Top Flite P51 - 65" span

[Peter Jenkins]

I finally found a use for my LiPos which are all at storage charge!

Underneath that pile of LiPos is the tailplane with the top sheeting glued in place and the LiPos making it stick! I shall steam a curve into the bottom skins so that there is less of a drama in trying to get the skin to adhere to the ribs and spars! Funny how much you forget if you don't build something for some time!

[Chris Freeman 3]

Hi Peter

I found that the manual was very informative and that the kit is very well thought out. A mustang is one of the few war birds that does not need nose weight so the retracting tail wheel is not a bad idea. I made fibre glass parts in the plastic kit parts, I just used PVA release and then made my parts in the plastic. They are a little smaller then but came out quite well and will not be as brittle as the plastic.

The main goal that I set myself with this build was to see what sort of finish I could get using plastic film and not going the glass and paint route which can add quite a bit of weight. By using the manual directions I got what I thought was a good finish and a great flying aircraft. The cowl was sprayed white and the black covering was ironed onto the paint and has stayed in position. A mustang really is a treat to fly if it is done right, even the retracts and flaps do not need mixing when used.

[Tim Flyer]

Very impressive indeed Chris.! I must say I also used the plastic film route on my recent Chipmunk build and it certainly was easier for me and I think was probably turned out better than my efforts at painting. By the way does anyone know if this Top Flite Mustang kit is still available. I have a spare OS95v and a Laser 80 so maybe I should have a go too ?  

Edited By Tim Flyer on 03/04/2020 10:21:00

[Peter Jenkins]

Oooh, that does look nice Chris. I see you went for the second u/c door. Any tips you'd like to share on doing the job once?

With an OS120 FS up front, a retractable tail wheel and doors should help with getting the CG in the right place. What engine are you using and did you have a custom silencer made?

[Peter Jenkins]

Tim, I've not been able to find any sign of TopFlite kits. I bought mine when it was offered on the BMFA Classifieds site. As it happens, I was offered the chance of buying a second one and did so. That's my insurance policy or my 2and airframe with a different identity.

[Peter Jenkins]

Today, progress was limited to fixing the bond between skins and some ribs as well as the skin to LE join. Needed my entire stock of spring clamps, so just ordered a load more. Used a razor plane to plane off the excess sheet to level with the LE.

3 ribs were clearly too low on the underside after taking off the tabs and sanding the ribs and TE and LE flush. So, some remedial action was required. Back to the left over balsa sheets to cut some soft balsa as packing. Next job will be to send the ribs to shape, cut out the sheet covering the balance tabs before gluing the 2 bottom skins in place. Hope to show some pictures after tomorrow's session.

[Tim Flyer]

Good idea to buy two Peter. I must admit to having done that before too

[Peter Jenkins]

Well, I finally managed to get some time in the shed today.

First of all, I was surprised to see that most of the ribs on the underside of the tailplane seemed to have very little camber. After consulting the plan it was clear that the tailplane was symmetrical so I grabbed a sheet of balsa out of which I'd popped the tailplane ribs and cut some 1/8" strip. I then cut these to size and glued them to the top of the ribs that seemed to have a flat undersurface profile as you can see below.

I sanded these to the required section and then cut out the sheet that covered the structure into which the balance tab would fit. I then glued on the previously glued together sheeting. It was only after the glue had set that I remembered I wanted to fit some balsa blocks to drill into for fixing the Robart hinges!!!!!

So, after carefully marking where I needed to cut our the sheet, I bit the bullet and cut! The 1/8" Robart pin hinges had arrived just after lunch and proved to be exactly the right size. So, I decided that 3 hinges per elevator would be fine and made up 4 blocks from 1/2" sheet to fit the space. The third hinge block will be the block I glued in to the inboard end of the balance tab cutout. Having got the blocks glued in it now looks like this.

Once the glue had dried, I added the last bit of sheeting and clamped it up to dry. While it was drying, I took the time to put the 3 sizes of hinge pin side by side and it quite surprised me how big the difference in size was! The grey one on the left is 3/16", the black one in the middle is the 1/8" one, and the black one on the right is the 3/32" one.

