# Helicopter Question **URL:** \ **Category:** Factual Questions\ **Created:** [April 3, 2006, 4:45pm UTC](https://boards.straightdope.com/t/helicopter-question/351097 "2006-04-03T16:45:01Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![rippingtons\_fan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rippingtons_fan/32/11767_2.png) [@rippingtons\_fan](https://boards.straightdope.com/u/rippingtons_fan)\ **Post date:** [April 3, 2006, 4:45pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/1 "2006-04-03T16:45:01Z")
Do the tail rotors on the AH-64 Apache contra-rotate? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [April 3, 2006, 5:16pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/2 "2006-04-03T17:16:07Z")
[No](http://www.b-domke.de/AviationImages/Rotorhead/1079.html). ---
**Author:** ![dmatsch](https://avatars.discourse-cdn.com/v4/letter/d/3be4f8/32.png) [@dmatsch](https://boards.straightdope.com/u/dmatsch)\ **Post date:** [April 3, 2006, 8:17pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/3 "2006-04-03T20:17:12Z")
> [@Johnny L.A.](#): > > [No](http://www.b-domke.de/AviationImages/Rotorhead/1079.html). I’m going to have to disagree with you here. They, in fact, do. [lookie here](http://www.aircraftresourcecenter.com/AWA1/001-100/walk072_Apache/walk072.htm) [QUOTE] The AH-64 was one of the first helicopters to use counter-rotating tail rotors, meaning that two of the blades rotate clockwise, while the other two rotate counter-clockwise. [/QUOTE. It would be counter-productive for them to rotate in the same direction. If they did, it would set up oscillations in the tail of the aircraft that would make it pretty much unflyable. This is the same reason that multi-bladed (tail rotor-less) helos have the main blades rotating in opposite directions. To counteract tourque caused by single-bladed helos. Sorry, Johnny. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [April 3, 2006, 8:28pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/4 "2006-04-03T20:28:54Z")
> [@dmatsch](#): > > I’m going to have to disagree with you here. They, in fact, do. I’ve only flown civilian helicopters, so I’ll retract my statement if I’m wrong. But look at the photo. The inner set of blades pass through a solid structure and the linkages to the outer set. Also note the blades themselves. It’s hard to see in the view, but the angled bits on the blade roots are not symmetrical. The angle on the trailing edges are longer than on the leading edges. You can see on the two blades on the left (one inner, one outer) that the leading edges are on the same side. [Here’s another view](http://www.b-domke.de/AviationImages/Apache/5323.html) that shows that the blades move in the same direction. ---
**Author:** ![Duke\_of\_Rat](https://avatars.discourse-cdn.com/v4/letter/d/ad7895/32.png) [@Duke\_of\_Rat](https://boards.straightdope.com/u/Duke_of_Rat)\ **Post date:** [April 3, 2006, 8:54pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/5 "2006-04-03T20:54:06Z")
I’d say no. The page **dmatsch** linked to, the authors said they weren’t really familiar with the craft and wanted some expert opinions. But looking at a close up of the AH-64 tail rotor assembly, the leading and trailing edges of all 4 blades are positioned to spin the same direction. This indicates that they are not spinning in different directions. Here’s a good [pic](http://members.aol.com/ah64info/airframe.htm) (scroll down to tail rotor) ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [April 3, 2006, 8:59pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/6 "2006-04-03T20:59:36Z")
> [@dmatsch](#): > > It would be counter-productive for them to rotate in the same direction. If they did, it would set up oscillations in the tail of the aircraft that would make it pretty much unflyable. I’m not sure what you’re saying here. It would not be counterproductive for anti-torque rotors to rotate in the same direction. [Here](http://www.b-domke.de/AviationImages/Rotorhead/3416.html) is a photo of a UH-60 anti-torque rotor. Four blades, but all in the same plane instead of on two teeter hinges. ---
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [April 3, 2006, 10:54pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/7 "2006-04-03T22:54:33Z")
They do spin in the same direction. Having the blade pairs cross at a 55 degree angle reduces noise from the blades. ---
**Author:** ![igloorex](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@igloorex](https://boards.straightdope.com/u/igloorex)\ **Post date:** [April 4, 2006, 5:54am UTC](https://boards.straightdope.com/t/helicopter-question/351097/8 "2006-04-04T05:54:53Z")
> [@dmatsch](#): > > It would be counter-productive for them to rotate in the same direction. If they did, it would set up oscillations in the tail of the aircraft that would make it pretty much unflyable. This is not accurate. A great many helicopters (including the H-60, in which I have quite a bit of flight time) have tail rotors where all the blades go in the same direction – in fact I can’t think of a helo with counter rotating tail blades. You are correct that vibrations are a problem, but that’s solved by strong materials, careful vibration measurement, and balancing of the rotor. Also, while all dual hub (ie having two main rotors, like the H-46 or H-47) helos have blades that rotate in opposite directions, most or all western single hub helos have all the blades going the same way, including the 4-bladed H-60, the 5-bladed H-3 and the 7-bladed H-53. The soviets built a few interesting helos with counter rotating blades on a single hub, such as the Helix and Hormone. The Kaman [KMAX](http://www.kamanaero.com/helicopters/kmax.html) has two main rotors on different axes that intermesh and spin in opposite directions. And, to address the OP, the picture linked by **Duke of Rat** clearly shows the leading edge of all 4 apache tail rotor blades facing the same direction, so they all clearly rotate in the same direction. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [April 4, 2006, 12:13pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/9 "2006-04-04T12:13:32Z")
