DB Spitfire Build

[Tim Flyer]

Hi all. After spending the last month finishing some never seeming to end household diy project I have finally decided to make a start on my DB Spitfire. I had bought it in the summer.. The purpose of this build is to make a good flying “stand off scale” model I’m afraid I don’t have the ability or time to create a masterpiece. It will be covered with film/glass and brush painted with enamel.

Getting started had taken a while as It wax interrupted you various other projects including an Acrowot XL that I hope to use for my B test once normal flying resumes.

Other considerations that delayed my start were the various decisions of powering this model plus thinking about what retracts I would like to use. I initially planned on using my trusty Laser 180 which was in my SG Hurricane and now happily resides in my Acrowot.

I have also been following Jon from Laser engines’ in line twin project with interest. With firewall modifications the new Laser 200 inline could provide a very impressive power plant for this model. Using a single cylinder inverted 180 glow engine would also provide some challenge in terms of needing to lower the fuel tank. I have looked at chicken hopper tank fitting or possibly pressure feeding. Jon has also suggested using his petrol 180 which removes the risk of tank height sensitivity. At the moment I’m undecided but due to simplicity and cost considerations I might well choose the 180 single cylinder Glow power. Anyway I have decided to progress the build and make major power decisions a bit later .
Here is the initial wing assembly. The kit quality is really excellent and all the balsa slots in easily. The strong ply bits needed some minor filing. Today I put the main wing frame together as below. I used brushed on PVA in order to give me enough time to slot it together and weigh it down on the table and put in the two wash out jigs.

excuse me if the build is slow but I will update as I go

 

 

Edited By Tim Flyer on 14/12/2020 21:21:48

[Nick Somerville]

Very interested in the build and especially how the engine choice pans out. Jon’s new inline design is going to require a similar commitment from builders as he has invested in developing them.

[Tim Flyer]

Hi Nick. Yes there will be a fair bit of modification if the new in line is used due to its length. One other factor for me is that I would rather try to keep the wing as a two piece unit due to storage limitations. With the in line the wing would probably work better structurally as a one piece.
When I make a final decision it will be based on all factors added together. The inline engine would certainly give much better performance with its extra power and would minimise the need for extra nose weight. However as you say that had to be weighed up against the extra effort to adapt the model plus the obvious higher cost .

[Tim Flyer]

I did a little bit more today putting in the front wing brace box and rear fixing.

Removing the small ply parts from their laser cut sheets had been a bit tiresome but I have now resorted to using my power scroll saw which makes it much faster and way easier.

I also had to adjust the retract plate as it wasn’t fully bedded in when I assembled the wing yesterday. As I did the wing gluing with PVA, quick application of the heat gun made adjustment and re gluing easy. The manufacturer recommends PVA rather than cyano and I have to agree. In areas where I may be sanding I will be using my usual Tite Bond aliphatic glue which is excellent. On high stress areas such as retract supports and the wing brace box I will also go over with epoxy later . By the way epoxy with ground cotton makes super strong light weight structural filler.

I also went engine shopping today . I bought a Laser 180 which will power this plane nicely. This will be my third Laser 180 so I must like them! The 180 will also fit in my Hurricane so reduces the need to swap the engine from my Acrowot. I chose the glow version as all my planes are glow and I like the simplicity. The install will also be simple and I will just lower the tank as much as is practical. I didn’t consider a two stroke as I prefer the four stroke sound.

Edited By Tim Flyer on 15/12/2020 17:20:36

Edited By Tim Flyer on 15/12/2020 17:21:08

[Tim Flyer]

I sheeted the top of the left wing today after finishing off the aileron.
The straight through wire design on the aileron is a good idea and better than hinges in that if allows relatively easy aileron removal for finishing and later possible repair.

Wing parts are well made in the kit and fit quite nicely. The trailing edge of some of the ribs in the flap area is extremely vulnerable to damage. Two ribs in particular have snapped a number of times. They can easily be repaired and are certainly not critical.

After hearing about the risk of sloppiness in the internal aileron mechanism, I decided to use simple direct actuation via a control horn and an external pushrod. I am by mounting the servos in plastic servo holders which I think are made by Radio Active and are quite a neat solution and save some time and weight versus making my own mounts . The model is going to be stand off scale so I do not wish to trade functionality for scale looks.

Here is a picture so far !

[Tim Flyer]

I will have plenty of trimming to do later on the sheeting !

