If you pick another speed, then you better not oscillate around it otherwise your average drag increases tough average speed stays the same
(for those who likes maths )

The problem you get if you start changing height too much to keep the same speed, then DA starts to matter, say the performance chart tells you 8000ft is sweet spot for min fuel (or 12000ft is sweet spot for max TAS), then changing your DA to keep speed will not help you optimizing your fuel

For fuel efficiency, you are convex on height/speed with min being DA = 8000ft and V = 60 IAS, then good luck figuring out which one matter most

For time efficiency, you will also have to include IAS speed vs TAS speed and power curve vs drag curve, which also depends on DA in a non-linear fashion…

Last Edited by Ibra at 11 Oct 15:52
EGSX, United Kingdom

Yes, to answer the two questions others have asked (1) my Autopilot is heading only (not altitude hold) and (2) I was thinking of minimum fuel used for a given trip.

I suppose these “waves” are similar to those on the sea so one might be better to go with the flow?

Thanks – Archer-181

United Kingdom

I would have thought going with the flow.

While it is a different medium, and different dynamics, when you race high performance sailing dinghies it is incredible the difference betwee someone one using the rudder too much and someone the minimium possible. OK it exaggerates the effect being water, but every movement creates drag, and saps speed, or, in this case fuel.

I guess “go with the flow” is the general advice you can do to save energy

You get the same while flying gliders on a weak ridge, you will stay longer if you keep stick inputs just the necessary minimum (also with too much playing on the stick, you will have more difficulty spotting lift location or guessing wind direction…)

EGSX, United Kingdom

Slow to make the most of the updrafts and race through the downdrafts.


Yes, butvthus applies only when looking for max range (or max glide) or max average speed on a given distance with external energy, some even call it McCready theory:

But I am not sure much of this applies when you look to “save your own energy” as you will gain/loose some of it from external thermal activity?

I guess the initial question from Archer-181 assumes hight changes are from speed changes not from external updrafts/downdrfats? Otherwise your advice kwlf will also apply

Given low cloudbase in the UK, you will be luck very finding any thermal activity at 5000ft tough

EGSX, United Kingdom
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