Tandem rotor question

[Shawn Adams]

Can someone answer this for me... I assume that a CH-47 Chinook has 2 swash plates. One for each rotor. Are both full / identical swashplates with cyclic and collective, or is forward motion achieved by more pitch on the rear rotor and less on the front, etc... without need for full cyclic control?

 

I have a guy that keeps asking and I can't give him a stright answer... He claims that there is no direct control to the rear rotor (which I have a hard time believing)..

 

Also.. I have heard of both. what is the difference from a SWASH plate to a SQUASH plate?

 

Thanks

[Gomer Pylot]

A squash plate is a plate full of squash. Nothing more. It has nothing at all to do with helicopters or aviation.

 

Chinooks maneuver by using differential cyclic and/or differential collective control, depending on the maneuver. See http://www.helicopterpage.com/html/tandem.html, which turned up in 0.17 seconds in a Google search.

[Guest pokey]

I have been in search of exactly how these 47's fly for years myself. 25 years ago i was building a R/C CH-47 (still have the prototype kicking around somewhere).

 

I talked to crew chiefs, mechanics, pilots, and even some factory Boeing ppls. The specific question i had was " do the swash plates tilt fore/aft"-----------I got varying degreees of "opinions".

 

We know that they rely mainly on differential collective for FWD flite, but the swash plates dont tilt FWD as in a conventional helicopter? They tilt sideways----of korse ! & that enables it in a hover to pivot around either front OR rear rotor.

 

Seems to me that if the swashplates could tilt fore/aft,,,, one would have more ability to control the "pitch" of the ship in high speed FWD flight,, & we ALL know these arent the "pokiest" of the helicopters in the air today.

 

Always been intreiged by these ships & in awe when i see on fly over

 

oh oh OH !!! and a "squash plate" is when an elephant steps on yer dinner

Edited November 21, 2006 by pokey

[Shawn Adams]

so.. Am I correct in assuming that a CH-47 has 2 swashplates that are only capable of left and right movement as well as collective? Forward / backwards flight is by more pitch on either the fron or aft rotor system?

 

Also. I am finding that there are some staements on the internet that refer to a Squash Plate (rather than Swash Plates). I've never heard of a SQUASH plate except through a few aquantences, but here is an article on a V-22 accident that mentions them (perhaps there is a wave of writers who have never clarified the correct term:

 

"This transition occurs automatically when the air speed is reduced below 160 knots to compensate for the lift loss from the reduced airflow over the V-22's fixed wing.

 

At 7:23:40 p.m. shortly after the nacelles began to transition from the airplane mode, a main hydraulic line ruptured that feeds the aircraft's left squash plate actuators.

 

When the flight control computers sensed the problem, they stopped the rotation of the nacelles. I'll show you a more detailed picture of the hydraulic system in just a moment."

 

Here's the link... http://www.defenselink.mil/transcripts/200...1_t405mv22.html

 

thanks

Edited November 22, 2006 by Shawn Adams

[Gomer Pylot]

The link you posted is a transcript someone did from listening to audio of a verbal briefing. The transcriber simply misheard what the briefer said, and didn't know any better. Thus misinformation gets spread around the world.

[Linc]

so.. Am I correct in assuming that a CH-47 has 2 swashplates that are only capable of left and right movement as well as collective? Forward / backwards flight is by more pitch on either the fron or aft rotor system?

The swashplates actually tilt in the fore and aft, or 45 degrees left of fore and aft. Phase lag / gyroscopic precession (whichever flavor you prefer) turns it into right and left tilt of the rotor discs. Forward flight is initiated with a combination of increasing the collective pitch on the aft rotor and decreasing the collective pitch on the front rotor. Likewise, rearward flight is initiated with the reverse actions.

 

Your initial question implies that the tandem rotor doesn't use cyclic pitch. I can only assume that you meant that it doesn't utilize cyclic pitch changes to induce fore and aft rotor tilt, which would be true.

[Shawn Adams]

The swashplates actually tilt in the fore and aft, or 45 degrees left of fore and aft. Phase lag / gyroscopic precession (whichever flavor you prefer) turns it into right and left tilt of the rotor discs. Forward flight is initiated with a combination of increasing the collective pitch on the aft rotor and decreasing the collective pitch on the front rotor. Likewise, rearward flight is initiated with the reverse actions.

