Top Flite P51 - 65" span

[Peter Jenkins]

Quite so Ron!

Not a lot to report on today other than to say that both wing panels are now ready for joining so that will be tomorrow's job. Will have to use the main building board to set up the correct dihedral angle with the supplied wing jig set up. They are, in fact, two 1/2" thick bits of balsa wood which go under the two inner most wing ribs!

I also checked out that both retracts worked without any binding. That did require a bit of relieving of the structure to allow the wheels adequate clearance to exit and enter the wheel wells cleanly. I have a video of the operation but cannot find a way of loading it and, in any case, it's no big deal. I will be working on the secondary doors while the wing join is setting.

[Chris Freeman 3]

Nice work, making good progress.

[DaveyP]

Great work Peter I'm looking forward to the wing joining, it's something I always seem to struggle to get right......

[Peter Jenkins]

Thanks guys. Davey, I'm not sure I'm looking forward to the wing joining! Still, one step at a time with a triple check, cup of coffee, further double check, before applying glue will work well - I hope.

[Peter Jenkins]

Today was focused on getting the wings glued together. Before that, I wanted to reinforce ribs 4 and 5 that carry the u/c rails with carbon fibre. Once I’d done that I followed the instructions for wing joining very carefully. I had let the ply reinforcement of Rib 2 block the gap the dihedral brace needed to go through to pass along the main spars to Rib 3. That was quite a tricky job to saw out the ply without damaging the wing spar or the rib! Doh!

Having dry assembled the two wings, the two dihedral braces, the ply cross member that joined the 2 wings towards the rear, the next job was to make two sticks to take to the two Rib 2s to make sure they were held parallel during the gluing operation. The kit is also provided with two die cut planks 3/8” wide, 1.5 “ deep and 7” long that are placed under both Rib 2s with the wing inverted. Both tips, which still have the rib tabs in place, rest on the table top.

As everything seemed to be in order, I mixed up some 30 min epoxy and put the plan into action! As soon as both wings and dihedral braces were engaged, I tack glued the two sticks at the front and rear of Rib 2 with cyano, pinned the tip ribs down to the board and left the assembly to dry! I hope that tomorrow, when I remove the pins holding the wing tips down they will still sit flat on the board! That will mean that both wings are at the same incidence. If for any reason they are not, then sheeting will provide another chance either to correct any warps or, heaven forbid, to introduce others!

You can see the carbon fibre on the inside of Rib 5 in this picture. Rib 4's reinforcement is not visible.

Tomorrow's job will be to build the ply centre section that will fill the gap you can see between the two Rib 2s and then jiggle that into place and glue it in. Should be another taxing task! If I have the time, I also want to have a go at making a dummy secondary main u/c door on my dummy wing section. That will allow me to try out the geometry and hinging as well as mounting a standard servo to operate both doors. That's the ideal with the fall back to use 2 mini digital servos.

What I've already found is that the power to the main u/c needs to be provided by a separate 2S Lipo as the additional voltage makes the u/c unit faster in operation and prevents the doors closing while the wheels are still in motion. However, it will all come down to seeing how it all operates on the test rig. I'll also have to take into account any drag from the u/c mounted doors that might cause the retraction to slow down a little and lead to a clash with the secondary doors.

As one of the RCU contributors commented, he'd never spent so much time on building the structure that was required to get a set of flying retracts!

[Peter Jenkins]

When I checked the wing this morning, it felt very much stiffer than it had - good! The box spar formed by the dihedral brace, two main spars and webbing on each side gives the structure a great deal of strength and stiffness - just as it should. The other good thing was that the wing sat flat on the tip ribs's tabs. So, no twist then!

The next task was to remove all the rib tabs and sand the wing down to make sure that any high spots were dealt with. In that respect, the 12 inch bar sander from Permagrit was a boon. If you don't have these tools they are really worth getting. They never wear out and the two grades of roughness supplied as standard do for all tasks except final sanding.

