Gary's BT 69" Spitfire Mk1

[dave parnham]

Anybody have experience with the CY models Retracts ? They look great but to be honest they are my First Retracts 😂

I keep hearing that they are not so good but i wondered why?

[Jon H]

2 hours ago, dave parnham said:

Anybody have experience with the CY models Retracts ? They look great but to be honest they are my First Retracts 😂

I keep hearing that they are not so good but i wondered why?

 

No experience with the CY gear but i have used the almost universally slated YT air retracts for well over a decade and had no problems with them so i would take negative reviews with a pinch of salt as you never know if the person writing it has done something wrong. 

[Gary Clark 1]

Disappointing evening as I tried to dry fit everything as all the ribs are wrong. the front half is correct but they are all short and that means the shape at the trailing edge is slightly off

 

 

It is easy to correct the outer ribs but the inner ones (1D to 5) will need re-cut I think. Really annoying when the laser cut parts don't match the plan. It shouldn't take long to do but I'd rather it was correct.

 

That is tomorrows job now though

 

Gary

[Gary Binnie]

The plan could be distorted, it looks like you have a copy of a copy.

 

The black lined border of the plan is for size checking, some designers write the size of the border on the plan.

 

My BT Spitfire wing sheet border is 32" high and 49" wide.

 

Without known border sizes you could check other smaller dimensions with a vernier caliper, the main wheel size on the plan should be exactly 3.75", I have even measured the width of strip wood parts on plans.

 

This size is written in the bottom right corner of a Chris Foss glider plan.

[Gary Clark 1]

Thanks Gary, I'll check tonight 

[Gary Binnie]

Was shuffling a few boxes earlier today and moved one of the BT Spitfire boxes, there are spare and unbuilt laser cut parts in there so I measured a rib for you.

Rib 1A is 319 mm or 12 9/16" in chord.

My laser cut parts were from Bob Holman in the States, the only accuracy problem I came across was some of the upper fuselage formers had been cut to the outside skin line, had to break them off and make replacements.

 

 

[Eric Robson]

Hi Gary, 

I bought the B.Taylor plan and laser cut parts from Sarik 2 Yr ago for the Tempest some of the fuselage formers were under size by 1/8". After contacting Sarik the fault turned out to be  the copying of the original plan, the centre line was out on the plan view and the set up for the cutter was taken from the centre line which run off.

Edited June 22, 2023 by Eric Robson

[Gary Clark 1]

Thanks Guys.

 

I think I came to that conclusion last night Eric but it is odd that the width is good but just short. I decided, rightly or wrongly, that this amount across the chord of a spitfire wing won't make much of a difference. I also compared the inner rib with the centre section and they match so I decided not to cut new ribs. The error seems to be the plan and not the parts. I double checked and my ribs are the same as yours Gary, thanks.

 

I got the bones of the stbd wing together and decided to do some gluing

 

 

I tried to take a photo to show the combination of sliced spars and the notch to show that it's pretty strong

 

 

I also add a gusset to certain areas to strengthen these parts despite not being called for in the plan. A little weight gain for a lot of added strength

 

 

Good to be building again.

 

Gary

[Gary Clark 1]

A little more done on the wing tonight. Cut the new ply brace and you can see how much I've extended it but all in front of the CG so not too much of a problem as far as weight is concerned. I love building BT aircraft but I do feel like a little reinforcing is required on his main spars. He was clearly a much better flyer than me as I reckon I'd snap the wings whilst having to use full back stick trying to recover from a very poorly flown barrel roll

 

 

I also got the 1/16 shear webs done

 

 

Finally tried the new dihedral brace with the centre section so that I could sort the final position of rib 1 which then lets me sort the first layer of the leading edge then sheet the forward "D" section to lock in the washout. I'll also sheet the area above the flap to add a little strength before I remove it and start the other wing. My plan is to get the other wing to this stage, try and sort the retracts, sort ailerons and flaps then finish sheeting.

 

Retracts 🤢

 

Gary

[Gary Clark 1]

No much done due to being away for a few days but managed to secure the end rib (1D) and added some balsa sheet as fillers to allow the dihedral brace to connect to the main spar (shown by red arrows below). you can see the size of the original brace (shown by blue lines) compared to what I have added. I've also moved it to behind the forward brace (green arrow) so i can connect it to the main spar

 

 

The overall "structure" of the wing is done, just sheeting some of the top before lifting it off the board

 

 

Gary

 

[Nigel R]

Nice job.

 

The original brace does look small. That said with good carpentry, you're essentially making a join between two spar pieces the size of a regular splice join. And the eliptical profile is meant to have very equal spanwise flight loads.

 

Note words 'good carpentry' - sometimes a bit of extra wood in a key area can cover a lot of sins (at least it does for me) 🙂

 

All that aside, I really can't see any rhyme or reason for only having it on the front aux/stub spar. Surely you'd want the main spar to be the, well, main structural member, right across the span?

 

Aileron hinges look interesting - are you sticking with the scheme shown on the plan?

[Gary Clark 1]

Thanks Nigel

 

I agree and that's why the balsa sheet is there to connect the two but it would mean a re-design of the centre section too if I wanted to attach to just the mainspar so this seemed like the easiest option considering the centre section is complete already. 

