|Sep 04, 2012, 04:50 PM|
Attach upper fuselage framework to lower fuselage shell
Before the lower shell was lifted from the building board, a series of jigs were built as advised described in the building notes on the plan. A pair equally sized legs were been used to support 6 spruce cross beams one at a time to make sure that they are all level. A series of vertical pillars have then been fixed from the cross beams to the bottom of the lower shell. The cross beams have been fitted below ribs B, E & H on both sides. These are fitted to ensure that when the half shell is released and turned the right way up that it will sit perfectly flat on the building board.
This is the first plan from Chris Golds’ that I have built and I am quite impressed with some of the details such as this one, which otherwise means you could end up with a slightly twisted model as there is some tension in the lower sheeting that will not be balanced until the upper surfaces are attached and sheeted.
The lower half shell was lifted from the building board and the sheeting edges trimmed back a bit. The completed half shell is relatively rigid from front to back and side to side but still allows some twisting as the ribs remain flexible. This is now turned the right way up on the building board to allow the next stage of the build to commence.
The plans said that the upper fuselage was attached to the lower half next and then the undercarriage mounting plates added afterwards. When I thought about the build through the next few stages I realised that after the upper fuselage framework was added I would not be able to get the nose leg plate installed. I am not sure if this is because of the changes I have made to the upper ribs and the facts I have built them as a single piece or if it is a minor flaw in the plan. Therefore, I moved on to building the nose leg plate, which also acts as the battery tray and the avionics (for want of a better word), the Rx, UBEC, ESCs etc. This is fabricated from the pre-cut light ply parts, forming a rigid box for the nose leg built into the plate.
The lower half of rib A (central rib) was cut along the designated lines (in fact laser stich cut already made this easy). However, when the plate was attempted to be fit it was noted that the front location shown on the plan would not allow the nose leg to locate at the position shown on the plan and the front of the plate would not sit low enough. Looking at the other build logs confirms other people have also moved the plate back or had to sand down the front of the plate. Therefore, I moved the whole plate cack about ¾” (20mm) and it now sat level on Rib A. Once glued into position a small cut out was made in the upper Rub A and the upper framework fitted. This confirmed it would have been impossible to add the nose leg plate after the upper frame was attached.
The upper framework was then glued into position and this was done stage wise to allow easier alignment. The first stage was to glue to leading edges together with PVA and much pinning / clamping. Once this was fully set, the trailing edges were glued again with PVA, there was enough flexibility to get the glue nozzle between the TEs, and once again the whole lot was pinned and clamped.
The rib halves were then progressively glued together using thin cyano to give the final completed fuselage framework with lower sheeting. Now the two sets of framework had been glued together the whole assembly stays much straighter than when they were both free, obviously the residual stresses in both halves has matched the other in opposite directions and cancelled out. The unit is very stiff in the side to side and front to back directions when exposed to bending forces, but it has some movement when twisted, which should reduce when the upper surfaces are sheeted.
|Sep 16, 2012, 06:01 PM|
There has been no progress on the build and there will not be for some time as work has taken me away from home for a while. In the interim here is an update on my recent purchase of fans for the model.
The prototype build in 1999 was based on 70mm Wemotec Mini fans driven by brushed motors, giving about 450g (1lb) thrust per fan consuming some 200W per fan. The weight of the original power system was 2.3kg (5lb 1oz) and the flying weight was 4.5kg (9lb 14oz). This gave the specific power as 80W/lb and a thrust: weight ratio of 0.40lb/lb. Both of these are low, but given the slow flying nature of the plane were OK and it has been said that the prototype did take off from grass.
I have decided to use 4 x 55mm fans to reduce the height and width of the nacelles as already seen in the build. I have bought 4 of the new all carbon rotor Vasa 55 fans which have been tested up to 750g (26oz) of thrust (http://www.rcgroups.com/forums/showthread.php?t=1549178). With 750g (26oz) thrust from each of the fans, the total thrust will be 3.0kg (6lb 10oz) or 65% higher than the original model. Getting up to 750g (26oz) thrust is going to require about 400W per motor, twice the power input of the original model. This indicates the one downside of reducing the fan diameter and increasing efflux velocity; 2.0 times the power input is giving only 1.65 times the thrust despite the efficiency gain of the brushless motor over the brushed equivalent.
