Optimizing power from rotors

[Hotel26]

10 hours ago, Geschosskopf said:

But I'll try 1 more time.

Me too, actually.

https://www.aopa.org/news-and-media/all-news/1998/november/flight-training-magazine/bernoulli-or-newton

[KerikBalm]

13 hours ago, Geschosskopf said:

Again, if it was all about the 3rd Law and thus reaction mass, you'd have AT LEAST the same downdraft underneath a wing as you do under a hovering helicopter.  More even.  If you're throwing air down to counter gravity, and air only weighs so much per unit volume, you have to either move the same amount at the same speed or less at a higher speed.  There's no getting around that of if you're relying on the 3rd Law.  As airplanes demonstrably do not blast away the air under them, they obviously do not rely on the 3rd Law for lift.

Please move this discussion here, where we can further discuss how wrong you are :p

 

 

[Phreakish]

19 hours ago, Geschosskopf said:

Again, if it was all about the 3rd Law and thus reaction mass, you'd have AT LEAST the same downdraft underneath a wing as you do under a hovering helicopter.  More even.  If you're throwing air down to counter gravity, and air only weighs so much per unit volume, you have to either move the same amount at the same speed or less at a higher speed. 

Air always has a downward velocity vector when leaving the trailing edge of the wing. 
I've physically stood under plenty of low flying aircraft and the downwash is definitely there. Helicopters *seem* to push more air because it's a constant downwash, not just a single pass of a blade - the downwash is spread out over the distance the aircraft covers. 

We also covered how propellers and rotors worked in University. Look up the Kutta effect, and even Nasa's explanation:
https://www.grc.nasa.gov/www/k-12/airplane/propth.html

Suffice to say, KerikBalm is correct. 

Edited June 24, 2019 by Phreakish

[Geschosskopf]

3 hours ago, Phreakish said:

Suffice to say, KerikBalm is correct. 

Um... nope.

But regardless of your misconceptions (or mine) on this issue. the fact remains that the KSP aero model, which is what creates lift and drag forces from moving wing parts, DOES NOT model the 3rd Law as it applies to using the air as reaction mass..  This is because the KSP atmosphere is NOT a fluid or even a substance, it's just a mathematical space wherein lift and drag forces are calculated solely due to the velocity and AoA of various craft parts. Hmm, that's just like how lift works in real life.  Fancy that 

So, as I've repeatedly said, the 3rd Law plays no part in how rotor-based props (or helicopter rotors) work in KSP.  Which is contrary to real life, where the 3rd Law dominates via the "paddling" mechanism.  If you can't understand the difference between a kayak paddle pushing flat-on to the water and a wing sliding edge-on through a fluid, I suggest you go rent a kayak and see for yourself.

[Phreakish]

14 hours ago, Geschosskopf said:

Um... nope.
...
 If you can't understand the difference between a kayak paddle pushing flat-on to the water and a wing sliding edge-on through a fluid, I suggest you go rent a kayak and see for yourself.

According to NASA, every aerodynamics TEXT BOOK I own, and years of personal professional experience in engineering. You're wrong. Kayaks have zero to do with wings or propellers. Propellers and rotors produce thrust the same exact way a wing produces lift. All the equations for calculating propeller forces are derived from the exact same equations used to determine lift. The air moves as a result of the lift created, not vice versa, and is the result of conservation of momentum (3rd law). 

There is no such thing as a 'paddling' mechanism in real lift. Ever. What you describe is a very old 'newtonian lift' principle that used to be taught to pilots because it simplified the mechanics behind the physics of flight. I learned the same flawed principle when becoming a pilot. Engineering school (and years of work in the field) have taught me just how wrong it is.

But please, keep telling people with professional scientific experience how wrong they are.

[Guest]

KSP isn't like RL when it comes to lift generation. It's an approximation, much like patched conics is an approximation of real celestial mechanics. The cracks show in edge cases and props/rotors are edge cases. (The link @Hotel26 posted explains the RL case well.)

Here's some more stuff I've learned building my recoverable Eve launch system, including a summary of stuff I may have posted here before -- and it's quite apparent that things aren't working as in real life, so here real understanding of aerodynamics might be a hindrance more than a help.

(1) Control surfaces are better for rotors than wing connectors. 

I surmise this is due to fudge factors SQUAD has added to make them work like they want them to work. I also surmise control surfaces are better as wings than wing connectors, except you can't build a wing out of them because they don't click together and look ugly.

(2) Propellers/rotors getter better as they get bigger, up to a certain point where there's no discernible difference.

I didn't see a noticeable difference in performance between a rotor that's all the way to the edges of the VAB and one that's well short of the edges of the VAB. However scaling them up from something you could land with while pointed forward to something that would fit on a wing when pointed up makes a big difference. Scientific, I know.

(3) Each rotor has an optimal lift/climb pitch, which depends on the celestial body you're on and does not vary much with altitude, except...

(a) if your rotor is so powerful that you climb speed requires you to adjust the pitch, for example when testing a variant of my Eve lifter on Kerbin it reached a vertical speed of over 50 m/s ASL, which meant it wanted a commensurately steep pitch -- around 50 degrees IIRC

(b) see "weird things" below

(4) If you want to go forward fast, variable pitch is a must. You will want 76-77 degrees or so to crack 100 m/s, whereas you'll want 30 degrees or so tops for a horizontal take-off, and if you're building a tilt-rotor, much less than that for hover.

(5) Weird things happen when the air gets thin.

At high altitudes, RPM/torque becomes more important. I built two variants of my Eve lifter, one powered with six rotors mounted on three heavy motors sitting on freewheels, and one with four rotors mounted on four heavy rotors. The former had much better climb performance through most of the atmosphere, averaging about 14 m/s after it was out of the really soupy lower atmosphere; the latter was chugging along at around 9-10. However, the former has a practical ceiling of about 27 km, whereas the latter kept going over 30 km. These had different ideal pitches for maximum climb also: the former's was 7 degrees, the latter's, 5.5. 

(6) Even weirder things happen when the air gets very thin.

My four-rotor version kept going past 30 km, and not far above that altitude, its climb rate started accelerating. Exponentially. At that altitude it was normally climbing at around 4 m/s, which accelerated to 10, 15, 30, 50, 75... at which point I aborted the experiment because I wanted to launch my payload. I know from other experiments that the rotor will spin faster and faster until it's no longer possible to stop it even by setting blade pitch to 90 and disengaging power, while braking would blow up the craft. This is the Rotor Kraken pulling you into the sky with His noodly tentacle.

Summary: if you want the most powerful rotor you can get, use the heaviest control surface your rotor can effectively spin and the biggest diameter you can fit onto your craft. KSP as it currently stands is not kind to regular propeller planes with normal-sized fixed forward-facing props, because they will be much less efficient at that size than you would want. So tilt-rotors and helicopters are the way to go at this point -- unless you want to ditch the standard parts and sculpt larger-than-kerbal-scale planes with commensurately bigger propellers.

Edited June 26, 2019 by Guest

[nhnifong]

Completely aside from how they work in theory or in real life, how do you keep a multi-part prop blade in KSP from stretching ridiculous amounts at the joints?

Edited July 10, 2019 by nhnifong