All flying tailplane  
View related threads: (in this forum | in all forums)

Tower Hobbies
Enter up to 4 keywords or Tower stock numbers
Logged in as Guest



Users viewing this topic: none
  Printable Version
       


Universal Cradle System
Seller:  flyn4wrd
Details:   $175.00   |  10/5/2008   |  Classified Ad
We will rotate YOUR AD in this spot if you select "Forum Featured" when placing or editing your ad!

All Forums >> RC Airplanes >> Aerodynamics >> All flying tailplane
Page: [1] 2   next >   >>  

Login
Message << Older Topic   Newer Topic >>
All flying tailplane - 3/4/2008 8:51:45 AM   
quentinmayberry


 

Posts: 27
Joined: 6/18/2006
From: Inverurie, UNITED KINGDOM
Status: offline 		Don't know why the previous posting was deleted, perhaps the person who deleted it could tell me why?

Anyway, I was trying to determine if it is better to have an all flying tailplane pivot along its aerofoil centre line or pivot from a position some distance away from the centre line as per the image. I am building a 1/6th scale BAE Hawk and the fullsize pivots along the aerofoil centre line. I have also looked at a couple of other similar models and they don't pivot below either.

I know the pivot method is used on some T tail gliders, but is it best practice for a heavy jet (20lb)? I am concerned at the extra possibly unnecessary force applied to the servo trying to move the tailplane with an already built in turning moment. I am told that jets suffer from large loads on the elevator, so anything that can be done to reduce that is welcomed.

It was suggested that being at the MAC point is more important than the pivot moment, is that because it reduces the moment, or what?

Attachments
Click to see fullsize image.
Click for fullsize
       Post #: 1

RE: All flying tailplane - 3/4/2008 11:40:55 AM   
da Rock



Posts: 6641
Joined: 10/11/2005
From: western, NC, USA
Status: online 		There was a problem with the site earlier. I noticed it had fallen back slightly, that it was showing me unread messages that I had read. Perhaps that's where your thread vanished. It's not uncommon that forums step back to fix problems.

Since I see nothing indicating your thread was moved or deleted, and I know that I didn't touch it, I'd suggest your thread was lost in a fallback.

(in reply to quentinmayberry)
       Post #: 2

RE: All flying tailplane - 3/4/2008 12:01:36 PM   
da Rock



Posts: 6641
Joined: 10/11/2005
From: western, NC, USA
Status: online 		As for an answer to your questions.....

What you've drawn is a bit of a puzzle. Where is the connection for the pushrod. That location might give a hint what the designer had in mind.

Quite frankly, if that stab pivots on the hole you show, then the designer has caused himself a number of problems. Are you building an existing model design? a kit? or designing it yourself?

Very often, the symmetrical airfoils used most often in model pivoting surfaces lead the designers to place the pivot point as near to the center of pressure as possible. That way, the lift created by the airfoil does not create a moment. And the forces required to drive the surface do not have to work harder to hold the surfaces, only have to work to change the orientation, and encounter almost no induced resistance thanks to the pivot location.

I'm not sure what you're saying with, " being at the MAC point is more important than the pivot moment". The mean aerodynamic chord point really doesn't have much to do with the problem. So I'm guessing there is a problem with terminology.

(in reply to da Rock)
       Post #: 3

RE: All flying tailplane - 3/4/2008 12:41:51 PM   
quentinmayberry


 

Posts: 27
Joined: 6/18/2006
From: Inverurie, UNITED KINGDOM
Status: offline 		Thanks for that Da Rock, must have been one of those cyberspace glitches with the original thread then!

The servo connects to the top of the aerofoil as per the second picture. The nose of the aircraft is to the left as you look at the picture.

This is a semi kit, glass fibre fuz and foam wings. The rest you build yourself with reference to supplied plans.

Re the MAC point, a poster added to the original thread, that it was more important to have the pivot as close as possible to MAC. I was curious why.

(in reply to da Rock)
       Post #: 4

RE: All flying tailplane - 3/4/2008 4:57:10 PM   
Deadeye



Posts: 3683
Joined: 7/16/2002
From: Conrad, MT, USA
Status: online 		The stabilator on the TF Arrow 2 has the pivot point at the stabilator's spar.

_____________________________

The USA is not a free country anymore

(in reply to quentinmayberry)
       Post #: 5

RE: All flying tailplane - 3/4/2008 5:22:07 PM   
Tall Paul



Posts: 4653
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline
quote:

ORIGINAL: quentinmayberry

Thanks for that Da Rock, must have been one of those cyberspace glitches with the original thread then!

The servo connects to the top of the aerofoil as per the second picture. The nose of the aircraft is to the left as you look at the picture.

This is a semi kit, glass fibre fuz and foam wings. The rest you build yourself with reference to supplied plans.

Re the MAC point, a poster added to the original thread, that it was more important to have the pivot as close as possible to MAC. I was curious why.


.
This one uses a Dr. Mark Drela design, which when properly fabricated is quite good. In this instance, the pivot point was well behind the 25% mac. and the airplane had a serious stop-to-stop oscillation on the horizontal, leading to a total crash.
The drawing shows the proper way to have done this.

