PeterF's Chris Golds Northrop B2-A stealth bomber
I have always been fascinated by unusual planes like the B2 model since I started flying about 12 years ago. The Chris Golds B2 plan has always struck my eye as something I wanted to build. Many years ago Traplet offered a free plan or plan at a reduced cost to readers of RC Model World and I thought about getting the B2 but in the end I went for something a little more normal as I was still in my beginnings of model flying.
Anyway, I do not know what prompted me to look into this again, I have just finished my Brian Taylor Mosquito and was looking for something different to build and I recalled the B2. I searched around and found several good links on RC Groups and also found SDB Models (who have now stopped trading) offering a laser cut short kit. Cutting sheet balsa and ply is my bête noir and I am always more than happy to go the short kit route rather than cutting myself.
One of the things I want to do with the build is to make the top section more scale like, with smaller engine housings both in height and width. I will not be aiming for real true scale because I do not want to compromise what is a well proven flying platform, so the majority of the build will be as per plan, just the upper surface being changed. The upper body section is definitely cartoon scale and in the Electric Flight International articles from 1999 Chris Golds makes no bones about this. This was a compromise required at that point in time to get a working fan system installed when power availability from NiCd cells and brushed motors was very limited. Making the fans sit lower with a smaller intake area could have compromised the available thrust, which was quite limited in the first place. I have attached a few photos of the original model that are scattered around RC Groups to show what that looked like.
I have done a fair amount of searching for information about this plane over the past few weeks as I looked into options for modern power systems and reworking the top of the plane. This has provided me with some excellent information and ideas about tackling various parts of the build. One thing I will blatantly copy from all of the builds is removable outer wing panels to make transport less of a headache. The builds show 2 different ways of achieving the break point, not sure which one I will follow yet. Another of the builds has given me some ideas on the modification of the engine housings to get closer to scale.
There are two good build logs for this model on RC Groups, one from LotusTRBO dated 2002 (http://www.rcgroups.com/forums/showthread.php?t=48476) and one from Boulybooly dated 2010 (http://www.rcgroups.com/forums/showthread.php?t=1343497). Both of these threads have helped understand the build of the plane but in both cases the build has followed the original plan. The first thread being from 2002 did not have much chance to improve on the power set up as brushless motors and Lipo cells had not developed very far at that stage. The second referenced build has however swapped the lower power 4 x 70mm fans from the original design with 2 x 70mm fans of higher power although the engine area has remained to the original plan.
There is a third reference to a build on RC Groups, but the build photos are not on RC Groups. This is by Peter Bech from 2006 (http://www.rcgroups.com/forums/showthread.php?t=572655) and the build photos can be found at the Danish RC Jets site (http://www.electricjets.dk/edfdata.htm#1:B2 e-konstruktion and http://www.electricjets.dk/pbilleser...bum/index.html). This is interesting because the fans have been changed to high power brushless units but the 4 x 70mm fan configuration has been retained. These fan / motor combinations generate a colossal 6kg thrust (bench data) for a 4kg plane in flying condition that is not a fast model. Anyway, the interesting thing with this build is that it shows the top of the model has been modified a little compared to the original plan to have some curvature over the engines and cockpit and a trough between the cockpit and engines. From the photos it appears as if the fans have been set lower down in the fuselage resulting in the top of the engine housings being slightly lower than the top of the cockpit. Lowering the fans down into the fuselage will probably reduce the installed thrust, but given the very high bench figures available this should not matter. However, having decided to retain the 4 x 70mm fan arrangement there is only a limited amount of change that can be made in the area of scale similarity.
Robert May has contributed to many B2 threads over the years with little snippets thrown in here and there including some thoughts on updating the model with newer power set ups. This included projections for 4 x Vasa 55 fans using rules of thumb confirming that a 3.4kg (7.5lb) model should be doable with the older version of the fan. Now with the new all carbon rotor in the Vasa 55 it is possible to go to higher thrust and consider powering this 96” model with. I will make a post covering the whole issue of power systems in the near future.
If any of the named past contributors come across this thread then I thank you for taking the time to put your thoughts down on RC Groups and providing useful ideas, information and inspiration for my build.
good luck i'll be watching ....i've been on the full scale for over 25 years. yeah the nacelles need a lot of work ,and it appears that the airfoil does'nt have enough reflex as they have a ton of up elev in it .you need to look at "Hepdogs " thread ...his was a pusher but his has really close scale lines to it . not sure what airfoil he used .i'll give you colors ...fs 36099 is the grey and the "walkway lines are FS 37200. the line on the real plane are 1" wide so scale that to what ever scale your build is .have fun ....some day i'll do one ...really scale ;) you might be able to fly this with just 2 70mm fans ...that would make scale nacelles easier to do .
