rotor collision... what went wrong

[AlphaOneSix]

Blade flapping.

[Bucic]

Let's do you both fast:

You people argue as if deadzones or curvatures actually change how the aircraft flies in game.

 

It doesn't.

 

The analogies are endless ... someone sees you fly and asks "Wow, what HOTAS do you use?" much like as a photographer I invariably hear "Wow, what kind of camera is that?" after looking at nice photo I had just taken. So I'll borrow an age-old adage from photography, and slightly modify:

 

IT'S NOT THE HOTAS, IT'S THE PILOT!

 

(From "It's not the camera, it's the photographer!")

The notion was "Curvatures may help overcome some sim controllers limitations" - not "It's all about the curvatures and dead zones". So you've just chimed in to tell us that the notion which hasn't even been made is not true. Congrats.

 

 

 

Blade flapping.

The blade flapping allows it to happen. It's not the cause. Or are you saying that at higher altitudes the flapping behavior is different?

[BBQ]

I was specifically responding to JG14_Smil, which one would think, was obvious, since I quoted him in my initial response.

 

However, I didn't mean to high-jack the thread.

[Bushmanni]

Really? This would be a new one for me.

 

1. There is no reason to disregard the upper vs lower rotor opposite dissymetry of lift (manual, 4-2).

2. I think you've mistakenly took the airflow to be like this (manual, diagram 5-2) in high-speed forward flight. It's like (manual, diagram 4-16), and I guess it's even flatter at 250+ kph. I'm not saying the induced flow is negligible here. Just saying that the dissymetry of lift is the dominant phenomena responsible for the blade collision.

 

I've checked the manual just now and there seem to be no section dealing with blades intersection. The chapter 5 AERODYNAMIC FEATURES OF COAXIAL

CONFIGURATION HELICOPTERS only mentions possibility of blade to tail boom collision possibility for normal helicopters (p. 5-2).

 

 

As for the blades colliding more eagerly at high altitudes. I can't think of any specific reason why this would happen, right now. Wait... Does it really happen? At higher altitudes blades collide at lower IAS?!

 

Recommended read:

http://simhq.com/forum/ubbthreads.php/topics/2888992/Feature_Series_DCS_Black_Shark.html

 

 

PS. Is the provided BS manual still outdated compared to the one available at http://www.digitalcombatsimulator.com/

 

Dissymmetry of lift causes the rotor disks to pitch up (both of them), so it can't be responsible for the left-right tilt that happens at high speed. While I respect Erik's knowledge in general the reason he's proposing for blade collision in his article can't be right from physics standpoint and helicopter aerodynamics handbooks also disagree with him. Because of precession any tilting of the rotor disk is caused by a force roughly 90 degrees before the highest point of the rotor disk. As the lower rotor spins counter-clockwise and right side is tilting up, the force causing it must be acting at the aft section of the disk. There's no obvious candidates what could be the source of this mysterious lift but it's real world phenomena as a NASA article about co-axial rotor systems confirms.

 

An article by Mangler and Squire from fifties has a mathematical aerodynamic study of helicopter rotor induced flow patterns in different flight conditions. It uses some very rough simplifications as they didn't have supercomputers at that time for flow calculations, but it does give a hint of what might be happening. In the appendix of the article there's some visualizations of the flow fields in different flight conditions, and you can see that there's some upwards flow at the aft section of the flow pattern at high speed. While in single rotor choppers this isn't able to overcome translational lift that causes the retreating side to rise, in co-axial choppers that might not be the case.

 

I'm kind of speculating here as I haven't seen any studies of induced flow patterns in co-axial choppers but it's clear that blade collision can't be because of dissymmetry of lift. The induced flow pattern is currently the only thing I know of that could be causing it. It's also worth noting that blade collision indeed happens at both lower TAS and IAS at higher altitudes as induced flow increases.

 

I repost the links to the NASA article and the other article I'm referring to.

 

NASA article:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970015550_1997024330.pdf

 

The article by Mangler and Squire:

http://naca.central.cranfield.ac.uk/reports/arc/rm/2642.pdf

 

This one is also very usefull.

Introduction to helicopter aerodynamics:

http://www.cnatra.navy.mil/pubs/folder5/th57/p-401.pdf

[Bucic]

Dissymmetry of lift causes the rotor disks to pitch up (both of them), so it can't be responsible for the left-right tilt that happens at high speed. While I respect Erik's knowledge in general the reason he's proposing for blade collision in his article can't be right from physics standpoint and helicopter aerodynamics handbooks also disagree with him. Because of precession any tilting of the rotor disk is caused by a force roughly 90 degrees before the highest point of the rotor disk. As the lower rotor spins counter-clockwise and right side is tilting up, the force causing it must be acting at the aft section of the disk. There's no obvious candidates what could be the source of this mysterious lift but it's real world phenomena as a NASA article about co-axial rotor systems confirms.

 

An article by Mangler and Squire from fifties has a mathematical aerodynamic study of helicopter rotor induced flow patterns in different flight conditions. It uses some very rough simplifications as they didn't have supercomputers at that time for flow calculations, but it does give a hint of what might be happening. In the appendix of the article there's some visualizations of the flow fields in different flight conditions, and you can see that there's some upwards flow at the aft section of the flow pattern at high speed. While in single rotor choppers this isn't able to overcome translational lift that causes the retreating side to rise, in co-axial choppers that might not be the case.