The next task was to assemble the rib W1A/B/C structure. This is W1A dry assembled

And this is W1B/C and the other parts dry assembled

Here they are, all glued and ready to go into the wing structure.

The instructions call for the larger rear part to be glued in first. You really need five pairs of hands to do this but eventually I got it all lined up at the expense of having cut the 1/8 spars on the left wing too short and broken one of the spars on the right wing! That will be the first job to make good these errors tomorrow. So, here's the wing with the rear section of W1B/C and the cross brace glued in and clamped up.

After gluing in the front section of W1, the next task will be skinning the wing! That might be OK or another task needing many pairs of hands. I will find out!

[Peter Jenkins]

I thought I'd finish off the wing ready for sheeting it but the little issue of how to operate the secondary u/c doors needed to be dealt with before I glued the front W1A rib structure in place.

I had the idea of using a single servo in the aileron position to operate the doors. My idea was to use a pushrod to go through the main spar assembly and connect to a horn that itself was shaped like a wire saddle you use to operate elevators. The ends of the wire would hold control horns that would then connect to the u/c doors. I expected to silver solder the horns onto the operating wire but this would have to be done in-situ! That didn't seem to good an idea. So my next idea was to use a pair of mini servos (I would have used micro servos if I'd had any to hand) to operate the doors. That then created the problem of where and how to mount the servos as space was very limited and I would need to access the mounting screws.

Whilst I was pondering this issue, I decided to make up the door and started with a 2 mm ply version. Having cut that out, it felt very heavy so I used the 2 mm door to cut out a 1 mm ply door. That was sufficiently flexible to be steamed into the complex curve that exists over this door. I then used some balsa wood to begin the construction of the inner door detail but just did the front edge before turning to the hinging problem. I had bought a load of 3/32 Robart pin hinges that I felt were too small for the elevator so decided to use them. As a quick fix I used some triangle stock to provide a base for the pin hinges and finally added a drop of cyano to hold the hinges in place. The door worked very well although I made the error of gluing the operating horn to the front of the door. The full size has a hydraulic ram at the rear of the door. So this is what the door looks like in the open position using my much hacked about dummy u/c structure>

You can just see the wheel position marked on the open door. Before adding any more scale detail to the inside of the door, I want to put the retracts back into the dummy and check how much clearance there is between the door and the retracted wheel.

This is what the door looks like in the closed position.

After much agonising on where to mount the door servo, I decided to use double sided padded tape to secure the servo and with that fitting there was room to mount it in this position which gives sufficient clearance from the retracted wheel. I will do a full dry run with the door servo hooked up to the door and the retracts in position. It will be interesting to see if the whole sequencing of door and u/c leg is as expected!

Once that's done, I can get on with finishing off the wing structure and start on the sheeting.

[Martian]

looks ingenious

[Peter Jenkins]

Not quite as ingenious as I had thought Martian! Turns out that what was lurking in the back of my head about adjusting servo travel, speed and so on, is cancelled out by the Hobby King sequencer that sits between the Rx and the servos. Anyway, I'm jumping ahead. I had come up with the idea of using servo tape to secure the door servo as there was not enough access to use a traditional servo mount. After some trial with the mini servo in different places in the wheel well, I finally found the ideal place for it.

What you cannot easily see in the photo is that the servo arm was very close to the wing skin line. When I connected the servo to the HK sequencer and turned on the radio, as is usual with the sequencer, the servo went to full travel for door open and fouled the rib. So, move the servo arm one spline and, you guessed it, the servo arm now in the door closed setting but was through the imaginary wing skin! Yes, the HK sequencer does the sequencing magnificently but you have no control over all the other usual servo attributes. What this means is that the servo travel for the secondary door servo is the full travel which, if the wing skin had been in place, the servo horn would have punched through the skin!