 

I'm going to have a separate servo for each aileron and flap rather than bellcranks. I will probably incorporate an RDS to prevent pushrods sticking out below the wing but at 69" span the ailerons are quite thin so if that is too tight I'll go with option B and have a little pushrod sticking out below which won't be that noticeable. 

 

I've completed the sheeting over the front and aft sections of the wing so I'll lift it and start the aileron next 

 

Gary

[Nick Somerville]

Hi Garry, good to see progress on your build. Twin servos is definitely the way to go for flaps and ailerons for simplicity and reliability, albeit extra expense. Both my P47 and Sea Fury utilise the midi sized KST 10 servos, which are pretty light so no weight penalty over single standard sized. At 8kg they are plenty powerful enough.
 

I used the rds system for my Sea Fury with excellent advice from Danny Fenton. However, despite every care taken in their construction they have developed a little slop over time that cannot retrospectively  be rectified. Also I had an aileron fail a few weeks ago and discovered that both grub screws that tighten on the torque rod had worked loose and I found them both stuck to the case of the servo where the magnet is inside! Fortunately I was able to land safely. I am considering changing to a traditional external pushrod but the slop hasn’t got that bad yet. If you have a removable hinged aileron then RDS is fully serviceable but on my Fury with the surfaces permanently attached it’s impossible to alter the setup. Also due to the length of the torque rod, removing it and the servo really needs a larger access hatch than normal. Just some things to consider,  but I am sure you can work it out. 

[Ron Gray]

An alternative to the rotary RDS is the IDS type commonly used in F5J type gliders:

 

 

 

[Eric Robson]

Hi Gary, I used standard servos on their side for my B.Taylor P40. The arm just sticks out and in may years I have not had any problems apart from a major repair pilot error, 

I used the hinge system on the plan only I used nylon tube instead of brass. A throw back to the early days of avoiding metal to metal moving parts to avoid interference. 

[Gary Clark 1]

Thanks guys, lots a good points. Not seen those IDS before Ron, looks like a great idea. Question I do have though, does the geometry need to pretty bang on? I'm guessing they have to be exactly 90 degrees from the hinge line? This would take a bit of work as the ribs aren't perpendicular to the hinge line like most warbirds so would take a wee bit working so the movement is the same both sides.

 

I got the bottom skins cut for both ailerons as it was easy to both at the same time and also both sets of aileron ribs cut. I'll be able to sort the hinges and the leading edge of the aileron but not the top sheet until I sort out the pushrod system.

 

The wing is at the stage below. Happy with it so far but sorry about the state of the plan, she's been through a lot! Haha

 

 

 

[Ron Gray]

9 hours ago, Gary Clark 1 said:

I'm guessing they have to be exactly 90 degrees from the hinge line?

The servo can always be mounted at an angle so that the pushrod is at 90°.

 

I did something similar with the rudder on my P51 as I didn’t want to see an externally mounted control horn. Works very well and after 30+ flights still OK.

 

[Paul De Tourtoulon]

I also use Rons rudder controls on some of my planes, but the kwik link isn't in the middle but the edge.

[Gary Clark 1]

I've used that too and the plan shows a system very like this but I find it can have a bit of slop in it which becomes flutter airborne. Either of you guys had this?

[Ron Gray]

I drilled out the 'eye' that the clevis fits in to and bushed it with some nylon (?) tubing the exact size of the clevis pin, no slop.

[Jon H]

28 minutes ago, Gary Clark 1 said:

bit of slop in it which becomes flutter airborne

 

If you have the space bolt through ball links can work well at reducing slop. They would be much better than a clevis but they do take up more space. 

[Paul De Tourtoulon]

I only use schluter ball links on them, as on anything that is fast and expensive.

[Gary Binnie]

IDS is a pain, I took two weeks fitting it to a NAN Explorer glider (two flaps, two ailerons) and got unequal travel of surfaces (mainly the flaps) but could dial it out with my radio. Most composite glider manufacturers offer to install it at the factory, they use precision jigs and would be the way to go if I buy another one.

The benefit of IDS (drag reduction) is debatable on a slow flying F5J glider, it's mostly psychological!

 

There are plenty of 'thin wing' servos used for gliders and also frames to fit them in.

 

Not sure if you've deliberately left off the anti-warp spanwise strip above the flap, you can see from my photo that the trailing edge is not supported once the jig tabs are removed from the ribs. 

 

[Ron Gray]

1 hour ago, Gary Binnie said:

is debatable on a slow flying F5J glider

Totally agree but on a scale model the benefit is hidden controls. If the control horns were based on actually aileron / flap ribs duplicated then the correct geometry is easier to obtain.

[Gary Binnie]

True, but on this BT Spitfire the controls are already designed to be internal.

 

The flap linkage replicates the full-size by pushing a spring loaded door open on the upper surface of the wing as an indication to the pilot that the flaps are down (as they are split flaps they can't be seen from the cockpit). Using IDS for the flaps would lose the scale detail.  

 

The undercarriage has a similar system with pins that protrude from the wing, not replicated in the model.

 

I've used a cut off servo horn for the aileron drive and the flap horns were made from Paxolin, I open up the linkage holes in stages to ensure there is no lost motion (0.1 mm at a time).