I also had a stroke of luck in the week as John at Unitract International mailed me to say that he finally had my retracts finished. That was great as I didn’t expect to get them in time for the build. Anyway they are beautifully made and a million times better quality than any of my other gear, The Oleos have excellent damping( my HK ones do not have any) . I think Unitract are one of the few manufacturers with properly damped Oleos. The other thing is that the geometry with cranked leg angle (pintle??) perfectly matches the DB Spitfire for which they are made.

[LHR Dave]

Watching the build with great interest, I too purchased this kit earlier in the year. I do not want to start until I have much of the hardware purchased ( engine, serves , retracts and gear etc). I have recently bought some Lado electric retracts from eBay but need to either make ir buy the Oleo,s. The retracts are 90 degree but really should be around 93 degrees, like you I don't intend on a scale masterpiece but would like it to be the best I can get it, What are your thoughts on the 90 degree units.?

Dave

[Tim Flyer]

Hi Dave. Thanks for your interest. Good to hear a fellow builder of this model . One thing I noticed on the retracts is that the are “cranked “ so will move forward when extended (I think that’s called “pintle angle” that should reduce nose over possibility. Here is a picture of them .

[Tim Flyer]

The oleo needs turning in the picture but you can see the angle. I don’t think the 90deg vs 95deg makes a big difference though.

[LHR Dave]

The Unitract retracts do look exceptional quality I guess you get what you pay for !. As I said earlier I didn't want to start my kit untill I had all the fixtures and fittings which are a big outlay for this Spitfire. Looking at the excellent Unitract retracts I am thinking now on selling my Lado electric ones and putting the money towards the Unitracts.I'm assuming that they are air operated ?. I also would like to purchase a Laser 180 but again will have to wait untill I have the funds available. Keep this excellent build thread going I will contribute wherever I can.

Dave

[Tim Flyer]

Cheers Dave will do . I will also post up pictures of the retracts going in etc when I get a bit further . They are specific to this model and given the time and effort to build it I think they will be well worth their cost. They are air operated and come with all bits minus the air bottle. John mentioned I could use a Coca Cola 1//2 litre bottle which I will try. This is the first time I have used air retracts. My other models have electronic HK retracts so this is also an education for me. The oleos are much thicker and stronger than HK ones. The quality of the Unitract gear is like Laser Engines! Obviously these units are dear but they are had made from quality aluminium. They also come with full instructions which is helpful.

[Tim Flyer]

I’m a bit further on now with the top of the wing sheeted wing tip glued . I built both wing tips at the same time. They are fairly simple laminated sections with a ply reinforcement around the outside. The part numbering was slightly different from plan but fairly obvious where each bit goes. Careful sanding will be done with the tip glued on the wing. I have also started on the retract well and cut the ribs and added the new balsa outer walls. On my bench I am curving the balsa wheel wells around some tape packages after lightly wetting it . The wheel size for this model is 4.5 inches which is good . I bought a pair of nice lightweight treaded Dubro wheels from RC World and they arrived rapidly as usual!

Edited By Tim Flyer on 30/12/2020 14:48:35

[Tim Flyer]

[LHR Dave]

Good work Tim, I was wondering how the retract system tubing would fit in with the wing attachments. One other question , the instructions mention glueing up the wing frame work in one session, did you do this or can it be done in a few sessions ?

[Tim Flyer]

Thanks Dave . I did do the basic frame as they suggested in one session. A very important thing is to use a needle file first and do a “dry run” with the plywood parts. I found they needed plenty of filing especially the main laminated ribs and retract support wedges. If you use pva like they suggest and make a mistake, a heat gun can be used to pull it apart. I found the retract platform wasn’t quite right so used a heat gun to ease the PVA and then was able to easily correct that and add more glue. Most of my gluing is done with Titebond aliphatic but I’m also using the odd bit of cyano which it great for tacking bits together and useful in the wheel wells.
I plan to test the air in the retracts very soon so I can check the wheel wells are deep enough. There is plenty of space for air retract tubing ( remember the upper part of the wing between the first two wigs isn’t sheeted).

Edited By Tim Flyer on 30/12/2020 20:10:28

[Tim Flyer]

I have glued in the wheel bay and sanded it flush and reduced height of internal ribs to clear the wheel. I will have further fettling when the retracts are mounted to give clearance when fully inboard. The other possibility is adding small ply shims under the retract plates. There seems to be plenty of room to do that and lift the aluminium mount plates a bit above the ply mounting . Before doing that I will need to connect up tubes etc and test the retract operation range. The full open position seems to be adjustable. I have also glued in my own aileron servo mounts . I’m certainly pleased I used that system rather than the more complex internal one.

