1. Reduction of induced drag - is it caused by the tip vortex axis deflection? Where can I read more about it? Some old NACA papers?
Tip vortex axis deflection? NACA Papers? You are examining the problem with a microscope, when all you need is a picture window.
There is no exotic or hard-to-understand subtle effect. Even in dead calm, for a loop, say, the Cl is lower (and thus generates less drag) at the top of the loop than the bottom. You have to fight 1 G of gravity at the bottom, you have to fight only .71G of gravity at the top. Thats why you can't hold your hand or constant control pressure going around a loop. That effect is greatly exaggerated when the wind is blowing, you have the wind unloading the wing at the top of the loops, and nothing at the bottom. If your engine put out constant power, that alone would greatly accelerate it.
Unload the airplane (require less lift) and the induced drag goes down. Reduce the induced drag, the airplane speeds up, to a degree determined by how the thrust from the prop reacts to the increased airspeed. Hint- the thrust goes down, but the engine takes a while to react, so the airplane accelerates.
2. What is the "dynamic soaring effect?". Again, perhaps some NACA publications deal with this issue.
Again, look at how the wind effects the airplane as you travel around a loop. On the way up, the airplane gets increasingly nosed up into the wind, slowing it down, on the way down, it gets pushed down, with a component down the tail, that accelerates it.
Additionally, there is almost always a lot of shear in the wind, where the velocity at low altitudes is lower than at high altitudes (like the top of the loops) That means as you go around, it tends to speed you up more than it slows you down. By the way, you can just type "dynamic soaring" into Google, the first link is this:
https://en.wikipedia.org/wiki/Dynamic_soaring Where is shows a lopsided version of loops where velocity is gained flying in and out of shear. This is the "head on" version, you have to do a little more work to put it into our rotating coordinate system, but otherwise, you are counting on the wind increasing your groundspeed at the top and counting on it doing nearly nothing at the bottom, that is the effect. It's not nearly as strong as the typical glider examples because you are counting on shear just from viscous effects and low-altitude turbulence.
"The prop and engine, correctly set up, inhibit it to varying degrees". How does this work?
Again, this is the most fundamental function of successful stunt engines. When the airspeed increases, that reduces the load on the engine, and when the airspeed decreases, that increases the load on the engine. The prop is harder to turn at low speeds than high speeds - I wrote a very long discussion of the effect here:
https://stunthanger.com/smf/engineering-board/effect-of-wind-on-maneuvers/msg568436/#msg568436 Any time it is generating more thrust, it is also generating more drag, which for a propellor, means the torque required to turn it at a particular speed is higher. That is "loading" the engine, the converse "unloads" the engine. This is *absolutely critical* for any stunt engine to work. Increase the load suddenly, and your engine had darn well better increase its output, because otherwise, you are just going to slow down.
This is the cause of the 4-2 break in vintage engine systems. It's running along in level flight with a mixture that is excessively rich, to the point it misfires on alternate cycles (4-cycles). Put a load on it, and that slows the rpm enough that it has enough extra heat to fire even the rich mixture every time - breaking into a 2-cycle. Conventional engine systems, the reduced RPM puts it closer to the tuned peak of the pipe, increasing the cylinder packing and increasing the torque, inhibiting it slowing down more.
Point being, these are the *very basic fundamentals* that are inherent to *all* stunt systems.
I don't want to pick on you, but please - stop worrying about minor trivial aerodynamic subtleties until you have fully understood the big picture, gross characteristics. You don't need to concern yourself with what angle the tip vortices are deflected or Prandtl tunnels to understand these, or NACA papers, if you haven't already fully understood the far more obvious larger effects. To first approximation, you don't even need to know the mechanism of any of these effects to use or be aware of the effects.
There should be plenty of competent stunt fliers in Poland that can explain it do you, if they are willing. That is generally much more effective than having me try to type out lengthy word explanations, although I am willing to do it. I certainly *did not* figure any of this stuff out originally - other people over decades had noticed the effect and come up with workaround or solutions, mostly without understanding the reasons why it happened, other than the most facile way.
My minimal contributions to this body of knowledge are more about the mechanisms behind it, but that is not at all necessary for knowing what to do in a practical state.
Excellence is a journey, not a destination. And you can't understand the end without first experiencing the beginning.
Brett
Topic: Large propellers and Stunt Flying (Read 3111 times)
