One of the rotors can hit the other one?

[Stratos]

Was reading the manual yesterday and in the technical part, they talk about how wonderful and how superior the coaxial configuration is over the normal helicopters. But I have one question for the experts, is not possible that one of the rotors hit the other during High-G maneuvers, strong winds or other similar situations?? Is a thing that can happen? And is modelled in the sim?

 

Thanks.

[Royale]

Yes, it can and will happen. Just kick in full right rudder while traveling near max speed.

[159th_Viper]

Yes, it can and will happen. Just kick in full right rudder while traveling near max speed.

 

Not entirely correct iro rudder-input :)

 

You can firewall left and right rudder all the time at 300+, provided that you are trimmed correctly and also dependant on collective input. It's how I fly: Flying it like I Stole it - always standing on the rudders at any speed and suffering no rotor disc intersection.

[Suchacz]

It is caused by dissymetry of lift on a retreating and advancing rotor blade.

 

Look at this great article at SimHQ! http://www.simhq.com/_air13/air_427a.html

[effte]

It happens, at high speed. Coning plus airspeed plus precession causes a rotor disc to tilt to one side with increasing airspeed. As the direction of tilt depends on direction of rotation, this effectively becomes an airrspeed limitation for coax systems. Try flying with the rotors at low RPM (i e more coning) if you want to see it in action.

[effte]

It is caused by dissymetry of lift on a retreating and advancing rotor blade.

 

Look at this great article at SimHQ! http://www.simhq.com/_air13/air_427a.html

 

That causes both rotors to tilt in the same direction (aft with increasing airspeed) so it is less of an issue as far as rotor interference goes.

[Bucic]

Is a thing that can happen? And is modelled in the sim?

 

Thanks.

If you ask that question you haven't done any flying, have you? ;) For me it was one of the first things... I was introduced to, let's call it this way. Then there was the vortex ring state :)

 

Flying it like I Stole it

Nice way to put it :) And perfectly legitimate.

 

Few tips:

1. high risk of intersection situations => reduce collective!
   
2. right rudder increases the risk of intersection
   
3. the 300+ kph airspeed for the high risk of intersection may become 290 kph during turbulent winds and/or jerky handling of the control stick / poor joystick
    
    
    
   

See what's possible:

 

And don't forget about

Rookie pilots - Quick Intro

[Xxx]

http://www.simhq.com/_air13/air_427a.html

 

This is a great article. Nice find.

 

If you use large rudder deflections and cyclic inputs simultaneously, without lowering the collective, the chance of Rotor Disc collision is greatly increased. I now know why I have had to eject so many times when getting over excited during SAM/AAA evasion:D I kept thinking that I had been hit.....nope...my darned rotor cones had intersected! Well, naturally I told the CO it was AAA that hit the blades..:music_whistling:

 

I fly like I own it! ( and have to do the maintenance;) ) That way I dont break it so often!

 

This is another heck of a great sim! No skill, no fly!:smartass:

 

Cheers.

[johnnychemo]

Loved the flying in the youtube videos. Any links for info on how to do that in the Shark?

[Slayer]

Also when you are in the sim just pan to the left there is a nice chart that shows max IAS based on weight. If you are exceeding these there is a good chance of rotor clash.

[Agg]

This is one of the biggest reasons why Black Shark has been collecting dust on my shelf since the release of Warthog :P

 

I hated always being nervous about the blades hitting each other, you have to be a bit careful with the Hog too, but not in the same way.

[Xxx]

Loved the flying in the youtube videos. Any links for info on how to do that in the Shark?

 

The skill demonstrated is very great! I think only a lot of practice is the answer.

 

Would be great if there was a voice over, Wags instructor style, during the manouvers. If you watch, the pilot is using a lot of left boot for much of the time. The loops and rolls are special. My rotors usually collide, which is probably due to incorrect collective movement during inputs....:(

 

Shows what you may achieve with this ship if you know the techniques.

 

Yes, good point re the weights. As we are usually at a high weight with a combat load. Also there is the crosswind factor! Add to that some nut is trying to kill you with SAM/AAA and you got a heck of a sim!:joystick:

[winz]

The main rules for such manouvers are - be smooth, be gentle, and drop your collective.

