Pitts Special S1 - 1/3 scale

[Steve Hargreaves - Moderator]

Those servo holders are rather nice Herri.....they DO look the business..

Interesting design though...if I understand you there is one servo for the top wing & one for the bottom.....the movement being transferred to each aileron via pushrods & bell cranks...is that correct? I would have thought one servo per aileron would have provided more power & less slop & have been easier to set up. Never having owned such a big model though my opinion is entirely untainted by facts or experience....

[Herri]

Yes Steve, that’s correct.

I was at first skeptical but look at this link, where someone has taken the trouble to work out the science.

**LINK**

Its in German but the guy has confirmed the fact, that its less effort to roll with one servo in the center.

The designer always said it was better to fly with just two central servos and he has been flying these Pitts since the 70’s.

Considering I am using Digital servos with 10 kgf torque, I don’t think it’ll be an issue.

The reduction in total weight was always an issue with this model to try and keep the wing loading within reason.

Not to mention the reduced effort/ cost.

I also only intend to use one elevator servo as it has been proven to be sufficient and the case for redundancy has been thoroughly discussed.

Time will tell.

Herri

[Steve Hargreaves - Moderator]

Interesting stuff Herri......as you say he has done the maths. So much of this hobby is experience based on prejudice with no real engineering knowledge applied......as the saying goes anyone can build a bridge that will stand up but it takes an engineer to build a bridge that will just stand up....

Most certainly keeping the aileron servos in the middle will reduce the rotational inertia of the model....big servos are what.... 60g each maybe? so thats nearly 120g saved per side ( yeah I know you'll have the weight of the bellcranks etc). A significant amount!!

[Herri]

I fitted the leading edge sheeting.

The first attempt, I did dry and had some problems getting the wood to bend correctly and stick. I used aliphatic with just a couple of centimeters at the tip end with cyano. I am allergic to cyano, so I don’t use it often and then with a table ventilator blowing the fumes away. I have tried the odorless but in my opinion, its not as good.

The second sheet, I wetted and shaped a bit by hand whilst waiting for the first one to dry.

This turned out as a good idea as it went on much better.

[Herri]

Next job was the trailing edge at the ailerons. These are made of ply and chamfered to the section in the drawing.

This is the aerobatic version as opposed to the real scale ailerons, which are frieze. That option is also on the plan but the recommendation is for this version.

I first of all marked the edge with pencil.

Then held the piece down and sanded up to the pencil line.

These are then glued 5mm from the rear spar front edge.

The center sheeting is made up of some 2mm balsa which just had to be fitted and glued.

The 2x4mm balsa for the rib cap strips was provided so it was a doddle to fix them to the ribs.

Tomorrow the wing can be removed from the board for the finishing work on the underside – exciting stuff.

[Martyn K]

Looking good - a proper build and its very helpful including snapshots of the plan. How are you going to do the aileron hinge? Or is that tomorrows installment...

BW

Martyn

[Andy Green]

Just seen this tread and registering my interest. This model might be my next build (with a DA65). Its either this or the Mick Reeve Mossie.

Dont suppose anyone wants to build one of those so I can compare!!

Andy

[bouncebounce crunch]

Hello Herri, this is a good build showing laminations so i am a fan, cannot wait to see a few more posts.

bbc

[Herri]

Posted by Martyn K on 10/09/2013 09:28:03:

Looking good - a proper build and its very helpful including snapshots of the plan. How are you going to do the aileron hinge? Or is that tomorrows installment...

BW

Martyn

Brass tube and wire rods Martyn. I shall cover that in detail later as I have never done it that way before.

Pleased there are a few of you guys interested.

Herri

[Herri]

Well, I removed the wing from the board.

I prepared the aileron push rods. These were 1.5mm rods, which were just pushed through the ribs to align with the servo/bellcrank.

The holes were slightly enlarged and plastic tubing added for strength.

This is the servo connection I made. On the plan, they still suggest bending the rod 90 degrees at the servo arm and connecting the second rod with wire and solder.

Although, I am sure it would work, I like this better.

I have used Loctite on the nuts and this red stuff to show up if there is anything loose.

The rod ends are araldited into the ball joints.

I will replace the servo arm screw with an Allen key type as this will be better to access later - if need be.

[Herri]

The lower aileron trailing edge gets a piece of ply fitted. I fitted this before gluing to the wing, although the instructions don’t fit it until later.

The lower leading edge sheeting is next with the center section to follow.

I wetted the leading edge sheet to make it bend better and used all my many clamps.

Later, when I started on the second side the first started to come loose (probably due to the afore mentioned wetting)

I then used some masking tape and hey presto, why didn’t I do that before. Works better than the clamps and does not damage the balsa.

I shall araldite the rods now and leave it alone until tomorrow.

Cheers

Herri

[Herri]

 

I have now sanded the leading edge strengthener. The leading edge had been glued on and tomorrow will be sanded to shape with the help of a little jig. I do love those long Perma Grit sanders, makes work like this so much easier (and accurate)

 

These little ply strengtheners need to be let in to the leading edge sheet. They are for the wing struts. I wish I had seen them earlier as it would have been easier to cut the sheet before gluing.

The center bottom sheeting is now complete and the wing just needs a bit of sanding.