Once the glue had dried, I used a razor plane to cut back the LE sheet so that it came level with the LE spar. It now looks like this, although since I've taken the picture, I've put a bit of light weight filler over the 2 cut outs I had to make to get the hinge blocks in! It now looks more like a stabiliser!

Tomorrow, I'll glue on the tip blocks, sand the LE to the required shape and figure out how best to sand the rear stab structure so that it has a concave structure as well as shaping the elevator leading edges.

[Peter Jenkins]

Just to clarify on the hinging of the control surfaces, this is what I am trying to achieve.  The black + marks the hinge point for the control surface.

As it happens, the 1/16" square stringers are not really needed so I will not be using them when hinging the rudder and ailerons. Using a bit of scrap, I mocked up what this might look like and checked that it worked OK and that 's the next photo.

It all works fine provided that you have properly radiused the control surface LE. As I was to find later, it is best done by trial and error! Isn't it always?

So, I then drilled the elevators using my home make centre drilling jig and then shaped the leading edge. The photo below shows one elevator LE shaped and one about to be. Using the long Permagrit sanding block makes the job very easy to achieve.

As you will see on the right most hole in the lower elevator, I have cut a clearance slot either side of the hinge pin head. That allows the hinge to operate without fouling the balsa.

The next job was to form the tailplane trialing edge to create a hemspherical socket for the elevator to fit into. This proved to be a real pain and that decided me on altering the construction for future control surfaces as I mentioned earlier. So, here's the first elevator in place with the pin hinges dry engaged. They won't get stuck in place till I've covered the Mustang.

As you can see, the hinge is pretty close to the full size and a lot closer to scale than the normal V hinge. Finally, here are both the elevators dry connected, the tips sanded to shape (final sanding still required) and the balance tab slots in the tailplane cut out. I will be gluing the balance tabs on in my next session and also sorting out the elevator saddle/horn and dry fit that. That needs to move aft by 1/8" so that it sits on the new hinge line.

Edited By Peter Jenkins on 06/04/2020 18:51:46

Edited By Peter Jenkins on 06/04/2020 18:52:41

[Ron Gray]

Nice attention to detail Peter

[Peter Jenkins]

Thanks Ron.

[Bruce Collinson]

Watching with interest Peter.

Those long reach spring clamps are not ones I have tripped over before but look useful. Where from please?

BTC

[Peter Jenkins]

Posted by Bruce Collinson on 06/04/2020 20:37:07:

Watching with interest Peter.

Those long reach spring clamps are not ones I have tripped over before but look useful. Where from please?

BTC

Sorry Bruce, I've had them for so long that I can't remember where I got them. They are prone to falling apart! That is the yellow pad tends to detach just as you are about to clamp something up! I prefer the ordinary spring clamp that you can see in some of the photos and will no doubt see when I start on the wing!

[Peter Jenkins]

There has been a slight delay while I ponder two issues. The first is the placement of the main retracts and the second is the issue of using Oracover Chrome. On the issue of the retracts, as I'm going to be using two doors, I need to make sure I have a workable solution before starting the wing build. This is what I'm going to have to accommodate:

The blue board, in between the 2 wheels, is the Turnigy retract and door timing controller while the red coloured box connected to the retract is the Robart control box for their electric retracts. After pondering how I might combine these two contollers, a light went on in my brain - yes, I know! The blue control box will control the door servos and provide the timing while the Robart control box will control the retracts and both will be on a Y lead originating from the Gear Channel of the Rx. The door servo will move much more quickly than the retract servo and there is a slight delay in the retract beginning to operate once the command has been issued.

To be absolutely sure this will all work in practice, I've decided to build a mockup of the part of the wing from the centre line to the end of the retract. This will allow me to check how deeply to position the retract so that the door attached to the leg lies flush with the wing and secondly to construct the inner gear door and test out the geometry for the servo linkage to open and close the door. My gut feel is that I might need a fairly powerful digital servo as the inner doors are quite large and may require a bit of force to operate in flight as well as to hold the commanded position against any blow back. That indicates a digital rather than an analogue servo to avoid any chance of blow back.

