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Top Flite P51 - 65" span

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Adrian Smith 129/05/2020 12:14:31
2470 forum posts
1273 photos

Like the progress, Peter.

I have a Robart Incidence meter and I wouldn't without it. I has thrown up some interesting numbers on one or two of my smaller ARTFs! That's why I like Extreme Flight's bigger ARTFs because enclosed in the box are the incidences of all wings of that particular kit.

Adrian Smith 129/05/2020 12:19:01
2470 forum posts
1273 photos

like so.


Peter Jenkins29/05/2020 12:32:27
1603 forum posts
266 photos

Yes EF are good in providing detail. Some F3A kits come without this vital information but I've not found that to be the case with the ones that I have had. As you may know, most F3A models come with wing and tailplane incidence adjusters and you can play tunes on the motor up/down and side thrust as well which is when having an incidence gauge such as Robart's is essential if you want to know your starting and end points.

I was just surprised at the accuracy of the Mustang's wing incidence. Of course, once the model is finished I shall just have to live with this as changing the incidence, if required, will compromise the scale appearance! F3A models are provided with adjustable wing and tailplane incidences on the whole although some don't necessarily come with the initial set up data! Resorting to the various fora does produce the answers although many will tweak the incidences to eliminate any aileron or elevator trim required to fly level. One can also play tunes on the motor side and vertical thrust lines much more easily.

It will be interesting to see how good the Mustang set up is once it's in flight.

Edited By Peter Jenkins on 29/05/2020 12:33:06

Peter Jenkins29/05/2020 21:53:03
1603 forum posts
266 photos

Not a very productive day today! It began by me tracking down a user manual for the OS 120 FS. It was clear that turning the carb through 180 deg to put the throttle arm in the correct place was relatively easy. So, order of work was to do that first and then sort out the firewall to get the holes for the engine mount drilled.

Having rotated the carb, I found that I had made the classic mistake of forgetting that the fuselage was upside down on the building board and that the carb had been in the right place all along. So, turn the carb back to the original position! While I had tools to hand, I decided it would be a good idea to check valve clearances. That gave the minimum reading for valve clearance so that was fine. Nothing like a bit of unnecessary work!

I then carefully marked up the firewall as per the instructions. There are punch holes for the two centre lines for the firewall and also the displaced centre line so that the spinner comes out in the right place given the side thrust. I'm using a Dubro Softmount for the OS 120 and after carefully examining the mount it looked like I would have to drill and tap the mount for the 5 mm engine retention bolts. That then sent me back to do a Google search for the correct size of drill required for a 5 mm tap. I had the tap but not the 4.2 mm drill. I took a look at the UK Drills website and as my Bosch drill set is now well past the time it needed sharpening, a bit of retail therapy had a boxed set of cobalt drills and a 4.2 mm drill (almost half the cost of the boxed set!) on its way. Hope they'll be here for Monday next so that I can then drill out and tap the engine mounting holes in the Softmount. Never having mounted an engine like this before, I shall have to make very certain that the holes are drilled in exactly the right place!

Back to the shed to drill out the holes for the Dubro mount and forgot they would, being American, be imperial sized. Still a 5 mm drill hole is not too far off the required 3/16" hole required. Having drilled the holes I used the T nuts mounted with the smooth side facing the bolt to bolt the the mount to the firewall. With both mounts "bolted" in place I did feel that the 3/32" firewall was a bit flexible. It will be glued to another 3/32" plywood former that I had previously glued to the fuselage. In the end, I decided that I'd like to make another firewall but out of 1/8" ply as that felt a little bit less prone to flexing. Currently, the combined formers will be 7/32" thick and will be backed up by some 1/8" ply pieces for the T nuts to bite into. I'll see how the glued up formers feel and will decide then whether an additional full width 1/8" ply sheet would be advisable to reinforce the firewall further.