 

Your initial question implies that the tandem rotor doesn't use cyclic pitch. I can only assume that you meant that it doesn't utilize cyclic pitch changes to induce fore and aft rotor tilt, which would be true.

exactly.. thanks.

[Linc]

The swashplates actually tilt in the fore and aft, or 45 degrees left of fore and aft. Phase lag / gyroscopic precession (whichever flavor you prefer) turns it into right and left tilt of the rotor discs. Forward flight is initiated with a combination of increasing the collective pitch on the aft rotor and decreasing the collective pitch on the front rotor. Likewise, rearward flight is initiated with the reverse actions.

 

Your initial question implies that the tandem rotor doesn't use cyclic pitch. I can only assume that you meant that it doesn't utilize cyclic pitch changes to induce fore and aft rotor tilt, which would be true.

Correction. Since one rotor turns clockwise and the other turns counterclockwise, the 45 degree offset for the tilt axis of the swashplate would actually be left OR right, depending on which rotor system. Apologies.

[Guest pokey]

Correction. Since one rotor turns clockwise and the other turns counterclockwise, the 45 degree offset for the tilt axis of the swashplate would actually be left OR right, depending on which rotor system. Apologies.

 

now i'm REALLY confused ! 45 degrees? is the geometry of the 47 rotor head such that the pitch horns are NOT at 90 degrees to the span of the blade (like my 300 & the 206) ? OR is the precession 45 degreees?

 

For the 206 & 300 ( & most ) helicopters, the tilt of the swash plate is the direction of flite of the ship, ( the piston Hiller being an exception- but its 180 degrees off) NOW 45 degrees? hmmmm OR would that be 135 degrees? PLUS ? add TWO rotors?! HOLY COW !!

[Linc]

now i'm REALLY confused ! 45 degrees? is the geometry of the 47 rotor head such that the pitch horns are NOT at 90 degrees to the span of the blade (like my 300 & the 206) ? OR is the precession 45 degreees?

 

For the 206 & 300 ( & most ) helicopters, the tilt of the swash plate is the direction of flite of the ship, ( the piston Hiller being an exception- but its 180 degrees off) NOW 45 degrees? hmmmm OR would that be 135 degrees? PLUS ? add TWO rotors?! HOLY COW !!

You mean, they should make every aircraft just like the ones you fly so you won't have to "lurn nuthin' new"?!?

 

Tilt of the swashplate in relation to the centerline of the aircraft is dependent upon when in the rotation the input to the P/C arm occurs. Anyways, it was an approximation. Maybe it is 60 degrees? I couldn't find it in the manual text and guessed off of the system diagram. It's certainly not inline with the centerline (resulting in left/right swashplate tilt). 60 degrees offset for the swashplate input sounds more probable on a three-bladed rotor system, since the P/C arm could at least be expected to be centered (30 degrees) in the angle between the blades. A 90-degree input would place the P/C rod at an extremely odd angle and may require much more "throw" than the swashplate tilt is capable of producing to effectively change the blade pitch.

 

"Most" helicopter designs without a teetering or two-bladed rotor system input their cyclic pitch changes at less than 90 degrees in the rotation from blade position and compensate by offsetting the swashplate inputs appropriately so that the blade changes take effect at the correct location in the rotation. My aircraft (OH-58D) has P/C arms that receive pitch change inputs at 45 degrees prior to the blade location (by my calibrated eye's calculation), and the inputs to the swashplate are at 45 degrees off from the centerline of the aircraft (fore / aft at 45 / 225 degrees, and left / right at 315 / 135 degrees, respectively). So, that's probably where my idea of 45 degrees came from. The blades would fly the same, so there's no worry for the pilot, it's just a mechanical accomodation for aerodynamics.

 

The Robinson R-22 has P/C arms that receive input at 72 degrees (18 degrees short of perpendicular) to account for aerodynamic effects on the rotor system. So, I guess that even the two-bladed systems don't always do it the way we may be familiar with.

Edited November 25, 2006 by Linc

[helisphere]

Yaw is acheived by differetial lateral cyclic.

 

Roll is acheived by lateral cyclic.

 

Pitching (By fore and aft movement of the cyclic control stick) is acheived by differential collective.

 

There is no fore and aft cyclic pitch apllied to either rotor by the main flight controls. There is however a need for a difference in the fore and aft cyclic between the two rotors as forward airspeed increases or decreases. This is acheived in the 46 by a manual switch controlled by the pilot called the speed trim. In the 47 this adjustment is done automatically and no pilot input is required for it.