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

I think a key to analysing this is that this is a small-signal effect. There is no doubt that a large-signal version (say, 1000ft excursions around the mean) will cost you MPG.

I too find that sometimes I can have the autopilot in PIT mode and the altitude stays constant, say within 20ft, for some minutes. This relates to this which is a similar thing and, looking at how it disappears with alt static selected, I am very sure it is a static pipe blockage.

Administrator
Shoreham EGKA, United Kingdom

What @Archer-181 describes sounds much more like a phugoid oscillation, than a wave of up- and downdrafts. Just over the sea not that disturbed by aoa-oscillations. During the phugoid, the AOA stays almost constant (+- 0.2 degrees usually) so chances are you’d over-control the changes and loose energy. The oscillation is not very hard damped and you don’t lose much energy overall.

mh
Aufwind GmbH
EKPB, Germany

mh wrote:

During the phugoid, the AOA stays almost constant

That is excatly what I meant when I say:
Ibra wrote:

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),

Yes, as far as the AoA is concerned, the aircraft does not oscillate (during the phugoid, the actually aircraft flies a flat track with respect to the medium!)

It is one of those hard thing to understand about aerodynamic trimming, from an engineering perspective, you trim for an angle of attack, the aircraft will try maintaining that AoA, if speed oscillation is dumped via the choice of the right attitude, then you are trimmed for a given speed and your rate of decent is fully determined by your power, from a pilot perspective you think the other way around, starting by guessing power & attitude and iterate few times until all settles

As pilot you can fly based on angle of attack (AoA) and Glide Path (GP), it could be very easy: set +14 on takeoff, -2 in cruise and +14 on landing following the GP, but most of us will not enjoy the instability of attitude & power that goes with this flying…the only guys who enjoy it are aircraft carriers pilots (they fly a constant AoA = 12 & GP = ILS or OLS until touch down, they call it “being on the ball, on speed, otherwise a meatball don’t hook”, while power & speed & attitude jump all over the place)

Paris/Essex, France/UK, United Kingdom

I think there was another discussion more recently on t he matter but I could not find it. Yesterday we did a flight from Murcia to Mallorca diagonally across the waves in 50 KTS winds at FL100.

Cruise IAS was changing from 120 to 170 KTS in circa 5-10 mins cycles as the autopilot was striving to maintain altitude. On the first half of the first cycle I was checking configuration, gear doors, and power setting as I watched IAS come down and A/P trimming up in an unnerving way, having me change power to ROP and ready to call for a lower altitude if unable to maintain, but once I saw it doing the opposite then I confirmed it was waves we were riding, all along our flight. Sat picture shows it clearly

ATC was complaining about us showing 100ft lower then higher than assigned altitude, and I had to explain we were riding the waves…they did not comment about our GS changing from 120 to 210KTS and back !

Antonio
LESB, Spain

GS and altitude trace

Antonio
LESB, Spain

Nice one, that was surely on a westerly winds? have you spotted any lenticular around?

Did you told ATC your total energy altitude is fix :)?

Paris/Essex, France/UK, United Kingdom

NW winds, so even though they were mostly 90 deg direct xwind, it did reduce my GS doubly when in downdrafts since the reduced GS needed xtra crab angle to maintain groundtrack in the 60kts wind.

Yes, there were some lenticulars but far and few. They were mostly St and StCu. I did go through a particularly turbulent one at 5c and it left a clean if wet airframe. Othar than that I kept mostly above cloud and turbulence levels, but having to keep watching airspeed, autopilot and LOP power settings all the time.

I should have tried getting a clearance for a certain total energy, yes…not that it would have helped with separation though!

Antonio
LESB, Spain

Problems like this cannot be solved unless you define a rather accurate cost function. Using the least amount of fuel (only) is not very relevant, because it means you can disregard the time aspects. If you disregard the time aspect, you will find you can do it without using any fuel at all. That’s what a glider would do. Or more precisely, the glider would do it in the least amount of time possible without using any fuel. So, the only answer you will get is the more fuel you are willing to use, the faster you will get from A to B. But, since a glider can do it without using any fuel at all, it is reasonable to assume that flying “glider like” you will use less fuel than flying in any other way and still use the same amount of time. However, if time itself also has a cost, then this assumption is not necessarily valid anymore, because flying “non glider like” could save you time at a cost that you cannot make up for by flying “glider like”. You need to know the cost of time spent

Anyway, the basic principle is to spend more time in updraft than in downdraft, and gain/store energy (altitude) in updraft that can be transformed to speed in downdraft. An autopilot will do the opposite. It will speed up in updraft and speed down in downdraft. You will spend a short amount of time in updraft and a longer time in downdraft, which is inefficient fuel vise, but not necessarily time vise. I would think if you also have an auto-throttle (not just auto-alt), then you could do it in less time and by using less fuel, but now things start to become more complicated because you have to know the efficiency, not only of the plane, but also of the engine/propeller.

My guess will therefore be, if you are constrained to an altitude, that you will roughly strike a balance of using the least amount of time and fuel by flying at constant airspeed all the time, at least up to a point.

The elephant is the circulation
ENVA ENOP ENMO, Norway

You should definitely follow the flow. Actually, you should exaggerate it, by pitching up in updraughts and pitch down in downdraughts. That way you will maximise time spent in upgoing air and vice versa. That is what glider pilots do in order to win competitions. It has a name, dolphin flying. Of course, this is neither comfortable nor safe or legal in many circumstances, but definitely efficient.

Last Edited by huv at 05 Nov 11:16
huv
EKRK, Denmark

The basic rule in gliding (which would be applicable to fuel efficiency) is slow down in lift, speed up in sink.

If you try to maintain altitude while crossing wave you would be doing the opposite – speeding up in lift (thus shortening your time in the lift, where you’re having energy added to your plane for free) and spending more time in the sink by trying to maintain altitude (slowing down, and having energy robbed from you for a longer time period). Just maintaining your speed will be better than speeding up in lift and slowing in sink.

Hypothetical case: 500fpm average up for 2nm, 500fpm average down for 2nm, a Cessna 172 which climbs 500fpm at 75 knots and cruises at 105.

In the up parts, the plane will get to around 120 knots by maintaining power and altitude, and will be in the upflowing air for 1 minute.
In the down parts, the plane will have to slow to 75 and use full power, and will spend 1.6 minutes doing that.
The total time spent to fly the 4nm will be 2.6 minutes.

If the plane simply maintains a cruise of 105kt and rides the wave, it’ll take just under 2.3 minutes to travel those 4nm and never had to use full power.

Andreas IOM
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