I then used the existing firewall as a template to cut a replacement one out of 1/8" ply taking the opportunity of making it a better fit than the original piece. I drew in the 2 centre lines and the displaced centre line. That allowed me mark out the required width of 46.6 mm for the OS 120 crankcase to be a snug fit between the bearers. I measured up and marked out the position for the four bolts on the firewall and trial fitted the soft mounts. Pleased to say that when looking down each mount hole, the cross marking the bolt hole was centred.

mustang 109.jpg

mustang 110.jpg

As you can see, I have also marked out the centre line on the 2 engine mounts as well as the bolt hole centres on the lower of the 2 arms.

Dubro state that before using the mount, all 4 rubber "grommets" should be removed and washed in glow fuel before being returned and bolted up. I presume that is to provide some form of lubrication to allow them to work more easily. There is no information about the life of these rubber inserts so that might be something to find out about.

Once I've got the engine mounted, I'll return to the manual and install the fin and tailplane.

Peter Jenkins31/05/2020 00:45:49
1603 forum posts
266 photos

I spent another day looking at the engine installation and, in particular, the routing of the throttle pushrod.

Productive work was limited to drilling all the engine mounting holes, which I'm pleased to report were all spot on - the first time I've achieved that! The increase in stiffness of the 1/8" ply was noticeable once I'd bolted the softmount to the firewall and then clamped the engine in place.

It was immediately apparent that my burblings of yesterday about the carb being correctly setup were wrong. It's amazing how difficult it is to get an inverted engine mounting sorted out when the fuselage is upside down! Anyway, I repositioned the carb for the 3rd time and once the engine was clamped into place it became clear that there was a problem with getting the throttle pushrod to the throttle arm. The tank and softmount arm are in the way of a direct route so after much deliberation it looks like I will have to misuse a servo arm to create a bell crank to route the throttle rod run through 180 deg. I haven't worked out whether to glue a small hardwood block to the softmount arm or drill and tap the arm to take a short bolt to act as the servo arm bearing. I don't like the idea of drilling and tapping into the metal arm so I think a hardwood block will be the way to go.

There is a cavernous space for the tank. I'm using a 12 oz tank as I don't really want much more than 12 min flying time. However, I do need to have the option to increase the tank size so my method of tank positioning is to use some blue foam (from packaging!) to construct a tank size hole that will position the tank in the right place. The trial fit showed that I need to have a piece of string tied around the tank to pull it back against the grip of the foam!

All that was left to do was to glue in the firewall and the third part of former 1 which gives the rounded top for the top cowling and the plywood supports for the T nuts.

mustang 111.jpg

I've left it to dry overnight and will then drill through the rest of the firewall and pull in the T nuts. Since the lower mounting bolts will be pointing straight at the tank, I might cut them to being only just longer than the T nut. That will allow the soft mount to be bolted on and the throttle linkage mocked up.

Ron Gray31/05/2020 07:03:16
2186 forum posts
942 photos

Peter, for your throttle pushrod tried Bowden cable instead as you can route that round obstacles.

Peter Jenkins31/05/2020 17:10:27
1603 forum posts
266 photos

Good point Ron. Have ordered a Bowden inner and outer combo.

Peter Jenkins31/05/2020 20:55:48
1603 forum posts
266 photos

I managed to finish off fixing the captive nut ply pads to the rear of the firewall and bolted the Dubro Softmount into place.

mustang 112.jpg

I then couldn’t resist plonking the OS 120 onto the bearers. Pleased to say that it drops in smoothly with minimal side to side movement. I have a 4.2 mm drill coming on Monday which I will need to enable a 5 mm thread to be cut in the softmount arms. I’ve never bolted an engine in place using a threaded metal bearer. I have some Nordlock washers that I will use under the bolt head to hold the bolt in place. For those who have not come across Nordlock washers, they have 2 parts that have a some ramps built in so rotation of the bolt or nut has to generate a lot of torque to ride up over the ramp before any rotation can occur.

By eyeballing the space available for the throttle push rod, using a bowden cable will solve all the problems I was considering before.  Thanks Ron.

mustang 113.jpg

Ooh! This is getting exciting!

Having now dealt with the front end engineering, I can now turn back to fixing the tail feathers in place.

