rrTV-PHOTO   New HD TV
HOME   rrTV-PHOTO   GALLERIES   MY GALLERY   HELP-FAQ
myHOME PM pmRR MEMBERS 782 ONLINE 20 EVENTS SEARCH REGISTER  START HERE
 
2 pages [ <<    <     1     ( 2 )    >    >> ]1307 viewsPOST REPLY
Boca Bearings . Modefo's RC Helicopters . XHELI.COM

.
.
Off Topics > CH53 Tail Rotor
 
 
KC
Elite Veteran
Location: WA

pancakes!
11-11-2003 Over year old.
HOMEPAGE  
 
 
melsman
Senior Heliman
Location: Atascadero, CA

Here's another 'tilted tail'.... The Commanche also has a fenestron that tilts down and to the left, just like the CH-53. That kinda blows that 'more weight in the tail' thing.

Actually, I flew many missions with 53E's, and my best friend flew them, so I kinda had the inside scoop. Of course, that was 14 years ago, so who knows what they're telling folks these days. (We used to tell airshow groupies that we folded our blades so that we could either a.) fly through narrow canyons, or b.) break the sound barrier.)

And no, I don't know how many blades the Commanche's fenestron has...

Ashley
11-11-2003 Over year old.
 
 
CK_
Senior Heliman
Location: Redondo Beach, CA

Think about it. The only helis with angled tail rotors are military helis. The tail is angled so that the heli is harder to "see" by certain weapon systems when looking at the heli from the side.

Chris
11-11-2003 Over year old.
 
 
HeliNerd
Senior Heliman
Location: Navarre, FL

What? do you have any idea what we are talking about??? Get a clue.
11-12-2003 Over year old.
 
 
KC
Elite Veteran
Location: WA

melsman, it sounds like you are saying the 53 tail is pushing air up / pulling down?

someone lay it down simple for me, the big rotor spins 1) clockwise or 2) counterclockwise when looking down on it from the refueler plank.

the little rotor 1) tractors or 2) pushes the boom thing.
11-13-2003 Over year old.
HOMEPAGE  
 
 
GMcNair
Key Veteran
Location: Birmingham AL

A friend of mine, a recently retired Blackhawk pilot, answered this question for me. He said the tail is canted to provide extra lift. In the case of the Blackhawk, they were told during initial flight training that the tail provided 15% of the helicopter's total lift. He did not know what the percentage is for the 53 model, but that the angled tail for stealth reasons is a crock, and he laughed out loud at the suggestion. His comment was "OK, so you angle the tail to hide it from radar. Now how do you hide a 54 foot diameter main rotor?"

The 53 i'm currently working on has a 17" diameter tailrotor, and the main head is 11" in diameter. The tail won't tilt on this one...it's a J model. See more info in my gallery and at http://www.mcgirt.net/RC/MH-53J/mh53j_index.html

Oh, and the mains spin counterclockwise when viewed from above, and the tail spins up into the downdraft of the mains.
11-13-2003 Over year old.
HOMEPAGE  
 
 
Doug
Elite Veteran
Location: Naples Florida....

I must disagree on the reason for the tilt. On the big "lifting" helies the "windage" on the tail boom imparts a nose up out of trim in hover that is proportional to the weight of the heli. The canted tr provices "lift" to off set the "windage"

First member of Member of Bearings Anonymous
11-13-2003 Over year old.
 
 
CK_
Senior Heliman
Location: Redondo Beach, CA

Quote 
What? do you have any idea what we are talking about???

Here's a link to a interesting article:
http://www.mitre.org/news/the_edge/...r_99/third.html

Excerpt from article:

A problem of great interest to many military organizations is the ability to detect and classify targets from remotely sensed data. Helicopter detection and classification is an important automatic target-recognition problem that falls into this category. Most of us can tell when a helicopter is in the area from the distinctive pulsating sound the rotor blades produce. Radar signatures from helicopters also contain distinctive periodic components know as blade flashes. These transient radar signature components occur when the helicopter rotor blades are aligned with the radar antenna producing specular reflections.

In other words, distinctive pulsing signature PERPENDICULAR to the rotor disk. A helicopter operating in a nap of the earth environment will have a damn near perpendicular shot to the tail rotor when targeted from abeam.

Quote 
OK, so you angle the tail to hide it from radar. Now how do you hide a 54 foot diameter main rotor?

You don't. But the signature of the main rotor when looked at edgewise is a hell of a lot smaller than that of a perpendicular tail rotor.

Doug, I like your explanation. Makes a lot of sense. Sometimes there's more than one reason something looks the way it does. It's funny though. Some of the biggest heavy lifters like the Skycrane, Halo, and Harke don't have a tilted tail rotor.

Chris
11-13-2003 Over year old.
 
 
KC
Elite Veteran
Location: WA

I'm f-cking confused now
11-13-2003 Over year old.
HOMEPAGE  
 
 
HeliNerd
Senior Heliman
Location: Navarre, FL

another reason that i have heard for the canted tail rotor is to simply make room for the massive diameter that the E models have on the tail. Their T/R is ALOT bigger than our M models. SO, to give them a little more lift in a hover and to make room for it's larger blades they cant the T/R over to the side. I love the way it looks, and from what the pilot's i have talked to have had to say they like the way it works.
11-13-2003 Over year old.
 
 
melsman
Senior Heliman
Location: Atascadero, CA

Doug, GMcNair, HeliNerd,.. We're all saying the same thing in a little different way.