[Tim Flyer]

[Jon H]

Looks great Tim, and coming on fast!

On your retracts, do you have the leg raking forward or back? I ask as many of these DB spits i see seem to have their legs pointing almost straight down out of the wing and that isnt right. Spits do have a decent fwd rake on them so im curious to see how it all works out.

Your servo hatch is also in almost the right place for the outer machine gun. If you wanted, you could make the servo hatch look like the gun access hatch on the full size and hide it that way.

You will need to be very careful sheeting the bottom of the wing. A guy i knew built one of these wings and the washout on each panel was miles off. I think one was a half inch more than the other wing and both were wrong. We had to strip the sheeting and do it all again.

When i build mine i will sheet the bottom first, pin it down on washout jigs, and then sheet the top. I have done wings this way for years and never had one go wrong. Sheeting the top on jigs and then trying to do the bottom in free air with no support is a recipe for disaster in my eyes as it would be very easy to pull it out of line with only the top sheeting in place.

Im super keen to get started on mine. I just dont have the room for it in the the hangar

[Tim Flyer]

Hi Jon . Thank you. Hopefully I can keep the washout in order or it might be the trouser press! My workshop is heated at fairly constant temperature so humidity isn’t a problem and as long as I don’t take too long over it I hope to get a reasonable wing profile. I do think your way logical , but as this is my first Spitfire built up wing I have been a bit of a slave to the instructions. Your suggestion to use a regular aileron actuator was much appreciated!

I think the retracts should angle nicely forward when extended as you mention. That should work better and also look better than straight versions. Unitract have worked out a nice pintle angle for them . I pleased I didn’t have to do it . It seems harder than understanding wheel travel on multi pivot suspension on bikes😬. I just need to get my plastic drinks bottle out and connect up and test the up and down to do the fine fettling .

Edited By Tim Flyer on 31/12/2020 18:58:45

[Jon H]

Its not the humidity that twists them its the forces applied when sticking the sheets down. Its really easy to add an unwanted twist.

In fairness though, i have never tried it myself, it just didnt look good to me so i stuck to the method i knew would work

[Tim Flyer]

I just started sheeting the bottom wing today . I have added an access bay where the aileron servo lead joins the extension as the routing is a bit convoluted and I didn’t want to install a paper tube lead tube . The retract bay cutting will be started once the glue has set and before the next sheets are added.

I will later on glass the inside surface of the upper sheeting in the split flap bay with thin matting to add rigidity as suggested in the instructions. I also plan to glass the wings on this model .

[Tim Flyer]

The top and bottom of the wing are now both sheeted . Next task is fitting the flaps . Then it’s on to repeating if all again for the second wing as I don’t have space to build two wings at a time.

One slightly frustrating thing is that I can’t really test the retracts yet. I would like to do it to test the angles and wheel well depth etc. The barbed air fittings the flaps came with are designed for a single fitting and did not include quick release “bayonet connections”. If I am to fit and test them I will probably need extra tubing plus quick release fittings etc . As I’m new to air retracts I didn’t have any spares. These fittings will be necessary on the finished model too unless I want to keep the wings on permanently ! The standard barbed connectors I don’t think would appreciate being connected and disconnected repeatedly.

Flap installation is slightly complicated but I have spent a few hours thinking about the installation. The flexible flap tube is held to the wing by “plastic coated electrical wire” and diagonal holes are drilled in the rib to accommodate this. I don’t like the sound of that and will probably make a small and narrow aluminium saddle clamp instead. I’m also using some plastic tubing to act as a bearing where the torque rod runs through the rib into the small flap dowel. As the instructions say the flap area will certainly need some reinforcement.

 

Edited By Tim Flyer on 13/01/2021 18:35:09

[Tim Flyer]

typo in my previous post should read.....The barbed air fittings the “RETRACTS “came with ar

[Jon H]

On the bench i use a syringe with some pipe on it to kick the locks out on air retracts. I can then move them by hand and use the syringe to kick the locks back in when i get sick of them flapping about. Poking them with a screwdriver can work, but you can end up scratching things up so using the syringe to give enough pressure to move the locks is a better bet.

Beyond that, i always try and mount my air tank and all the other retracts gubbins in/on the wing as it means i can work on the retracts without the fuselage falling all over the work bench. With the whole lot contained within the wing its so much easier to deal with and saves a pointless connection (aka, point of failure) to the fuselage.

[Tim Flyer]

Thanks very much Jon for you’d good advice . As I mentioned these are my first air retracts so it’s a learning experience. Sounds a very good idea keeping them connected. My split wing might complicate it a bit...