 

I'm kind of speculating here as I haven't seen any studies of induced flow patterns in co-axial choppers but it's clear that blade collision can't be because of dissymmetry of lift. The induced flow pattern is currently the only thing I know of that could be causing it. It's also worth noting that blade collision indeed happens at both lower TAS and IAS at higher altitudes as induced flow increases.

 

I repost the links to the NASA article and the other article I'm referring to.

 

NASA article:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970015550_1997024330.pdf

 

The article by Mangler and Squire:

http://naca.central.cranfield.ac.uk/reports/arc/rm/2642.pdf

 

This one is also very usefull.

Introduction to helicopter aerodynamics:

http://www.cnatra.navy.mil/pubs/folder5/th57/p-401.pdf

 

Man, if this is true, I'll stand corrected and it will be a nice lesson of humility. A second one this week for me! I'll take a look at it later.

[BBQ]

Here is another excellent resource -- a real gem that I only found a month or so ago, having no idea that it existed. Very well written -- a whole article on the topic -- but you'll regret not reading the entire "series" of articles that are linked at the bottom!

[Mike Busutil]

: Thumbup:

: Thumbup: Edited July 7, 2013 by Mike Busutil

[BBQ]

Yup, that's the way it's done! That video inspires me every time I see it: pure, undiluted, 100% talent. What a show!

[AlphaOneSix]

A rotor blade on the advancing side flaps up so it generates less lift (reduced angle of relative wind ). On the retreating side, it will flap down to increase lift (increased angle of relative wind). So on a coaxial helicopter, this means that as airspeed increases, one rotor disc tilts more to the right, and the other tilts more to the left. One side get a smaller gap between discs, and the other side gets a larger gap.

 

At higher altitudes the angles of attack of the blades increase, resulting in the blades flapping higher on the advancing side and lower on the retreating side...given the same airspeed.

[MaverickF22]

As for the blades colliding more eagerly at high altitudes. I can't think of any specific reason why this would happen, right now. Wait... Does it really happen? At higher altitudes blades collide at lower IAS?!

 

 

I think you're too fixated on forces acting on the blades. Let me put the relations we discuss here this way:

the dangerous upper to lower blade tip distance <- opposition of upper rotor vs lower rotor dissymetry of lift <- dissymetry of lift phenomena

Ok..., i think i have the answer after thinking a bit, yet i didn't take the time to read what you've shared, but i'll read it too! Thanks!

 

At lower air density(higher altitude), now it doesn't matter if the helicopter moves in horizontal plane or not (regardless of IAS on blades and airframe), so let's say it's standing still..., as the angle of attack increases, so does the air rotation component driven by the upper rotor blades (due to higher drag created on them), will further increase the horizontal component of the air motion that takes effect on the lower rotor, increasing it's angle of attack way more than the higher rotor does.

 

So it's simple, as the blade's angles increase (geometrically from a reference plane) and evenly for both rotor blades, the upper rotor will have a linear increase in angle of attack, which will also give a linear increase in lift (which rushes the air down towards the next rotor with a linear increase in speed) until the curve before the critical angle of attack, and an exponential increase in drag (which exponentially increases the rotational component of the air on it's way to the second rotor), that gives an exponential increase in angle of attack for the second rotor.

 

This, from my point of view would be the greatest influence for the increasing difference in rotor's lifting force as the angle of attack increases.

 

 

A good day to everyone!:thumbup:

[Bushmanni]

http://www.dynamicflight.com/aerodynamics/flapping/

 

[seikdel]

http://www.dynamicflight.com/aerodynamics/flapping/

 

 

I... I just don't... understand gyroscopes... at all. This is a little much for my brain o.O

[justoc]

I... I just don't... understand gyroscopes... at all. This is a little much for my brain o.O

 

In essence, you can split a rotor into two sections; the advancing side and the retreating side. If you are going 200km/h, the speed of the advancing side will be 200km/h + the speed of the blade(because it is advancing with the aircraft), and it is the opposite with the retreating side (200km/h - speed of blade, because it travels in the opposite direction). If the sides do not equal the same speed, the helicopter will lift unevenly, so the blade will flap up on the retreating side to attempt to generate more lift on that side(engineers intentionally do this).

 

Also, because of gyroscopic precesion(basically any force you put on a rotor/gyroscope will take 90 degrees to have an opposite force take effect. If you tilt over a vase from the top, it will tip 180 degrees from the force you exert on it, its just 90 degrees in the motion of the rotor instead) more lift is generated on the advancing side, and since the force takes 90 degrees to take effect, the helicopter pitches backward. A sort of natural speed limit.

Edited July 10, 2013 by justo(c)

[AlphaOneSix]

so the blade will flap up on the retreating side to attempt to generate more lift on that side(engineers intentionally do this).

 

Good explanation, just this part is backwards. The advancing side flaps up to create less lift, and the retreating side flaps down to create more lift.

[justoc]

Good explanation, just this part is backwards. The advancing side flaps up to create less lift, and the retreating side flaps down to create more lift.

 

Ah, thanks for that clarification.