The Tail Rotor

[sling load]

A question for Nick Lappos,
normally we could ask this question on rotorheads, but I didn't want to get swamped with Zuckerisms.
Very little information is written about the tail rotor and its various designs. I have two keepsakes from aircraft ive flown, one is a Bell 206 tail rotor the other is a Fenestron blade from the 365 series. The 206 blade is symetrical, no twist or taper (that I can see) and the wear strips on the edge indicate the speed the outer portion rotates compared to the inner portion. Is this lift difference taken out by flapping? I havn't seen the 206 for a while so my memory is vague, but I am presuming the lead and lag is accounted for in some way, the root has two holes in line with each other to fix it to the tail hub. Or, am I completely wrong and the tail rotor is designed differently because of the short rotor diameter.

The fenestron blade has about 7degrees of twist, and the blade is assymetrical, there is the ability for the pitch to change, but flapping would be nearly totally eliminated. Is this the reason of the two different designs, the ability to flap or in the case of the fenestron, built in twist and assymetrical design.

Ive searched the library at Uni and very little is written about the tail rotor. Ive gone back to my flying experince to track down the reasons behind the designs of tail rotors and fellow pilots seemed to get stumped as well. One helo I noticed with a tapered tail rotor was the S 55 in an old photo, so far, thats the only one. I hope you could enlighten the subject for me. Thanks Nick

[Lu Zuckerman]

To: sling load

Since I could not contact you directly via email you are going to get stuck with another Zuckerism.

You got it right regarding the Bell tail rotor and it applies to most if not all-tail rotors. When they flap they change pitch thus increasing or decreasing the lift for each blade and in the process equalize the lift across the disc. If this provision were not incorporated the bending loads on the tail rotor drive shaft and the gearbox would be excessive. There is no lead and lag on the Bell tail rotor. If there is any tendency to lead or lag the loads are absorbed by the tail rotor and the hub and finally reacted by the drive quill.

Regarding the S-55 tail rotor it has the same pitch flap coupling as on the Bell. The reason the blades are tapered is to eliminate bending loads. The lift increases at the square of the speed of the tail rotor blade and the tapering of the blade decreases the lift the further you get out to the tip. Main rotor blades are twisted to effect the same conditions.

I’ll leave the Fenestron blade question to Nick.

[ 16 December 2001: Message edited by: Lu Zuckerman ]

[Nick Lappos]

Sling load,
The tail rotor is made purposefully quite simply, and more robust because the weight trade-off against the cost of the extra components makes simplicity a virtue. The typical tail rotor has no lead-lag and no cyclic so it is stronger relative to its thrust than a main rotor. One means of relieving the flapping forces due to forward flight (disymetry of lift) is to incorporate lots of delta-3 coupling (pitch-flap coupling). Note that the blade flapping (or teetering) axis induces considerable washout of the collective pitch. This is done by making the flap occur at a different place than the pitch change horn. As the blade flaps, the pitch horn doesn't move, so the collective pitch is changed. In a hover, little flapping occurs, so this feature does not reduce thrust. In forward flight, the forward sweeping blade builds more lift, flaps away from the vertical tail, and so gets its lift reduced. This balances out the forces, and relieves most of the bending that the shaft and gearbox would otherwise see.

The blade is symetrical, mostly for simplicity, and it has little twist because of the simplicity. Also, it must work to thrust in both directions, and the twist would hurt the right pedal thrust.

The fenestron has many blades packaged tightly, and would be a problem to allow flapping. Also, the fenestron needs tight tip clearance for high efficiency (almost touching the shroud). This would be a problem if significant flapping were allowed. The shorter blades are more easily stiffened against bending, as well. But the biggest reason why the fenestron blades need no flapping relief is that the duct shields the fan from the disymetry forces, so the disk gets almost no differential bending due to forward flight. The fan behaves as if it is in a regular hover, even at high speed. That is one of the reasons why Comanche has a fan, and why it can do the "snap turn" at such high speeds. The fan sees almost no bending, and so does things that would very highly stress a tail rotor (such as the big sideslips of the snap turn.)

The fan's blades are highly assymetrical and twisted, for very careful efficient design in hover, where thrust is most important, and where the small disk of the fan would otherwise cost lots of power. The opposite thrust (right thrust for American designs) is not as efficiently developed as a result, but this is only needed at autorotative descents, where a bit of extra power penalty is hardly noticed. :)

[sling load]

Nick,
Thanks for your response, very little is written about the tail rotor, and I got some pretty blank expressions on the faces of other drivers who couldn't explain it either,being an engineering student and a rather curious person, my enquiries drew a blank, particularly the flapping.

On the Dauphin, stationary, the tip clearance around the fenestron varies, like an oval, in flight this distortion is evened out by the distortion of the fin.

In Commanche, is this the case, where flight loads 'twist' the tail and change clearance of the blades, or has technology beaten the old French design?