The other advantage with the modern power system is the reduction in weight. My power system weighs 1.25kg (2lb 12oz), a saving of 1.05kg (2lb 5oz) over the prototype. Therefore, I have set 3.63kg (8lb) as a target weight, which gives a specific power input of 200W/lb and a thrust: weight ratio of 0.83lb/lb, both of which are much more than required for this model. But it will leave me with thrust to spare in case I do have problems with the air flow into the fans with the scale engine housings or other problems / weight gain.
The only thing I am not sure of yet is should I cut the intake lip off and flair the intake into the main fan body as the entrance to the nacelles will be rounded. How I have all this nice carbon it seems wrong to cut it. I could try and leave the bell mouths on and taper the inlet ducting into the bell mouth.
|Oct 27, 2012, 03:02 PM|
Start wing panel framework
Well I am back home and into the build again and have moved on to the two outer wing panels, which I am going to make removable so the plane will fit into my car. The wing framework is quite straightforward in that it has a leading edge strip, a pair of main spars and a pair of trailing edge spars all connected together by a number of ribs and a wing outer edge strip.
The main difficulty though is the note on the plan that it is very important to have ¾” (19mm) of washout at the trailing edge of the wing tip. I wanted to build my wing panels upside down so I could sheet the bottom surface whilst they are still on the building board, which means that upside down washout means the wing tip trailing edges need to be lower down. Therefore, I have set the leading edge strip ¾” (19mm) above the building board on a number of blocks. The outer edge trip is then glued onto the leading edge strip, with the trailing end on the board. This gives me the correct washout in the correct direction when the wing is turned the right way up. The inboard rib is set into position with its centreline horizontal. The lowermost spar is then glued between the inboard rib and outer edge and packed up so that all the other ribs then move steadily lower at the trailing edge to give a smooth increase in the washout along the length of the wing. It just so happens that this means the lowermost spar is horizontal along its length. A couple of packing bits were required between the spar and some ribs as the cut outs are not of uniform depth.
Once the lowermost spar was set, everything could be glued in place to complete the right hand outer wing panel ready for sheeting.
|Oct 30, 2012, 06:33 PM|
Complete wing panel framework
Thanks for the comments, really appreciated
The next stage was to sheet the underside of the wing panel whilst it was still held down to the building blocks to hold the shape. PVA (white glue) and lots of pins and some lead did for this. I then completed the framework for the other wing built over a copy of the plan on greaseproof (Butchers) paper and got that to the same stage. When the sheeting was dry I lifted the shells from the building board and then built the outboard elevon directly onto the sheeting as this was already formed to the correct shape, especially the twist of the washout. Trying to build these flat would have caused some mismatch. I have also built the lower drag rudder half. The elevon and drag rudder could then be detached from the wing.
There was a bit of cutting and sanding to get the elevon leading edge to fit, the sides were tapered to fit against the sheeting as was the trailing edge. When initially gluing the elevon and drag rudder thin cyano was applied in very small amounts from a fine nozzle just to tack everything together and hold its in shape. I did not want the cyano wicking through to end up gluing the elevon leading edge to the wing traling edge. The elevons and drag rudder were separated by simply running a long scalpel blade down the join as not glue had penetrated. One split off from the wing then the main gluing could be completed.
The plan does not call for a trailing edge on the drag rudders but I have added one for a bit more strength. The plan also shows some diagonal members on the elevons to keep them from flexing, but the plan left them as open structures tissue covered and I am sheeting mine so these are left off.
|Nov 04, 2012, 11:46 AM|
I realised that I needed the upper drag brakes completing, I could not wait till I sheeted the top wing, I needed them before that for servo mounting before the top wing is sheeted. Therefore I clamped the elevons back onto the wing panels and then pinned down a spare piece of sheet onto the wing to follow the correct curve and then built up the drag rudder frame onto the sheet. As before, this was tacked on with cyano using a very fine nozzle and then properly glued once it was released from the wing. I believe this is better than building them flat due to the washout in the wing.
I then cleaned up the wing panels, all the control surfaces and all around the fuselage as I had left the sheets protruding from the framework. I then clamped everything together for a first trial assembly in the workshop. 8ft (2.4m) wing span puts my work area into perspective. Things look good but one thing is evident, the outermost ribs on the fuselage are not vertical, they lean in a bit and the outer wing panels are not horizontal. That will be easy to solve with an extra layer of rib added on and then sanded vertical.
|Nov 05, 2012, 05:31 PM|
Thanks for all the compliments, I am enjoying the build. Thinking about the wing joiners next, then onto u/c, servo installtion and then the fans so a bit of a change of direction for the remainder of the year.
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