[effte]

http://www.simhq.com/_air13/air_427a.html

 

This is a great article. Nice find.

 

Unfortunately, it is also incorrect. As I hinted at before, the rotor is essentially a gyro. A gyro has precession. In other words, if you apply a force on one side, the effect will be as if the force was applied 90 degrees later in the direction of rotation on a solid disc.

 

More lift on the advancing side of the rotor means the forward edge of the rotor tilts up - the rotor tilts aft and not to the retreating side as the unsuspecting layman might assume. Disymmetry of lift means both rotors tilt aft. I'll have a go at a simple explanation:

 

Forward airspeed means there is more lift, as compared to the average over a full rotation, on the blade all the way from where it is pointing directly aft to the point where it is pointing straight forward. This means the blade will flap up all the way from directly aft to straight forward, reaching its highest point when it is pointing straight forward.

 

Coning combined with forward airspeed on the other hand means the blade has an increased angle of attack in the forward half of the rotation, meaning it generates more lift and flaps up to reach a highest point when pointed straight to the retreating side - the rotor tilts to the advancing side.

 

 

(In the real world our wonderful theoretical 90 degrees will typically not be exactly 90 degrees, but we can safely leave that aside for now. I'm also thinking rotor clashing could be remedied, at least to a degree, by reducing blade pitch during the forward portion of the rotation for both rotors - i e effectively giving the rotors left/right cyclic respectively. I wonder if that's done to any degree. There's probably a complication I'm forgetting - there always seems to be when dealing with the whirlybirds. :))

[johnnychemo]

The skill demonstrated is very great! I think only a lot of practice is the answer.

 

 

Practice for sure, but some guidance in terms of how to execute the maneuvers would be helpful, it would tell me WHAT to practice!

[159th_Viper]

Practice for sure, but some guidance in terms of how to execute the maneuvers would be helpful, it would tell me WHAT to practice!

 

Which maneuvers specifically?

[Mustur]

Should of seen some of my first flights, I was wondering if it was possible not to collide the rotors. :lol:

 

Then I started paying close attention to the rotor RPM gauge. It seemed that whenever I started to pull some Gs, rotor RPM would increase rapidly.

So instead of lowering the collective I started raising it, thus keeping rotor RPM under control. Then when leveling off I lower the collective. That and not having a jet fighter mindset also helped.

 

I know this might seem obvious, but to me it was all new stuff. Never flown a helicopter sim before so it was a nice learning experience.

 

Edit: Oh and trimming, that took a while too.

Edited December 18, 2011 by Mustur

[AlphaOneSix]

Unfortunately, it is also incorrect. As I hinted at before, the rotor is essentially a gyro. A gyro has precession. In other words, if you apply a force on one side, the effect will be as if the force was applied 90 degrees later in the direction of rotation on a solid disc.

 

More lift on the advancing side of the rotor means the forward edge of the rotor tilts up - the rotor tilts aft and not to the retreating side as the unsuspecting layman might assume. Disymmetry of lift means both rotors tilt aft. I'll have a go at a simple explanation:

 

Forward airspeed means there is more lift, as compared to the average over a full rotation, on the blade all the way from where it is pointing directly aft to the point where it is pointing straight forward. This means the blade will flap up all the way from directly aft to straight forward, reaching its highest point when it is pointing straight forward.

 

Coning combined with forward airspeed on the other hand means the blade has an increased angle of attack in the forward half of the rotation, meaning it generates more lift and flaps up to reach a highest point when pointed straight to the retreating side - the rotor tilts to the advancing side.

 

 

(In the real world our wonderful theoretical 90 degrees will typically not be exactly 90 degrees, but we can safely leave that aside for now. I'm also thinking rotor clashing could be remedied, at least to a degree, by reducing blade pitch during the forward portion of the rotation for both rotors - i e effectively giving the rotors left/right cyclic respectively. I wonder if that's done to any degree. There's probably a complication I'm forgetting - there always seems to be when dealing with the whirlybirds. :))

 

Totally wrong! Blade intersection is caused by blade flapping as a result of dissymmetry of lift!

[effte]

Totally wrong! Blade intersection is caused by blade flapping as a result of dissymmetry of lift!