I’m quite pleased up to now and obviously the top wing should be a doddle.

Herri

Edited By Herri on 11/09/2013 15:04:24

[Herri]

I planed the leading edge to shape using this little jig I made.

I finished off with the sander and I’m pleased with the results although I will need a bit of filler here and there.

[Herri]

I cleared up the mess and fastened the building board down without dihedral.

The hardwood spars are pinned down with balsa underneath to give the correct angles. (see plan)

These pieces were pre shaped, no work involved.

The top spars have to be shaped as the wing is swept.

I did the bottom spars over the plan. I should have done the top spars at the same time as trying to line it up later is very difficult.

[Herri]

Tip 1:

This is the recommended way to fasten the spars down:

Using steel pins, push the pin in as far as possible.

Cut off the top and hammer in the pin until flush.

When you come to remove the wing from the board the pins pull out with the wing and can be removed with a pair of pliers.

I hadn’t tried this before but I am now hooked.

[Herri]

Second tip:

The cut outs for the spars can be very tight. To avoid damage, take a bit of spar cut off. Chamfer the sides a mm or so at the end.

Then push the bit of wood through the spar cut out, which enlarges the cut out to a perfect fit.

Again something I had not seen before, but it works!

Hope these tips are useful to somebody

Cheers

Herri

[jeff2wings]

Some nice woodworking skills there, I’ve never been that keen on foam/fiberglass models myself there’s something very satisfying about standing ankle deep in wood shavings !

However, just a couple of personal observations with regard to the control installation

1. I would be inclined to add local ply reinforcement to the ribs where the rod passes through apart from the rib nearest to the bellcrank/servo to allow for sideways/vertical movement
2. .The servo arm arrangement you have will add an additional element of differential movement and reduce the overall amount of travel available
3. I would use a single arm, or for max strength a solid servo disc, with a ball link each side with a through bolt

I have used a Great Planes dual end ball link connector that works well ,might be worth looking at ?

[Herri]

Well I'm pleased somebody is paying attention Jeff.

I saw the other day, that my way was not going to work (I hadnt tried the servos), so I altered it to this:

This looks better although I am still a bit concerned about the movement.

I see in the plan there should be 22mm at the aileron edge. I have looked at two other German builds on the web and they seem to have more or less done it this way (as in the plan) with quite small servo arms.

I will take your comment about the strength on board, thanks.

Herri

[Herri]

 

I wont go into too much detail with the top wing as it’ll otherwise be like the BBC – all repeats.

There are quite a few pre-cut pieces of hardwood to join the wing and they need to be identified first.

These are glued in with (30 min.) epoxy

In the original instructions you are warned at this stage to make sure the wing build is close to the edge of the building board as there is some drilling to be done before the wing is lifted off (see later)

This piece must be sanded to accept the bracket (these are the brackets, which will hold the top wing to the center section struts, just fore of the cockpit)

These are then drilled as later, when the rounded trailing edge is fitted, you cant get at the spar anymore.

 

Edited By Herri on 19/09/2013 14:04:56

[Herri]

 

 

A servo tray was made and the center ribs fitted to accept the tray.

 

The tips were fitted as per the lower wing.

Ply aileron strengtheners were fitted to the rib next to the aileron. The holes for the wire hinges was already predrilled so it was just a case of making sure the hole lined up over the plan.

The leading edge sheeting was completed

The center section needed to be sheeted.

I used paper to get the correct shape.

I then used the tape method, as learnt here on this forum to make large sheet.

Join the sheet pieces with tape.

Turn them over and put glue in the cracks.

Lay flat.

Wipe away the glue.

Put something heavy on it until dry.

Remove tape – done.

Edited By Herri on 19/09/2013 14:06:43

[Herri]

Ignore this, I somehow posted twice

Edited By Herri on 19/09/2013 14:08:26

[Herri]

This picture shows both brackets and the rear round leading edge fitted. The rear bracket will be visible when finished as per original a/c.

[Herri]

The ailerons are next.

There are a few precut bits as well as the rear pieces cut from the wing ribs, which had to be saved.

This is the cross section of how it will (hopefully) look when finished.

The leading edge is preshaped and a strip od 5x2 has to be glued on.

The ailerons are then built over the plan.

But note there will be cap strips added later, so there needs to be 2mm under the ribs at this stage.

After drying the cap strips are fitted to both sides.

They will then be sanded to the correct shape using the wing to ensure correct fitting.

The wire hinges will be epoxied in and made removable for covering.

[Herri]

The top wing before sanding

I now have two wings and just the ailerons to sand/fit.

I am looking forward to starting with the fuselage as I find that is the most interesting bit.

Unfortunately, it'll be a few days before I can start as there are some non-modelling commitments - shame

[Herri]

The next job in my case was to make the firewall, which was missing in my kit. (I bought the kit secondhand)

It is 9mm ply so I laminated two 5mm pieces I had.

I made a mistake here. There were two different versions of the firewall as a drawing. I didn’t read it properly and (murphy law) picked the wrong one. I then thought I had the incorrect crab manifold, so mailed the designer.

 

He put me right and this is what it looks like.

The carb actually pokes through and the main needle needed to be altered from the original “T” piece to a screw slot, (see pictures) so as to be adjustable.

 

Edited By Herri on 02/10/2013 15:23:28