There is no clear statement as to whether Oracover Chrome will block the radio signal so I will build a small box, cover it with Chrome, install the Rx and aerials inside it and then see is the Rx range is still the same as the original range in range check mode. If this fails, then the option will be to hide the Rx aerials on the external skin of the fuselage using some of the markings to hide the aerial. My first thoughts are to put some black trim to simulate the canopy track behind the canopy and the black squadron numbers to provide the orthogonal requirement for aerial mounting. The only problem will be the secondary Rx which on my 11 Ch JR DMSS set up has no external aerials. The secondary Rx also provides the telemetry transmission so that will let me know if the Rx transmission is working as well during the test.

Edited By Peter Jenkins on 09/04/2020 23:17:00

[Peter Jenkins]

I did some more research on the use of Oracover Chrome and found the following thread on another forum that indicates that practical experience with a "covering" called Flite Metal (which is real aluminium sheet) found that:

After the initial test of the Futaba© system it became apparent receiver & antenna placement was of little concern because evaluation results were the same as when the system sat outside of plane on a board. Please re-read that sentence again before continuing.

I will be doing my own tests but it is good to see that others have found virtually no effect on 2.4 GHz radio when enclosed by aluminum sheet itself.

[Peter Jenkins]

Well, after a bit of guesstimation, looking at a small drawing of the underside of a Mustang, I drew up a design for the inner door. That then allowed me to set to and build a mock up of the wing where the retract sits. That's when I found that all the balsa I had didn't quite allow me to go ahead and skin the top and bottom with 1/16 sheet as the design calls for! This shows the beginning of the mock up.

This is what the mock up now looks like, with all the spars and LE glued in. I've also glued in a thick balsa sandwich next to the root rib and will use that for the Robart Pin hinges that I intend to use to hinge the inner doors.

My final job was to use the actual ply ribs that support the balsa ribs to hold the retract servo mount and then lay one of the retracts in position. It all looks quite good.

Once the glue has dried overnight on the mockup structure, I'll set to and carve out the ribs inboard from the retract mount to allow clearance for the leg door. I'm also looking at how best to reinforce the u/c bay and to place balsa supports for both the leg and inner doors while adding some strength to compensate for the large amount of skin and structure that will have to be removed. I was impressed with how strong the mock up felt although without the skins and shear webs it was relatively easy to twist. Once sheeted with the webs in place, I expect the torsion strength of this part of the wing to be high. However, the transmission of landing loads to the wing structure is worrying me a little and I am considering reinforcing the ribs holding the retracts by using either some ply or carbon strip from under the retracts to the rear of the wing to distribute the load more widely.

Once I get the retract and wheel in the next task will be to check that the door servo and retract operate in the right order and that there is no fouling during operation. Then, I'll be able to start on the wing construction proper.

[Tim A]

Hi Peter, watching with interest, I have an unfinished TN P51 shelved for the last two years while other jobs took precedent. I have the same HK servo timer as you, it does work well in sequencing retracts and gear doors including closing/opening inner doors with retracts up/down. The project stopped at this same point you are at now, trying to work out how best to arrange inner door mounting/hinging. OCD doesn't help either!

Thanks to lockdown I hope to get it back on the bench soon.

[Peter Jenkins]

Tim, that's interesting. It does take a bit of thinking about in order not to make a hash of things like this. It is all too easy to make an assumption and find that was wrong just as the glue sets! Good luck on getting going again. I have to say, that's one of the reasons I started this blog so that I would be forced to keep going! The other one was the lock down of course and not being able to fly!

[Tim A]

Peter, two years of thinking & a viral lockdown might be stretching it! I will be watching.

[Peter Jenkins]

The mockup is as complete as it's going to get and so I installed the retract with strut and wheel. After a bit of butchery to the 2 ribs to the inboard side of the u/c mounting I had the u/c in place.

The value of this trial fit was immediately apparent as you will see from the next photo.