Would I be correct in saying that Frank Piaseckis flying jeeps were the same principles of two fenestrons turned over, or where they different too?

[Genghis the Engineer]

I don't have copies of either to hand, but I seem to remember a fair bit about Tail Rotor design in Ray Prouty's "Helicopter Aerodynamics", and Simon Newman's "Fundamentals of Helicopter Flight". Both should be in your University library.

G

[Nick Lappos]

Sling Load,
Yes, those fying bedposts had nicely shaped ducts (at least to y inschooled eye) so they were similar, as is the Sikorsky Cypher RPV.
The duct shapes the fan output flow and allows it to spread out coherently. The result is that the thrust of the ducted fan almost doubles, relative to the small diameter (picture the exhaust plume fanning out to an equivilent larger disk).

[Cyclic Hotline]

Interestingly, Sikorsky also produced (perhaps) the most complex tail rotor of all with the Sikorsky S-56 (H-37). This tail rotor was fully articulated, complete with dampers!

The S-56 represented a collosal leap in Helicopter technology, when you consider how fast helicopters came from the R-4 and in such a short period of time.

This tail rotor even made it from the H-37 to the prototype CH-54 (Skycrane), although by the time it reached production it was a very simple tail rotor, (similar to the S61 but like the S61R, with oil lube). The CH-54 was an evolutionary derivative of the H-37, employing essentially the same rotor system (6 bladed instead of 5) and the defunct H-37 inventory became a common source of parts to keep the Cranes going over the years.

In the process of the evolution of the H-37, two major modifications were attempted. The first was the incorporation of a 6 bladed rotor system (same as the crane). The second may have been the most complex tail rotor arramgement in the history of the helicopter;
Twin Tail Rotors

The prototype XHR2S-1, USMC No. 133734, was modified to have a "V"-tail with two tail rotors. The original version using two four-bladed tail rotors first flew on March 2, 1956. Subsequently, two five-bladed tail rotors were flown on April 11, 1956. The rationale for this modification was to increase the center-of-gravity range and allow more indiscriminate loading. With two canted tail rotors, the individual tail rotor thrusts could be adjusted to provide both the required anti-torque thrust and different levels of lift. In this way, centers-of-gravity that were further forward or further aft could be compensated for by decreasing or increasing the combined tail rotor lift component by retaining the same anti-torque sideforce. The project was successful but never adopted for production. My guess is that the additional weight and power required were too high to make the system viable. http://www.big-deuce.de/d_story05.htm

Does demonstrate that in the evolution of helicopters, virtually every possible design is considered and sometimes utilized.

Fortunately, perseverance may demonstrate that simplicity is a virtue!

(I suppose the most complex tail rotor must lie with the Boeing twin rotor products!) :p

[ 17 December 2001: Message edited by: Cyclic Hotline ]

[sling load]

Ghengis,
Thanks, the library is bias towards fixed wing, I will search their database for those.

Nick,
Regarding the Commanche, does the duct alter its shape in flight as the 365 duct does?

[Nick Lappos]

sling load,
No our helicopters don't bend in the wind.
;)

[The Nr Fairy]

As an addendum to the twin tail rotors post, the Apache, if you look closely, has two separate two-blade rotors on the same hub, offset by something other than 90%, with separate pitch links.

When I noticed this on an Apache on static display at Middle Wallop earlier this year the nice man in a green suit said it was for survivability, and each pair of blades has sufficient authority to control the machine should the other pair be lost. Some interesting design criteria there.

I've tried to find static piccies of the t/r, and can't - all the piccies I've seen are of the beast in flight . . .

[Cyclic Hotline]

http://www.ncguard.com/pao/graphics/130AVN8A.jpg

[sling load]

Looks like the Kamov design bureau designed that!

Nick, glad to hear that! The Dauphins aren't to good at snap turns! :)

[RW-1]

Neat, but for survivability, are they independantly driven as well?

(While I typed that above, I know it would be difficult to do so, perhaps a shaft within a shaft, but that would still leave the area vulnerable to a hit wouldn't it?)

So what is the survivability criteria of the design? (If a hit takes out the drive system ...)

[Lu Zuckerman]

The tail rotor gearbox and the intermediate gearbox are capable of taking a 23mm hit and keep on ticking. The gearboxes are not lubricated with conventional oil but very viscous non-hydrocarbon grease. The helicopter should be able to fly for thirty minutes providing the gear mesh was not destroyed or impaired when hit by the 23mm round. If a tail rotor blade is hit it should be able to survive as can the main rotor blades. This is due to their construction. The main trannie can also survive a direct hit and operate for thirty minutes even though the oil had drained out. Inside of each gear set is an absorbent material that retains oil so when the gear case is compromised and the oil runs out the oil retained in the respective gear sets is forced out by of all things, centrifugal force into the gear mesh. However if a tail rotor blade is lost due to a direct hit then all is lost and the pilot if he can should immediately set it down on the ground.