Attachments
Click to see fullsize image.
Click for fullsize 		Click to see fullsize image.
Click for fullsize


_____________________________

Sparky Paul
http://www.angelfire.com/indie/aerostuff

(in reply to quentinmayberry)
       Post #: 6

RE: All flying tailplane - 3/4/2008 6:53:09 PM   
BMatthews



Posts: 8943
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline
quote:

ORIGINAL: quentinmayberry

.....Re the MAC point, a poster added to the original thread, that it was more important to have the pivot as close as possible to MAC. I was curious why.



I didn't delete it either so it must have been the ups and downs yesterday. Lost a couple of posts myself.

Anyhow back to the fun. When they said to put it at the MAC they forgot an important part. You want to put it at or slightly ahead of the 25% chord point of the MAC.

Why you ask? Symetrical airfoils tend to be neutral to weathervaning if they are pivoted at the 25% chord point. If you put the pivot so that there's equal area ahead and behind like you may think you'll find that it wants to strongly pvot up or down rather than politely align itself with the airflow. If you put the pivot right on the 25% point the loads will pretty much balance and it won't kick around much. If it's slightly ahead like at 22 or 23% it'll show a very slight self centering action.

Although with the pivot below the surface like this it'll have some drag and that will put some very variable loads on the servo I would think.. But in the airfoil or below you want that pivot point to be at or a % or two ahead of the 25% chord of the surface's MAC.



_____________________________

Bruce-
Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

(in reply to quentinmayberry)
       Post #: 7

RE: All flying tailplane - 3/5/2008 8:26:55 AM   
quentinmayberry


 

Posts: 27
Joined: 6/18/2006
From: Inverurie, UNITED KINGDOM
Status: offline 		Thanks for all your inputs guys, I will re design the pivot and make sure it is on the aerofoil centreline and 25% forward of the MAC

(in reply to BMatthews)
       Post #: 8

RE: All flying tailplane - 3/5/2008 11:44:23 AM   
da Rock



Posts: 6641
Joined: 10/11/2005
From: western, NC, USA
Status: online
quote:

ORIGINAL: quentinmayberry

Thanks for all your inputs guys, I will re design the pivot and make sure it is on the aerofoil centreline and 25% forward of the MAC



Good idea to put it on the centerline.

To help a bit with the terminology, you actually wish to place the pivot at a location that is at 25% of the MAC. Designers figure out where the MAC lines up relative to the centerline of the surface and then measure back 25% of the MAC's chord. That would be 25% back from the LE of the chord of where the MAC would be if it were projected to be at the center of the span.


(in reply to quentinmayberry)
       Post #: 9

RE: All flying tailplane - 3/5/2008 2:28:36 PM   
quentinmayberry


 

Posts: 27
Joined: 6/18/2006
From: Inverurie, UNITED KINGDOM
Status: offline 		Err confused now, so where on this sketch should the pivot be as I have indicated or am I the wrong side?


Attachments
Click to see fullsize image.
Click for fullsize

(in reply to da Rock)
       Post #: 10

RE: All flying tailplane - 3/5/2008 6:17:03 PM   
BMatthews



Posts: 8943
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		An old concept was called the Center of Pressure for an airfoil. This was where the airfoil could be "suspended" such that all the forces in front and behind that point were balanced. In other words it didn't try to pivot at all. This concept fell out of favour because at the zero lift angle for any of the airfoils the Cp could be shown to be infinitly to the rear of the airfoil. And physicists don't like dealing with infinite values at all.

The old Cp concept was replaced with a standard using the aerodynamic center that is located at the 25% chord point and then the airfoil is given a pitching moment value or torque value that describes now much it wants to try to diverge when pivoted at that point for various angles of attack.

Now here is the reason you want to pivot it at this point.... on almost all sort of normal symetrical airfoils the pitching moment is very close to 0 over a wide range of angles of attack. That Pm = 0 means you will see the minimum load on the servo and that's a good thing.


_____________________________

Bruce-
Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

(in reply to quentinmayberry)
       Post #: 11

RE: All flying tailplane - 3/5/2008 6:23:33 PM   
Ron S



Posts: 1061
Joined: 12/12/2001
From: Fort Worth, TX, USA
Status: online 		I modified your pic. You don't want to go aft of 25%mac, or you'll have an unstable control surface. You can go slightly forward though, and as a positive effect, the surface will become slightly stable. On the other hand, you'll be starting to make the servo work a bit harder. Also, to your original question, the F-100 Super Sabre has a physical pivot point above the flying surface itself... It then had an actuator that would move the surface fwd/aft (following a slight circular arc around the pivot point.)

Attachments
Click to see fullsize image.
Click for fullsize


_____________________________

-Ron S.

(in reply to quentinmayberry)
       Post #: 12

RE: All flying tailplane - 3/5/2008 8:38:46 PM   
quentinmayberry


 

Posts: 27
Joined: 6/18/2006
From: Inverurie, UNITED KINGDOM
Status: offline 		Thanks Guys

Ron, that clears things up, many thanks!

(in reply to Ron S)
       Post #: 13

RE: All flying tailplane - 3/6/2008 3:55:20 AM   
otrcman