Thanks for the information on colours, that is useful and I will have a look at Hepdog's build.
2 x 70 mm fans would certainly fly this and is one of the options I am considering, they would still need installing deeper into the fuselage than the original plan to get the top of the nacelles lower down. I am also looking at 4 x 55mm fans as another viable option and need to spend some more time looking into this. More on that choice as the build progresses.
I had also wondered about reflex, because the build does not have any apart from some washout in the wing tips and it states the elevator neutral is set in line with the wing. There is a flying shot in the Electric Flight International article that does not show reflex on the elevons and the photo on the grass shows the elevons at neutral. Then the photo I found on tarmac shows a whole bunch of reflex on the elevons and the EFI article has similar photos with the reflex. Most confusing. I am up for modifying the nacelles but I am not going to start messeing around with the airfoil to add reflex because we know it flys even if it is not perfect aerodynamically, if it needs it it will have to be on the elevons.
Preparation and start of the build
The first stage was to obtain the laser cut wood set and plans and at the same time find and purchase back copies of the original Electric Flight International articles in April and May 1999 along with plan release in June 1999. The EFI articles are not detailed build logs but have a large number of photos that help in understanding the build and the plan and give some useful guidance in some areas. Some areas of the plan are not terribly clear and these articles help.
There are a set of build notes on the 2nd sheet of the plans containing a lot of useful bits of information. However, I agree with some comments, this is a builders kit for experienced modellers not a perfectly executed plan that holds your hand all the way. There is even a comment that builders need to be experienced and capable of “cutting and gluing their way out of trouble”.
Additionally there are lots of notes jotted around the main plan that need to be considered if this is going to fly right covering some of the things people have fallen foul of with B2 models. Getting the C of G correct, making sure the undercarriage legs are in the noted position so that the model rotates, making sure the washout is in the wingtips etc.
The plan calls for the fuselage section to be built in 4 quarter shells and the EFI articles show this approach. However, following the example set by Peter Bech (http://www.electricjets.dk/pbilleser...bum/index.html) I am going to make the fuselage as two half shells, an upper and a lower shell. The main reason for this in my mind is that the central sheeting will not have a join at the centre of the fuselage on the bottom of the model, increasing the strength and making for a neater build.
I have started off by making a building board to match the size of the fuselage section and attached this to a stiff frame. Shims have been placed between the frame and board to make sure it is flat. The half plan has been attached to the board and greaseproof paper used to trace the reverse plan for the right hand side of the plane. The whole thing then covered with plan protector.
The first part of the plane to be built is the lower half shell of the fuselage. As the ribs are so long at the centre of the fuselage they have been laser cut in two parts, a front and back section so these have been joined first. A totally conventional build then starts with the lower rib halves pinned down to the plan (upside down) along with leading and trailing edges. A few of the ribs had notches for the stringers missing, these are not on the plan so the laser cutting has not missed them. This post ends with the leading edges notched for the ribs and the first trailing edge section having been cut. One of the photos shows most of the ribs set vertical into the leading edges to give some idea of the size of the build, the board is about 90cm x 130cm (36” x 52”). I wish I had a central bench in my workshop that I could walk all the way around, it would make this easier, but I am sure I will manage.
Lower fuselage framework
The remainder of the trailing edge sections were cut, well oversize because at this stage I was not quite sure how they height varied along the body, it is certainly not a constant shape along the length. Once completed all the ribs and leading trailing edges were pinned down and glued. I have a sheet of allegedly glue proof plan protector over the plan, it is quite glue proof but not totally glue proof as I learned later on.
The stringers (or are they spars on this build ?) were then glued onto the ribs to keep everything straight and start to build up the rigidity. The section between the main body of the plane and the wing section has quite a steep profile change between the 5th and 6th ribs and the 1/4" balsa did not want to bend to follow this so I lap jointed two pieces and will profile them later. All gluing has been with cyano to keep the added weight down.
Changing the engine housings will only affect the upper fuselage shell so there are no changes to the lower ribs / stringers.
How about adding the engine aux air doors? That would allow for a more scale intake design since they would allow for extra airflow to the fans, kind of like a cheater hole, but not really.:D
Don't be forgetting to build in Gold's "signature" 1/4" control surface gaps :-)
Jet ...out here everybody that works on the north side of base call it a "cockroach" cuase all they can see is the top of the fuse from over there.( it looks like the top of a bug )yeah im at south base ,if you get here come over and ask for a tour:rolleyes: maybe it could happen.