Edited By Peter Jenkins on 31/05/2020 20:56:35

Edited By Peter Jenkins on 31/05/2020 20:58:36

Peter Jenkins04/06/2020 23:49:46
1603 forum posts
266 photos

Well, despite wanting to get going on the tail feathers, I thought I'd face up the problem that I was not looking forward to dealing wtih. The soft mount has a sloping bottom to the beam so it looked like I would have to drill and tap a 5 mm hole and bolt the engine to it That means that any error in drilling the holes spells trouble unlike a traditional bolt retained with a nut - you can just drill a slightly bigger 'ole!

I thought I'd practice with the plastic engine mount supplied with the kit. I used my Dead Centre drill to mark the two holes on one side and then drilled the holes. Unfortunately, this proved that the Dead Centre wasn't - or rather my use of the Dead Centre was not accurate enough - clearly I wasn't holding it absolutely vertical. So after two failures, I resorted to measuring and drilling and that was much better but still not exactly spot on.

Taking a deep breath, I took the first soft mount and, after a great deal of measuring and line drawing, I eventually drilled the first hole. I decided to start with a 1.5 mm drill as any inaccuracy in that one could be sorted with the next size up. The holes were lined up at 5 mm in from the edge of the bearer facing the engine. The holes are 25 mm apart. Having drilled out to 4.2 mm it was time for the tap to be brought into play. After a bit of faffing around, I eventually had two 5 mm threaded holes. I offered up the engine and found, to my great surprise and relief, that both bolts screwed in without any distress!

mustang 114.jpg

The second bearer was a bit easier to complete but it was still a bit nerve wracking. Pleased to say that side also worked out fine and better still, with both bearers bolted into place, all 4 bolts went in with just fingers!

mustang 115.jpg

I only had one bolt of the right length so the others stick out a bit!

With the bolts out, this is what the hole alignment looks like.

mustang 117.jpg

mustang 118.jpg

I'm greatly relieved with the outcome.

I had one job to finish off on the wing and that was to sort out the ailerons. After cutting them to fit between the flap and wing tip, the next task was to drill the holes for the Robart pin hinges in the wings and ailerons. If you remember, I'm aiming to have near scale hinging so the wing sheeting extends approx 1/8" beyond the end of the wing in the aileron bay. I then rounded off the front of the ailerons, dry fitted the hinges, relieved the aileron to allow the hinges to move within the LE and plugged the ailerons into the wing. I had to sand a bit off the ailerons and the wing in order to get the fit right. Here's what they look like now.

mustang 119.jpg

mustang 120.jpg

Both move smoothly and have enough movement to suit even the high rates quoted in the manual.

Hinging will be completed after the model has been covered.

Ron Gray05/06/2020 06:52:52
2186 forum posts
942 photos

Nice accurate work Peter.

Adrian Smith 105/06/2020 08:48:11
2470 forum posts
1273 photos

Precision drilling there, Peter. It's a definite art form and something you can be proud of.

I well remember dreading that job when I was flying I/C aircraft and there were a few hash ups on the way. embarrassed

Peter Jenkins05/06/2020 09:53:36
1603 forum posts
266 photos

Thanks Ron and Adrian.

It helps enormously that I have a pillar drill but it's a cheapo one and you can just see the tip of the drill wobbling slightly. I often wonder if it drills straight so this was the ultimate test which it seems to have passed. Will give it a pat on the drill head later for a job well done!

Nigel R05/06/2020 10:37:48
3938 forum posts
687 photos

Good work Peter. Getting the holes just right in a mount can be a touchy process.

I'm a little surprised the dead centre didn't work out for you, this is really its raisin d'etre.

I use a scribe (I have a very old compass with a long point that works well) to trace the inside of the mount hole, on plastic and the glass nylon mounts, then put a centre dint in to start the drill in the right place. Even after that the drill sometimes wanders a little, so as you do, I start small and open up in several goes.

That said, I've never tackled an aluminium one.