 

Sorry mate, blade flapping it is but the flapping due to disymmetry of lift is not the main culprit here. Time to go review your notes and/or literature, methinks. If that doesn't do it, read what I posted above. That's physics, and arguing with physics is setting yourself up for a fall. Newton has a habit of smiting those who attempt to oppose Him. If still in doubt, point out the bits which you think are in error and we'll try to sort it out.

 

Unfortunately you wouldn't benefit much from the reference books I have at hand. First, you wouldn't be able to get hold of them and second, I think the language would present you with some difficulties. Third, they are in a box somewhere and I really don't feel like going on an archeological expedition in the basement trying to find them. :)

 

While I'm normally hesitant to offer the FAA series of handbooks as a reference as they have a history of being riddled with implicit simplifications, peculiar definitions unique to the FAA and outright errors, they are easily available and printed in a common language - significant benefits which are hard to ignore. You could have a gander at the FAA Rotorcraft Flying Handbook. More specifically pg 3-6 and the section "Disymmetry of Lift" (and stop by pg 3-4 and gyroscopic precession on your way there).

 

As shown in figure 3-15, as the rotor blade reaches the advancing side of the rotor disc (A), it reaches its maximum upflap velocity.

...

During aerodynamic flapping of the rotor blades as they compensate for dissymmetry of lift, the advancing blade achieves maximum upflapping displacement over the nose and maximum downflapping displacement over the tail.

 

(My boldface.)

 

IOW, disymmetry of lift causes both rotors to tilt aft, as I already described above. Not nearly as bad as one rotor tilting left and the other right, as I also described.

 

You can also consider the case of cyclic inputs. Which side of the swash plate goes down (increased AoA/lift, assuming trailing blade edge linkage) if you give forward cyclic? Which way does the rotor tilt?

 

Googling is always interesting. I found this page, which describes it nicely, at least judging from a quick read-through.

 

The total result of this action is a rotor tilt to the rear which is completely independant of any additional cyclic stick action and which causes an angular separation between the control axis and the thrust axis of the rotor.

 

There is yet another periodic force with a phase-displacement angular separation of 90 degrees. This one arises from periodic longitudinal forces which result from rotor coning while the helicopter is in directional flight and causes the rotor to tilt to the side.

 

(My boldface, first paragraph is on disymmetry of lift due to retreating/advancing blade.)

 

Still not satisfied? IIRC, you're working in a helo operation? That leaves you with the final option of calling your OEM tech rep and making sure to get an engineer from flight dynamics on the line.

 

Cheers,

Fred

[wickedpenguin]

For the new pilots, mastering this bird is such a rewarding experience.

 

One of my favorite maneuvers is I guess what you'd call a fast stop. I'll be screaming along at max speed, then dump the collective (no rotor strikes, please!) and haul the nose back until near vertical. Essentially I'm air braking using the wings' and airframe's drag. As the speed falls to zero I lower the nose and pull in some collective to keep from sinking. After a little practice I could do it without losing or gaining altitude.

 

I know it's nothing like Frazer's videos, but hey, I'm having fun and I'm in control of the helo. :)

 

One other practice technique I use is hover taxiing around the airport taxiways at about two feet off the ground, with the landing gear raised. I'll go inside hangars, maneuver around parked vehicles, etc. It really hones my feel for the aircraft.

[EtherealN]

I hated always being nervous about the blades hitting each other, you have to be a bit careful with the Hog too, but not in the same way.

 

No reason to be nervous about it, just learn good flying habits. It's not more complex than learning to not cross rudders at low speed in a fixed-wing. (Unless you are using it to adjust your descent rate, but you'll want to really know what you're doing in that case.) :P

[Krebs20]

No reason to be nervous about it, just learn good flying habits. It's not more complex than learning to not cross rudders at low speed in a fixed-wing. (Unless you are using it to adjust your descent rate, but you'll want to really know what you're doing in that case.) :P

 

Lol lesson one: how to perform a forward slip to land.

 

Lesson two: Never perform a forward slip to land with passengers. It freaks them out.

 

 

 

That's about a word for word what my instructor taught me. Funny shit.

[Stratos]

Amazing all the info you added guys, I'm now browsing trought it. Thanks a lot.