The solution was to move the servo down towards the door and that seems to work fine. I then placed the port retract into position and operated the retract and door servo. This showed that unless I took the option to power the Robart controller with a 2S Lipo, the retract speed was a bit too slow to have allowed the wheel on retraction to be sufficiently far down not to have the secondary door potentially to cause a clash with the wheel! The Robart manual states that you can use up to 9 v to power the retract but 8.4 or probably around 8.0 v was sufficient to have the wheel 20 deg further into its retraction before the door servo sprang into life.

I know that you chaps with Taranis Txs will say you can do all of this in the Tx but as I have a JR and have every intention of keeping going with JR, now that they are back in business, that is academic. However, I'm pleased that my investment in a "test stand" has once again proved very helpful in sorting out the retract and door issues. I've also decided to use the 1/8" Robart pin hinges as the 3/32" ones (see photo above) look a little puny.

That then allowed me to glue the final part of the W1 rib construction into place on the wing followed by the balsa centre leading edge die cut part. The centre section now looks like this:

The next task will be to skin the wing top and bottom. That should be fun!

[Peter Jenkins]

Been tied up on life for the past couple of days but got back to the Mustang today.

First, I found a bit of a boo boo when I put the retracts into position in the wing. They were at different distances from the centre line! I eventually worked out how that had happened - managed to rotate the rails through 180 deg when fitting them! So, drilled new holes for the port side units, plugged the redundant holes with epoxy and re-bolted the u/c units into place. They were now equidistant from the centre line and, when extended, they were actually in line with each other! Oh joy! I also dispensed with the packing washers under the outboard end of the port retracts and both units now sat vertical when compared with ground reference. Another oh joy!

The other thing to double check was the clearance to the wing skin. Now, the centre section has to be sheeted with 1/16" balsa before the wing skins are glued on so the photos below will benefit from the inboard end of the wheels having an additional 1/16" of vertical space. Those are the ribs that glue into the piece of wood called CLE or centre Leading Edge in the photos below.

I then turned to gluing together the 4 wing skins. These use the 1/16" x 3" x 36" sheets in the kit. Firstly, I weighed the sheets on my electronic weighing machine which is accurate to 0.1 gm. This showed that there were sheets which fell into the 12 gm bracket and the majority into the 10 gm bracket. So, I selected heavier and lighter sheets to make up each skin and had four bundles at the 46 gm mark. At least this avoids the situation where all the heavy sheets are on one side leading to a lateral unbalance.

The instructions tell you to use a straight edge to true up all the sheet edges before joining with cyano. For once, I used thin cyano as it would instantly hold the edges together as I worked down the sheet edges. The instructions then tell you to put the sheet in place on the wing and then draw around and cut out the wing sheeting with 3/8" extra at the wing tip. Now that I have this sheet, I need to glue up the other 3 skins and transfer this pattern to them as my starting point.

So, tomorrow, I will start gluing the bottom sheets in place. I will then reinforce the skins around the u/c ribs with carbon fibre. I'm not sure how much room there will be to put similar reinforcement on the top skins but I'll cross that bridge when I get there.

[john stones 1 - Moderator]

Coming along well Peter, some work in it.

[Peter Jenkins]

Has its moments John!

[Peter Jenkins]

Life got in the way for a bit but I've now been able to get back to the Mustang. It must be said that I now have an additional reason for cracking on with this build as I've just had one of Richard Wills' Spitfire IX kits delivered!

I've now managed to glue together the 4 pieces of 1/16" sheet to form the 4 wing skins. I've marked out and cut 3 of the 4 skins. The instructions say that you can cover the flaps separately and leave a gap between the flap skin and wing skin to make it easier to cut the flap out. So, that's what I did.

Rather than cover the bottom surface first, I decided to do the top first as then I would be able to add a bit of carbon reinforcement to the upper wing skin that will be in tension when the wheels touch down. Below, you can see the starboard flap skin being glued in while the port skin is already fixed.

I looked at various ways of gluing on the first wing skin. I eventually settled for gluing the leading edge in place and holding it down with some 1/4" strip taped over it to the bottom of the LE. That didn't work so I decided to clamp the 1/4" strip down using the small spring clamps. That didn't work so I ended up using larger clamps you see in the photo below.