Peter Jenkins05/06/2020 16:56:40
1603 forum posts
266 photos

Hi Nigel R, yes, I too was surprised that the dead centre didn't! I think it might have been that I was a tad off centre while holding it as the top of the OS 120 cylinder is quite wide. That coupled with the thickness of the mounting lug is probably all that's needed to push you out a tad which is not accurate enough when making a tapped hole in metal for engine mounting. I didn't want to get the drilling wrong and have to buy a new soft mount so went back to measuring and marking and that worked very well.

The dead centre works well for servos as there is no difficulty in getting vertical. That and the thickness of the servo mount being a lot less than that of the OS probably accounts for it. There is the other view, I just made a pigs of using the dead centre!

Tim Flyer06/06/2020 09:37:11
1290 forum posts
236 photos

Nigel . When you are drilling engine mounting holes I hope you are using centre drill bits to start the hole such as this ?

The problem with engine mount drilling is often the Mark is in the correct spot if a @dead centre “ marker was used, but thin drill bits will wander when starting a new hole. If you use a proper centre drill bit on a pillar drill and gradually increase diameter to the one required you should have no problems with drill bits wandering off line . Centre drill bits are really important bits of kit as starting a hole in metal plate with a standard drill bit invariably will lead to an off centre hole due to the torque of the drill( very similar to why our planes try to drift leftwards on takeoff) .


Edited By Tim Flyer on 06/06/2020 09:38:39

Edited By Tim Flyer on 06/06/2020 09:45:01

Peter Jenkins06/06/2020 10:37:29
1603 forum posts
266 photos

Good call Tim. Does starting with a 1.5 mm drill to then step up via 4 increasingly larger drills to 4.2 mm provide the same accuracy or does the increased stiffness of a centre drill with a 1.5 mm starter but, say, a 4.2 mm finisher have greater accuracy as well as reducing the task to drilling once?

Edited By Peter Jenkins on 06/06/2020 10:37:51

Ron Gray06/06/2020 10:54:17
2186 forum posts
942 photos

Peter, when I'm drilling on the lathe or pillar drill I always use a centre drill as Tim has noted. You cannot guarantee accuracy when starting with a thin drill bit and stepping up, for the very small cost involved invest in some centre drills, you won't regret it!

Tim Flyer06/06/2020 14:41:07
1290 forum posts
236 photos

As Ron said the first starter hole is most important. If that’s wrong all the other holes will be out. The centre drill bits are really important as thin drill bits rarely go in straight into metal . As a centre drill is short and fat with an exaggerated pitch it does go in much much more accurately than a standard drill bit. Another tipis also using a centre punch to properly mark the point after the initial mark is made. That way it’s easy to start drilling on the right spot.

Peter Jenkins07/06/2020 02:24:20
1603 forum posts
266 photos

Thanks guys. Centre drills ordered. Tim, I know I should have centre pucnched the bally thing but I couldn't find my centre punch at the time! No doubt it will turn up when I'm clearing up after the buld!

Peter Jenkins07/06/2020 02:33:22
1603 forum posts
266 photos

I thought you might be interested to see the engine bolted down with screws of the proper lenght! They arrived a couple of days ago courtesy of Model Fixings. I bought two lengths: 20 mm and 25 mm given the slope to the bottom of the engine bearers. Looks like the 20 mm one is fine for the front but the 25 mm one is needed for the rear.

mustang 121.jpg

It's amazing how many little jobs keep cropping up and need to be sorted before I get to gluing the tail plane to the fuselage!

I had to silver solder a threaded end onto the elevator pushrod after cutting it to size. The next jobs were to do the aerodynamic balance for the elevator and rudder and hinge the rudder. I've already posted the tail plane and elevators so here's the fin and rudder. I also took the opportunity to drill some holes in the heavy sheet that forms the rudder shape. Forgot to weigh it before and after the drilling so I don't know how much this saved but it is as far back as you can get so any saving must be good!

mustang 122.jpg

If I can't find anything else that needs doing I might actually manage to glue in the tail plane and fin tomorrow!

My Bowden cable has arrived so I can at last tackle the throttle control run as well.

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