I had thought about adding the aux air doors, couple of micro servos programmed to high throttle, a switch or landing gear down, I love little gizmos and things like that, will keep it in mind as I go along.
As for the hinge gaps, I'll probably give that one a miss, I had thought about doing concealed hinging rather than the top edge hinging on the plan but with the inboard elevon being tapered along its length I am not so sure yet, need so furtehr building before I decide.
Complete lower fuselage framework
Once the framework was released from the building board, the LE and TE strips were planed to shape, the Les curved and the TEs tapered.
The final stage for the upper fuselage framework was to make the lower half of the elevons, so pin the framework back down over the plan and then make up the elevons from ribs, LE and TE pieces, pretty simple really. The only issue is keeping the LE shaped correctly as it does not taper linearly along its length.
Once completed I decided to check if I needed a new car or not. Alas, I will have to keep the car for a few more years as the new model fits. However, it does demonstrate that I will need to have removable outer panels unlike the original plan which is built as one piece.
Start upper fuselage framework
I have started on the upper fuselage framework. As the photos in the 1st post show, the model is quite boxy at the top, having a flat upper surface from the centre of the fuselage out to the formers outboard of the fan units. The 1st photo of the ribs before joining the front and back sections shows them in order from the mid line down to the outboard rib on the fuselage section. The 4th rib is the one outboard of the engine nacelle and the inner 4 ribs are all of the same height and flat topped.
The second photo shows the cut lines I have developed to make the model closer to scale – not exact scale but it will be much more convincing. The mid line ribs are OK apart from at the nose where they should have some aerofoil section before the cockpit rises upwards rather than the cockpit sloping directly up from the leading edge. The third ribs down are the ones on the centreline of the nacelles and these will be cut lower as the nacelles are lower than the central cockpit bulge. The ribs either side of the nacelles (2nd and 4th down) are made much lower, closer to the aerofoil shape on the body.
The final photo shows the ribs cut down. I still need to do some work on the front of the rib on the mid line of the nacelle (3rd down) to get the ramps down to the fans arranged as a single ramp down with both inlets in line rather than staggered ramps as per the original model.
Start assembly of upper fuselage framework
At the time of writing Scott at SDB models has one of the laser cut short kits left if anyone is inspired to try this model. See http://sdbmodels.com/For_Sale.html
The upper framework construction pretty much follows the same procedure as the lower framework and was built as a single contrusction over a doubled up plan rather than as two halves. Therefore, both A ribs were glued tgether to start the build followed by cutting out the leading and trailing edges, notching them for the ribs and assembling them over the plan. Then the spars are added to firm up the framework. There is some difference to the bottom half of the fuselage in that the rear spars cannot cross the engine exhausts. In addition, with the aim of making the nacelles more scale like I am going to have to improvise with the spars over this area.
At the moment I have just put some bits of ¼” strip to show how the top of the plane will flow from the wing, over the nacelles and then to the cockpit. This is clearly different to the original plan and I am making some of this up as I go along.
A few mods I have made and I should mention are visible in the final photo.
1. The front of ribs A to D between the LE and the spar have all been sanded to he same height and shape as rib D so that the aerofoil in front of both fans is level over thw widht of the nacelle, otherwise the inner fan would have had a smaller opening as the ribs got taller towards the centreline.
2. I have added an extra short strip of ¼” strip balsa at 90° to the ribs betwen ribs B and D to act as the front support for the ramps down into the fan intakes.
3. I have cut a small hole through rib A and let the front spar into this hole, gluing the ends of both spars together in this hole.
Continue assembly of upper fuselage framework
One thing I have not mentioned but is obvious from the photos is that the ribs are supplied complete with all of the lightening holes cut. As I have cut down the ribs, I was faced with either needing to cut a lot of my own ribs or the choice I have made to glue sections of the discs back into the lightening holes. I am not sure I need to worry too much about stress raisers at some of these odd shapes. These shapes would be a real point of crack initiation in a real plane.
The other thing I forgot in the last post was a close up of the joint between the front spar and ribs A on the centre line as I am keeping the leading edge airfoil on the centreline unlike the original plan. I have attached it to this post.
