I posted a vaguely similar Q here
But here I am talking about short term pitch (altitude) variations.
Obviously if you go uphill then that’s costing you. But maybe the subsequent downhill bit cancels out the cost – so long as the drag curve is fairly linear, which it should be for any small change.
Peter wrote:
But maybe the subsequent downhill bit cancels out the cost – so long as the drag curve is fairly linear, which it should be for any small change.
I don’t think you can measure it in fuel consumption, as there are much bigger influences and you have a fairly high margin or error, but the phugoid is a damped oscillation. Regaining what you invested would destroy entropy.
Isn’t that the same discussion as wind? Flying a given distance, then back again, a headwind will cost more on the outbound than the corresponding backwind will gain on the return.
Regaining what you invested would destroy entropy.
That’s why I think there is a net loss. But maybe it is too small to bother developing a more precise pitch control system.
Isn’t that the same discussion as wind? Flying a given distance, then back again, a headwind will cost more on the outbound than the corresponding backwind will gain on the return.
The result may be similar but the reason is probably different. But maybe the maths behind it is the same in the end. With wind, there is always a net loss because – so long as there is some wind – the average wind is a perfect crosswind, but unfortunately even with a perfect crosswind you end up flying a longer distance.
Peter wrote:
That’s why I think there is a net loss. But maybe it is too small to bother developing a more precise pitch control system.
I remember the original performance and flight profiles which existed before we had a crowded sky. That is times of the Comet I for instance and, later, but in a different scope, Concorde.
Cruise was flown then in a very different manner than today, namely it was based on a drift profile which had the airplane increase altitude gradually as it burned off fuel from an initial level to a final level in a continous cruise climb as far as I recall based on very precise pitch settings. This did optimise the fuel consumption.
I could well imagine that something similar could be true for our simple GA planes too, particularly for those which are performance optimized in their aerodynamics. However, that is not what today’s ATC requirements usually allow. In VFR it could be possible theoretically to still fly such a profile, but then again, the autopilots we have are not capable for that.
However: I don’t think this is what you had in mind Peter.
I would think there are two aspects of this. In Altitude Hold (manually or automatically flown) the airplane will oscilate slightly whenever the air is not totally calm and whenever it is not perfectly trimmed out. In our airplanes, we fly with constant power and fuel flow, so I would think these oscillations may have a small effect on TAS, but should be negligible all in all. If we had automatic thrust control, things might be different as the automatisation would try to regulate either thrust or IAS, thereby such oscillations would cause fluctuations in engine power setting as well. Each acceleration would cost in terms of fuel flow, each reduction would reduce the fuel flow temporarily. I would imagine that in such conditions, if they prevail over longer periods of time, a cost increase in total consumption would be noticable.
For me, the primary aspect of flying a proper cruise is to have the airplane trimmed out as perfectly as possible so as to minimize the oscialltions in the first place. While they will still happen in turbulent air more than in still air, they would minimize. What I am wondering in this regard is what autotrim will do in such conditions. Will it actually help or make things worse? Autotrim will trim the airplane as close as possible to perfect in calm air (as I’ve seen many times with the S-Tec 55X version) but it will work quite relentlessly in turbulence, so it will chase the altitude or VS selected. This could well mean a penalty in speed and therefore consumption if the trim excursions are extensive.
The gradual climb profile is still what most engineers would love to have and with more technology and possibly free route airspace it might become possible again. There is some research about this topic.
What you can do to get a more effective aircraft: Load the aircraft so that the elevator has no (or as low as possible within cog-limits) aerodynamic lift (in neither direction). The elevator is very inefficient in producing air forces and thus no lift equals lowest drag.
mh wrote:
What you can do to get a more effective aircraft: Load the aircraft so that the elevator has no (or as low as possible within cog-limits) aerodynamic lift (in neither direction).
I tried that with a case of wine in a C172, which I loaded in the aft part of the baggage compartment. It made for an “exciting” lift-off. Later, I found out that in the POH, they have two stations for the baggage compartment, but in the Excel sheet of my aeroclub which I used for W&B, there was only the main, forward part of the baggage compartment. The plane still flew fine after all, and I think we didn’t gain more than maybe 1 knot. Interesting lesson about verifying your information sources, though.
Yes, you have to be careful with that. But one knot just by distributing the load isn’t too bad, is it? Compare this to the cost per knot by flap gap seals, speedsloped windshields, gear doors and stuff like that :-)
Rwy20 wrote:
Interesting lesson about verifying your information sources, though.
And it shows that you should read all of the POH before flying…