The tail rotor is in a "pusher" configuration and rotates up into the main disk for greater efficiency. The main rotor, turning counter clock-wise, has a significant amount of torque, which is off-set by the tail rotor. However, the force vector from a normally vertical tail creates such a side force vector that the helo has to tilt to the left to off-set the side force and thus stay stationary in a hover. Because the torque with the 53E is so great, the tilt is relatively steep. And as many of you know, tilt in landing or take-off leads abruptly to dynamic roll-over.

The solution is to angle the tail rotor down slightly so that the tail rotor now has less of a side vector, and a slight downward vector (or 'lift' if you will...) Because the tail rotor is set up high on the fin, this thrust imparts a slight torque to the fuse off-setting some of the left tilt, (and pushing the nose down slightly) thus somewhat flattening out the hover.

It is possible that the downward vector adds to the overall lift, thus gaining additional lift where there was none before. Maybe this is the reason why Sikorsky has continued to include it on it other models (such as the Blackhawk and the Commanche.) However, the explanation I'm relaying was one given to me by the 53E pilots I flew with. They never mentioned any additional lift benefit.

As for adding to the 'avoidance' capabilities, I see your point, CK_, but even at an angle, there will still be plenty of aspects where the tail rotor disk is perpendicular to the observer. So I don't think it would have a significant impact on avoiding targeting. One way this is addressed is through the use of a fenestron, as on the Commanche. It also has the additional benefit of being a bit quieter and better suited for higher forward speeds, as well as higher sidewards speeds. (The main limiting factor for sidewards speed is the tail rotor.)

If there is still some doubt here, I'll be more than glad to e-mail my buddy who flew 53E's and ask him to clarify the issue. Heck, I think I'll just shoot him an e-mail anyway, since all of this chatter has made me question my own memory. And since I have a hard time remembering what I had for breakfast...

Ashley
11-13-2003 Over year old.
 
 
GMcNair
Key Veteran
Location: Birmingham AL

All I know is this is a PITA to build, and I thank God David chose the j model without the canted t/r. Ashley, that's a great signature by the way. And i'm with ya on the fading memory. What was this topic about?
11-13-2003 Over year old.
HOMEPAGE  
 
 
CK_
Senior Heliman
Location: Redondo Beach, CA

melsman,
Quote 
...but even at an angle, there will still be plenty of aspects where the tail rotor disk is perpendicular to the observer

Actually there's just one. Think of it this way. Draw vectors perpendicular to the tail rotor disk of the 53 in both directions. One vector will point to the sky. The other will terminate at one point on the ground a short distance from the heli if it is close to the ground. If the tail rotor is vertical, the normal vectors will go from horizon to horizon. Anyone along that line to the horizon will be almost perpendicular to the tail rotor.

Chris
11-13-2003 Over year old.
 
 
donlynn
Key Veteran
Location: New Zealand

Canted tail rotor

Gidday

I too thought it was something to do with the unsymetrical (no torque compensation at the front /nose rotor ) un balanced sideways thrust / translating tendancy interaction reaction jiggery pokery carry on.
So I canted the tail over on my rappy 30 taking up all availible slop in the tail box holes and the slots at the front, tried it so it produced a partial down thrust at hover then tried it the other way didn't make any difference to cyclic required on lift off or landing I could tell !!
I also tried it on my older cyclone too, I was able to achieve ~20-30 degrees offset,
same deal no difference I could pick, maybe I need a pylon to mount the tail on then it'll rock !!

-looked cool though :-)

Regards Don
11-13-2003 Over year old.
 
 
melsman
Senior Heliman
Location: Atascadero, CA

OK, guys. My buddy who flew 53E's responded to my e-mail today. I had sent him a copy of my last post and he responded with this...

Quote 
Your explanation is generally correct. The torque arm of a 53E is
pretty large. The 53E still hovers at about a 5 degree left wing down,
8 degree nose up attitude, but that is a lot better than what it would
be if the tail rotor was not canted. There is "some" additional lift,
but NATOPS says it is less than 5% of total.



He also offered to send along the section of the NATOPS manual (written by the Sikorsky engineers) that explains this. I told him his word was good enough. NATOPS manuals are notoriously boring...

BTW: The NATOPS manual in Naval Aviation is the "owner's manual" for the individual helicopters.

GMcNair- My 'tag line' is another 'unofficial motto' of the Marine Corps. Gotta love it!

Ashley
11-14-2003 Over year old.
 
 
2 pages [ <<    <     1     ( 2 )    >    >> ]1307 viewsPOST REPLY
Autography FlightPower . Advantage Hobby . Revolution Models

.
.
Off Topics > CH53 Tail Rotor
 PRINT TOPIC Advertisers 

Subscribe to This Topic

Wednesday, December 3 - 9:55 pm - Copyright © 2000 - 2008 runryder.com | email | link to rr | runryder needs cookie