A CL of 0.4 would be a C172 flying at 100 kts, which is not exactly best L/D.
And I’m not claiming any particular aircraft has or had this feature, I just wanted to find a possible physical explanation.
Peter if you check the chart you will see the steep nearly exponential rise is lift dependant or induced drag, it just happens to be on the non lift dependant side of the typical drag curves found in the ATP factory mill textbooks.
This drag bucket needs some further background, will check it out in Introduction to Flight my usual go to source. Le Sving if you have more background on the phenomenon please post it.
This Nasa document may provide some further pointers, in particular linking the notion to Laminar Flow wings.
The supposed effect is better explained by a lift drag polar with hysteresis – one that has a different shape on the way up to on the way down. Another way of saying it is that the shape of the airflow has some kind of memory and is uniquely determined not just by the power setting now, but also by what the airflow looked like in the past. That’s way outside the regular simple aerodynamics.
That way you can also postulate an airflow pattern you can only obtain by diving to altitude and not by accelerating in level flight with excess power.
That way you can also postulate an airflow pattern you can only obtain by diving to altitude and not by accelerating in level flight with excess power.
No. The flightpath relative to the ground has no relevance to the aerodynamics. Only the AoA is relevant, and that is still positive in a decent.
We are talking here about the possibility of temporarily needing excess power, beyond that shown on a conventional power-required curve, to achieve a given cruise equilibrium. At least, that is how I understand the originally posted question.
Aside from the scenario where you need to trade excess altitude (potential energy) for airspeed, because you are planning to cruise at full power, which is a red herring as nobody does this in the real world, altitude changes are not relevant to the debate.
I still don’t get this, since all the functions describing flight in the cruise region context discussed here are monotonic
Peter, OK, but can we at least agree that the parabolic curve you are talking about is not the “whole truth” in the real world? It is a function only of the zero lift drag coefficient, and the induced drag coefficient. And it assumes that those coefficents are constant. For a start, the coefficient of drag varies with true airspeed (Reynolds number), so it’s not really a constant at all.
Whether the differences between a real world curve and a simple parasitic+induced drag curve are signficant enough to explain the effects observed is still up for discussion of course. But there is physics at play outside of your monotonic curve.
David/Jarvis thanks for the reference/comment. I may be garbling the reason but the Russian use of circulation theory (Kutta Zhuvosky) was premised on the fact that the maths worked even if the physics might not be explainable. Also the fact that the Navier Stokes equations are incomplete (there is a $1million prize to complete them?) I use to explain being a rocket scientist is easier than the world of fluid dynamics.
And in any case the curve for a laminar flow wing (for which some kind of step is possible in laboratory/wind tunnel conditions) is NOT monotonic (it has the drag “bucket”)….but as the NACA paper explained, even laminar flow wings do not achieve laminar flow back to 50% of the chord per their design due to surface imperfections (dirt, dings etc.)….So again, we are back to the point made at the outset of this thread: in the real world (flying a real airplane in normal operation) there is no step effect.
No. The flightpath relative to the ground has no relevance to the aerodynamics. Only the AoA is relevant, and that is still positive in a decent.
We are talking here about the possibility of temporarily needing excess power, beyond that shown on a conventional power-required curve, to achieve a given cruise equilibrium. At least, that is how I understand the originally posted question.
Aside from the scenario where you need to trade excess altitude (potential energy) for airspeed, because you are planning to cruise at full power, which is a red herring as nobody does this in the real world, altitude changes are not relevant to the debate.
It depends on what you understand by “using excess power”.
If there is hysteresis in the lift/drag curve, that is, the lift to drag ratio is different at a particular value of drag depending on the direction from which the equilibrium drag point is approached (AoA increasing vs AoA decreasing).
In a descent you momentarily reduce the AoA below the cruise value and approach equilibrium AoA from below.
You could demonstrate this same effect using only excess power, but you would have to use the excess power to generate airspeed beyond the cruise speed, and then throttle back to get “on to the step”. I believe this is the first time in this thread that has been suggested. But maybe I misunderstood or misremembered what others intended.
I don’t believe in this kind of aerodynamics – it certainly didn’t feature in any postgraduate fluid mechanics courses I took – but we recognize “new” physics first by saying what they would predict if they did exist even while stating our disbelief.
Jarvis, I understand what you are saying now, but ultimately a descent and adding excess power are aerodynamically equivalent, no? They both provide an extra force in the thrust vector direction. One is gravity, the other is engine thrust. But otherwise, aerodynamically equivalent.
I’m not aware of the hysteresis effect you describe but that may indeed be another secondary aerodynamic effect at play here.
But I think we are getting too hung up on low level theory of airfoil performance when instead we should be focusing on the real world effects on the overall airframe.
I have put forward a few such effects already (P-factor, trim drag, viscocity), but there are more. Hysterises may be one of them.
To give another example of a secondary effect, normally angle of incidence is equal to cruise AoA such that fuselage pitch angle at cruise is zero or “straight into” the airflow. If you got stuck at a slower airspeed with a slightly higher AoA, the fuselage pitch angle or “fuselage AoA” goes from zero to a positive number. That’s a lot of drag which could sap a lot of your excess power.
