All flying tailplane
 

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?

RE: All flying tailplane
 

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.

RE: All flying tailplane
 

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.

RE: All flying tailplane
 

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.
http://www.qservices.biz/downloads/tailplane.JPG
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.

RE: All flying tailplane
 

The stabilator on the TF Arrow 2 has the pivot point at the stabilator's spar.

RE: All flying tailplane
 

.
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.

RE: All flying tailplane
 

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.

RE: All flying tailplane
 

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;)

RE: All flying tailplane
 

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.

RE: All flying tailplane
 

Err confused now, so where on this sketch should the pivot be as I have indicated or am I the wrong side?

RE: All flying tailplane
 

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.

RE: All flying tailplane
 

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.) [8D]

RE: All flying tailplane
 

Thanks Guys:)

Ron, that clears things up, many thanks!

RE: All flying tailplane
 

Lots of very good comments here regarding placement of the pivot relative to the stabilizer planform. One thing that got left out was mass balancing of the surface relative to the pivot. One reference was made to a stop-to-stop oscillation which resulted in a crash. This was almost certainly flutter, which is a phenomenon related to both aerodynamic distribution and mass distribution. To be on the safe side, be sure to balance the surface so that it's CG is slightly ahead of the pivot point. And because the surface is swept back, be sure to make it as rigid as possible.

Dick Fischer

RE: All flying tailplane
 

Thanks Dick, I hadn't forgotten about the balancing, it was the pivot that was causing me some concern.

I appreciate all your help guys [sm=thumbs_up.gif]

RE: All flying tailplane
 

Just working out the MAC, and have attached a photo of the plan. Should the pivot line be the red line or the green line?

(Excuse the line drawn, not exactly straight. The mac line is also out of alignment to the airflow I assume)

RE: All flying tailplane
 

Just the way you show it.
Both halves are rigidly connected to the horn.
The pivot looks OK, but I'd align it across the airplane, not with the slight angle you show.

RE: All flying tailplane
 

This is not my design! It came as a partial kit.

I started assembling it and it looked wrong, which is what started the thread. I assume that a pivot point along the red line is required to be at 25% MAC, not as shown on the original plan drawing, or am I wrong with that thought?

RE: All flying tailplane
 

As shown on the plan the pivot is actually ahead of your layed out 25% mac so all would be well with the original design in that regard. It was your original sketch that got the wolf pack all riled up. The picture of the plan (complete with glue blobs :D) shows that the designer wasn't on drugs and all is well. The sweepback working to move the MAC to the rear enough that the pivot is now comfortably in front of safe point.

The hinge point above the surface is still a pretty funky way to do it but it would definetly work. It's just that the surface would sort of rock back and forth a little like an upside down cradle instead of just pivoting. No ills would come of that other than a slight extra servo load. As long as you're using something strong both in output and gear toughness it would be fine with no redesign. In fact since this is for a ducted fan or turbojet I'm guessing it was done like this to keep the linking rods below the outlet pipe or duct.

RE: All flying tailplane
 

Oi, leave the glue blobs out of this :D I had to rummage around to find the plans as the tailplane had already been built months ago, just have never been happy with the way it fitted.

Yes it is for a turbine, so lots of heat just below the servo. There will be plenty of thermal insulation around that section.

I am slightly concerned, as a few of these kits have lost their tailplanes in flight, no one really knows why either. Another simply nose dived in to the tarmac! Not wishing that to happen to mine, I was suspicious about the connection and the whole teatering tailplane idea.

So, would it be better to have the tailplane hinge point/pivot along the red line, as opposed to the design shown on the sketch? It would certainly make a much stronger hinge point as the thick ply cross bearers could be used as opposed to the thin 1/8th inch ply plates supplied.

RE: All flying tailplane
 

OK, I'm editing out my advice.

As written it sucks. And is wrong.

I got hung up on the leverage and the pivots. Thank goodness we got better advisers than me. Sorry

RE: All flying tailplane
 

I've drawn the pivot line on the plans. It is designed into that stab pivoting hardware to be perpendicular to the joint the stab makes to the fuselage.

You can think of it as if it were a hinge line. If the stab was actually a stab/elevator with a hinge line, it would be along the heavy green line I've added to the plans. And as shown, it passes at the .24C MAC point.

I did a quick proportion check of the plans and the point on the MAC the pivot line runs through shows that point to be 30/125 or 24%.

The main problem with the design is that the pivot axis is actually not in line with the actual rotation of the stabs that are being held and rotated in a different plane.

You are correct to dislike the design. It shows little understanding of the geometry of the tail. And is quite a lot of work to do, that could be done more easily in a number of ways.

RE: All flying tailplane
 

The hardware used to connect it is 1/8th ply root doubler, and a 5mm bolt running through a bearing. One side is shown on the original plan, obviously there are two one each side.

I have attached the side view of what it all looks like and you can see it sits a few fractions of an inch above the jet tube and all that heat!

It wouldn't be hard to make the whole lot pivot/hinge along the centreline of the aerofoil using the joining bearers as hinge/pivot mounting points.

RE: All flying tailplane
 

Edited out some really stupid advice. Poorly written and not at all to the issue of the redesign. Sorry, I got caught up with the detail of the pivot only and the location of the pivots in particular.

My bad................

RE: All flying tailplane
 

honestly -I don't see anything I like about the servo/linkage setup in your drawing:
That servo travel to surface travel is fine for a very slow foamie but will cause problems on a faster model
a ratio of say---25% stabilator to servo rotation would be a lot smoother setup -at least to start.
The servo needs MAX holding ability which means the holding/moving force --vs throw is to be maximized.
Yes I have done flying stabs and typically ONE DEGREE of surface movement is a LOT.
any servo slop is a no no -
On a setup like this I would use an extremely high torque servo - a precise radio and maximize servo rotation-to throw of stabilator also the pivot for stab and servo is best if on a common plane -providing a linear movement. But then I am an old guy with pretty hard headed ideas ---