As I hope you can see the slope of the retract mount, in the foreground, has resulted in the wheel being at the same angle and therefore the rear part sits above where the wing sheeting would be! My first thought was to twist the wheel to bring it level but that would mean the wheel when extended would have a fair amount of toe out, like about 15 degrees!

So, the solution seems to be to alter the slope of the retract mounting and that will mean dropping the position of the rear mounting to allow the retract to sit a bit more level and thus get the wheel inside the wing and allow space for the inner door to close. That will mean having to modify the two ply ribs that are currently die cut with the existing sloping arrangement.

I have a template for the inner door and will cut that out in card initially and then in 1 mm ply to start the process of working out how that will be hinged and actuated.

[Ron Gray]

Nice work Peter and shows the value of a mock-up. Be careful if you lower the rear of the retract unit as the angle of the u/c leg, when down maybe too far back, you may have to not only lower the retract unit but rotate it as well.

[cymaz]

Would having a slightly smaller and thinner wheel go some way to dropping the height, or is that too much to ask of the design?

[Peter Jenkins]

Hi Ron, thanks for that warning. However, luckily the main axis of the retract is parallel to the main spar so it can be rotated without affecting the fore/aft position of the wheel - unlike a Spits for example. I will do some measurement of the required angle and then draw up what the new u/c formers should look like. I can then cut out the existing formers and build the new geometry into the mock-up and check that works as intended.

Hi Cymaz, good idea but the wheel is already smaller than scale in order to fit the Robart struts. The scale size is 4" and these wheels are 3 1/4 ". It would also lose the scale effect of the wheel tyre and spokes. As I noted above to Ron, a bit of a geometry change should do the trick. I might also lower the u/c mounting by either a 1/16" or 1/8" to give the leg door a chance to sit flush with the wing skin. Re aligning the mounting rails will also mean the leg door will sit at the wrong angle so may need the extra thickness to allow them to be sanded down at the front to follow the wing skin profile.

[Peter Jenkins]

Here's a head of view of the mock-up showing the problem.

[Peter Jenkins]

Having carefully measured the slope of the main wheel against the rib behind it, it looked like a 10 deg difference. In theory, therefore, tilting the retract support rails by 10 deg should produce the required effect. Here's what one of the retract support formers looked like against the original when I'd finished.

Having carefully parted the retract bearers from the two ribs, to allow the rest of the surgery to proceed, I then bolted the retracts to the bearers and offered them up as a dry fit to the mock-up and kept making minor adjustments to clearances to clear the bits hanging below the retracts until it all fitted. So, from the top it looked like this:

The retract strut came that length and as the full 4" wheel plus its strut is longer I have got a compromise in the strut length so that the set up is now between the 3 1/2 " and the 4 " wheel positions. The main thing was that the wheel was now within the wing completely and there were no bits of retract hanging below the ribs so the bottom (or rather the top) skin would be untroubled.

This next view as from the retract end to show the wheel clearance I now have.  Incidentally, the wheel is now far enough into the wing to allow the retract to drop the wheel slightly from the position you see as when the wing is the right way up the weight of the wheel will cause the retract mechanism to droop slighty.  Phew!

Finally, I checked that the leg door would sit on top of the wing unhindered - which it did!

The only issue is that the door is 1/8" and the skins are 1/16". I've just sent off for some 1/6" ply to make up some new doors and to make the inner doors.

I spent some time last night reading a thread on the RCU forum on the 1/7 TopFlite P51D. They had exactly the same problem with the retracts but I also picked up some info on making doors using fibreglass. The trick was to put some Monokote on the wing and then build up a fibreglass sheet above the covering. Once the fibreglass was dry, you iron the whole lot and and peel off the Monokote and there you have 2 doors that are to the correct wing contour! As I have some carbon cloth to hand, I might try using that in lieu of fibreglass.

Well, that's a major hurdle cleared for me. I still have to sort the inner gear door and its hinging. An interesting point from the RCU thread was that to avoid exhaust ingress to the wing centre section, the builder decided to hinge the inner door from the W1 rib - not scale but saves a lot of aggravation! That might be the way to go. We shall see!

Edited By Peter Jenkins on 14/04/2020 16:13:10