As I'm using aliphatic glue, I left this for a couple of hours to dry before gluing the rest of the skin down. My, what a performance that was! At least another pair of hands would have been helpful!

This time, I could only use one clamp as none of them had long enough jaws to enable them to be positioned appropriately. So, it was back to pins. As you will see from the photo above, I cut the wing sheet to leave a very small gap between it and the flap sheeting. I've also left a small (1/8" overlap of the sheet in the aileron bay. This will provide the necessary shroud for the aileron front spar to operate within, as per the elevators.

I had run out of my superphatic glue and was using aliphatic glue which turned out to be much more viscous compared with the superphatic sort. This has pros and cons. The glue is much more viscous and therefore harder to squeeze out of the bottle in the confined space below the wing skin. The good news was that it wouldn't run down the skin because of its increased viscosity. There is clearly room for both types of glue!

[Peter Jenkins]

Today, the postie brought me the decals I'd ordered from Pyramid Models. I'd sent them some photos of the Mustang in 303 Sqn markings and they provided a set of 1/7 scale decals.

The first roundel is for the wings (2 roundels on top and underneath), the yellow edged roundel is for the fuselage, RF D is the Sqn identity, KM 112 is the RAF aircraft number, then the fin flash and at the bottom the Sqn badge and the Polish national marking that was positioned just under the front of the exhaust stacks. I'd better get a crack on then!

Today, I glued on the 2nd top sheet, and then reinforced the sheeting above the u/c fixing rails with carbon fibre. I also finished sheeting the flaps top and bottom.

Before sheeting the underside, I must remember to put in some drawstrings for the Flap, Aileron and u/c retract and door servo leads. Not a big issue if I forget as a small nut on the end of a piece of string will do the job. I have not weighed the wing but it is beginning to fee a tad heavy with half the sheeting on.

You can see the carbon reinforcement in the photo above. The other thing I learned was that I had insufficient weight on the skins to ensure the skin stuck to the ribs around the u/c fixing ribs. I think I might steam the top skins and bend them to shape before gluing them down. Unlike the bottom sheets, I won't be able to sort out any glue failings with both sets of sheeting on!

[Richard Clark 2]

Peter,

Whatever you do keep the weight down. These TopFlite kits are grossly over-engineered and can turn out very heavy. My Spitfire MK9 said 8 pounds on the box but at the end of the instructions they said theirs, covered with film, turned out at 11 pounds. Even the owner of the model shop, an experienced flyer himself, warned me about it but I ignored him. It's very inaccurate too, which the P51 may not be.

My Spitfire, with a tissue and dope finish, was 11 1/2 pounds with an OS91 FS pumped, and flies like the brick it is. As result I don't fly it much. Which renders the whole thing pointless.

Personally I would discard the flaps, secondary u/c doors, and retractable tailwheel,, and thus their servos and the sequencer and if you have it, the cockpit kit.

You may do what you want of course, but I'm real serioua about this.

[Peter Jenkins]

Hi Richard,

All good advice. I have been weighing the parts I have completed to date. The separate wing panels before joining and once the sheeting has been finished I will weigh them again. Your point about about an 11 1/2 lb Spit with a 91 in it are borne out by my own experience flying such a beast for a friend. Basically, a 91 in an 11 1/2 lb aircraft is going to struggle and you can forget looping from level flight without first taking a good run at full throttle or diving slightly before commencing the loop. That was why I went for an OS 120. The fall back will be an OS 155 if it's needed. On the RCU forum there is a very long thread on the TF P51 which I have read pretty much all of. It is clear that with a 120 even 11 1/2 lb has been found to fly well. I am aiming for 11 lb. I won't be glassing just using Oracover. I'm not going over the top on scale detail just the ones I think are very visible (secondary doors, retracting tail wheel and scale control surface hinging).