There would be no advantage to having the tail rotor blades counterrotate - they all have to produce thrust in the same direction, and that thrust balances (and is balanced by) the torque from the single-rotation main rotor. The advantage of having 2 counterrotating mains is that you don’t need a tail rotor, and all of its weight and complexity and energy consumption, at all. But counterrotating rotors in any location are mechanically complex (i.e. less reliable) and heavy. ---
**Author:** ![Loach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/loach/32/350_2.png) [@Loach](https://boards.straightdope.com/u/Loach)\ **Post date:** [April 4, 2006, 12:46pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/10 "2006-04-04T12:46:27Z")
I was crew on scout helicopters and I spent a good bit of time in an Apache attack helicopter battalion (1-227th AVN, Ft Hood Tx). The blades of the tail rotor all spin in the same direction. ---
**Author:** ![dmatsch](https://avatars.discourse-cdn.com/v4/letter/d/3be4f8/32.png) [@dmatsch](https://boards.straightdope.com/u/dmatsch)\ **Post date:** [April 4, 2006, 2:45pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/11 "2006-04-04T14:45:27Z")
I stand corrected in the face of expert opinions ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 4, 2006, 3:31pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/12 "2006-04-04T15:31:34Z")
> [@Johnny L.A.](#): > > [http://www.b-domke.de/AviationImages/Rotorhead/1079.html](http://www.b-domke.de/AviationImages/Rotorhead/1079.html). That’s amazing. What do all those linkages do? Do many helicopters have this much control stuff on the tail rotor? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [April 4, 2006, 3:58pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/13 "2006-04-04T15:58:34Z")
> [@scr4](#): > > That’s amazing. What do all those linkages do? Do many helicopters have this much control stuff on the tail rotor? That one is a bit complicated because it has two teeter hinges. My rides were (I’m not current, but I’m preparing to become so) the Robinson R-22 and the Schweizer 300CB, which had two-bladed anti-torque rotors. The Apache’s system pretty much looks like two of the Robbo’s or Schweizer’s system. The linkages change the pitch of the blades. You’ll notice that they are connected to a round bit at the bottom. It goes up and down on the shaft (horizontally, in relation to the aircraft) and changes the pitch of the blades through the pitch-change links. The teeter hinge allows the two blades attached to it to ‘flap’ as a unit (as opposed to fully-articulated rotor systems, which have flapping hinges on each blade) to compensate for aerodynamic loads. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [April 4, 2006, 4:14pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/14 "2006-04-04T16:14:34Z")
Oh – If you’re wondering why rotors need to flap: [Dissymmetry of lift](http://www.dynamicflight.com/aerodynamics/dissymmetry/) [Blade flapping](http://www.dynamicflight.com/aerodynamics/flapping/) ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 4, 2006, 10:19pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/15 "2006-04-04T22:19:56Z")
Thanks. I understand the need for flap on the main rotor, but what exactly happens if you don’t flap the tail rotor? There’ll be a torque around the roll axis, but couldn’t it be counteracted by flapping the main rotor? Is it just to reduce stress on the tail rotor boom? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [April 4, 2006, 11:37pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/16 "2006-04-04T23:37:29Z")
> [@scr4](#): > > Thanks. I understand the need for flap on the main rotor, but what exactly happens if you don’t flap the tail rotor? There’ll be a torque around the roll axis, but couldn’t it be counteracted by flapping the main rotor? Is it just to reduce stress on the tail rotor boom? The tail rotor must flap for the same reason the main rotor does: disymmetry of lift. The anti-torque rotor (tail rotor) counters the torque of the engine. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 4, 2006, 11:44pm UTC](https://boards.straightdope.com/t/helicopter-question/351097/17 "2006-04-04T23:44:41Z")
What I meant was, couldn’t the main rotor flap compensate for the dissymmetry of “lift” on the tail rotor? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [April 5, 2006, 3:56am UTC](https://boards.straightdope.com/t/helicopter-question/351097/18 "2006-04-05T03:56:39Z")
> [@scr4](#): > > What I meant was, couldn’t the main rotor flap compensate for the dissymmetry of “lift” on the tail rotor? Dissymmetry of lift is between the blades in one rotor system. The tail rotor is there to counteract torque (and for yawing, of course, since it can produce more thrust in either direction than there is torque to counter). As you can see by the links, the retreating blade has a lower airspeed in directional flight than the advancing blade. Flapping and feathering (and rigging) counteract this dissymmetry so that the lift on either side of a helicopter is more or less the same. But you still have to look out for retreating blade stall. Retreating blade stall happens when the helicopter is in directional flight at such a speed that the airflow over the retreating blade – even with its flapping and feathering – is not enough to generate sufficent lift. So helicopters have V[sub]NE[/sub] that will keep the blade flying under most conditions. Exceed that speed at your peril. Anyway… Yes, if the tail rotor did not flap there would be a rolling moment. I suppose the main rotor could be rigged to counteract the roll caused by the tail rotor. I’ve never really thought of it. But the tail rotor must produce enough thrust to counteract torque (hence its name). If the retreating blade does not generate sufficient ‘lift’, then you’re trying to do the work of the whole rotor with only half of it. Interesting question though. IANA aeronautical engineer, and my thoughts about the anti-torque rotor run toward ‘Is everything hooked up and functioning correctly?’ and ‘The emergency procedures for Loss Of Tail Rotor are…’ I would assume that in addition to loss of efficiency there would also be increased vibration and twisting loads on the tail boom. Speaking of tail booms, the Robinson R-22 Pilot’s Operating Handbook discusses low rotor RPM and ‘tail boom chop’ (by the main rotor). It actually uses the word ‘doomed’. I’ve always liked that. But then, I’m a bit morbid.