The other area I have been working on recently is continuing to add the spars to the upper framework and making the cardboard templates for the spars that will go over the cockpit and nacelles. My intent to cut these shaped spars from ¼” ply as this will be better at transferring the flying loads over the top of the fuselage. I had thought about making them up from sections of ¼” balsa carved to shape, this will be a fallback if the ply option does not pan out.
The nacelles are still a little wider than scale even though I have reduced them down a lot from the original plan. I could make them narrower to get closer to scale but the area will then start being reduced below the FSA of the fan. At the moment I am aiming for 100% FSA in the inlet area, which allows for the fact that there will be a ramp down to the fans that starts in front of the nacelle entrance. I could go to a lower fraction of the FSA but as this is a slow flying plane I am more interested in Static and low speed thrust rather than dynamic thrust so I am trying to keep the flow areas high.
Complete upper fuselage framework
The next stage was to cut out the shaped spar sections from the cardboard templates to go over the nacelles and cockpit. When I thought about the sections over the cockpit I realised that as these are very curved they would not transfer the flight loads from one side of the plane to the other very well. The original design has a large flat top over the whole of both nacelles and cockpit. Therefore, I let in a section of ¼” square hard balsa at the same level as the nacelles, passing through ribs B, A and B and being a straight piece will be OK for flight loads. The photos show how I have let in the straight section for the loads.
As the section over the engines has to clear the fans when installed I had to make them ¼” square and profiled underneath, the picture will show what I mean. For these sections to be able to carry the flight loads successfully I cut them from ¼” birch ply. These are then epoxied onto the spars to give a good joint even if there are some small gaps.
The shaped sections over the cockpit are then only required to support the sheeting over the cockpit and not provide strength, so these have been cut from ¼” light ply. I have made the shaped formers for the removable hatch at the front of the cockpit section to allow access to the batteries and these are put away until later.
Having completed the shaped spars over the nacelles and cockpit the upper framework has been lifted from the board. This has allowed the upper and lower fuselage frames to be dry fitted to see how they marry up, and they go together well. There are a couple of small mismatches that will need to be sorted out but nothing troublesome. I have then shaped the Leading and trailing edges ready for sheeting. It is starting to look good and the difference from the original plan is starting to be visible.
After making up the spar sections from ply and all the work on the ribs I was wondering if this should be in the scratch built section, it is sort of a hybrid.
Sheet the lower fuselage framework
Attention has now returned to the lower fuselage section which now needs to be sheeted. The build notes on the plan and QEFI articles state that the sheeting is made up in one large section for the left and right hand sides and to paraphrase “beaten into submission” to get it to fit to the ribs. There are some compound curves on the bottom of the fuselage section which to my mind made this potentially difficult for my skill level. Therefore, rather than sheeting each side with a complete sheet I have decided to build it up piecemeal. Close inspection of the QEFU article also showed that the sheeting was made up with all of the sheet grain running in one direction, with the join on the centreline. My plan as described and shown in the photographs has the grain in several directions and importantly, the main bottom section runs over the mid line, hopefully strengthening this area.
The first section to sheet was the leading edges, with the sheet running parallel to the main spars and the leading edge. I have made the front sheet from 1/16” (1.6mm) medium balsa rather than the soft balsa listed in the plan to add some more strength to the body. The rest of the sheet is soft balsa. The framework was lightly pinned down to the building board at the leading edges and PVA was run over the ribs and spars. The sheeting was then applied to the framework and formed around the curved shapes, pinning it down as I went along. The worst point was where the deeper body section blends into the shallower wing section. In this case I have used some 2kg (4.5lb) persuaders to keep the sheet pressed into the PVA on the ribs and the ribs grounded against the building board.
The second central sheeting area was attached next and this used 1/16” (1.6mm) soft balsa as per the plan with the grain at 90° to the formers. As the rear sheeting was going to follow the direction of the trailing edge this central section was diamond shaped and needing careful trimming / sanding to fit against the leading edge sheet. Small balsa tabs were glued to the rear edge of the first set of sheeting to ensure that the second sheet butted neatly to the first sheet (see photo). As the second sheet was applied the butt joint between the two sheets was glued with cyano to ensure that the two sets of sheeting did not pop apart or form a step joint if things relaxed while the PVA was setting. The rear edges of the second sheet were also tacked down with cyano.
The third set of sheeting followed the same method as the first two using the1/16” (1.6mm) soft balsa. I ended up with one split in the rear sheeting, which I feel is much less than would have occurred had I tried to sheet each side with one single sheet.
The sheeting of the lower fuselage half is now complete and ready to be lifted back off the building board for attachment to the top half of the fuselage framework.
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