Before folk jump in and say a 120 is way too big, I would not dream of flying a 2 mtr aerobat (my usual mount) with a 120. It would be at least a 180 and that would be a YS. In fact, I fly electric and my electric aircraft have 2,800 watts (or 3.7 HP) actually available in flight although I rarely use more than 2,500 to 2,600 and that in a vertical climb. I very rarely need full throttle but this amount of power allows large loops to be flown at constant speed. Throttle control is one of the most difficult controls to master in aerobatics and one which I find most newcomers to aerobatics struggle with. So, having power available is for vertical manoeuvres and not full power dives leading to excessive speed leading to structural failure or over powering the elevator servo causing a dent in the earth!

The wing is beginning to feel substantial and I will be looking to save as much weight in the fuselage as possible. The wood in the kit for the wing sheeting is a good balance between lightness and strength. The fuselage, on the other hand, is stuffed full of ply and thick balsa, most of which gets removed it has to be said. However, substituting some of the ply formers with thinner ply or even balsa is on the cards plus judicious use of lightening holes.

Paradoxically, a heavy model is more likely to benefit far more from flaps than a light model where one would hardly need them for landing as the landing speed unflapped would be so low anyway.

So Richard, thank you for your experience with heavy warbirds. I will continue with my plans but be taking great care to lose weight as much as possible especially in the aft fuselage. How many times in a crash has the aft fuselage survived with barely a scratch!

[Nigel R]

Looking good so far Peter, lots of work getting the details in place before closing up the wing sheeting.

How are you planning on keeping the wing straight while doing the underside sheeting?

[Peter Jenkins]

Hi Nigel, there is only the wing jig (2 bits of sheet balsa under the R2 ribs) to keep the wings straight. Still, the top sheeting had no support at all and seems to have worked out OK. I'm hoping I don't induce a warp in either wing at this stage as it will be impossible to cure with the wing fully skinned.

After the struggle with the joined together wing skin on the bottom, I decided to try and bend/warp the top skins. As it happened, some time ago, I'd bought a small steam iron device for pressing clothes that were hanging up. I'd never used it for its proper purpose so decided to hunt it out and try that on the wing skins. Worked a treat! So, having got the skin bent to shape, I squeezed aliphatic all over the ribs, spars and leading and trailing edges and went for it. Using the steam "gun" to help in bedding down the skin worked well but I decided to pin the skin down till it dried. This worked very much better than the TF recommendation so, the port wing skin will be done the same way.

The hand held steamer is the grey and white thing at the middle bottom of the photo.

Tomorrow, the last wing skin goes on and then it's a case of filling in all the gaps in the sheeting with what's left over from the main skins, adding the wing tips and test fitting the ailerons. I will dry hinge them to check that all is well there. The wing gets put to one side after that to build the fuselage and gets completed after it's been mated to the fuselage and wing bolt holes drilled.

[Peter Jenkins]

Oh forgot to say that I just remembered to add the strings for drawing the aileron, flap and retract servo leads through to the centre section. It wouldn't have been a disaster if I had forgotten but nice to have remembered to do it!

[Chris Freeman 3]

Hi Peter

Looking great, the mustang and the P47 are the 2 Top Flight warbirds that carry the weight better than the others. The flaps really make a huge difference to the flying and well worth the effort and weight.

[Peter Jenkins]

Chris,

Thanks for that information. I am committed to working flaps as, like you, I think they will make a big difference. The full size had flap deflections up to 50 deg so I will aim for that. I will continue to build as light as possible by looking carefully at the fuselage construction. I know from building an 80s kit, Gangster 75, they seemed to be built to have crash resilience! In the case of the Gangster, it was both the fuselage and foam wings that were heavy. No doubt a built up wing and less balsa in the fuselage would have produced a sparking performance on a ST 90. The performance for club aerobatics is fine but not for F3A style aeros.

Peter

[Peter Jenkins]

Didn't manage to get much time with the Mustang today. Finished off the wing sheeting with the Port top sheeting. The technique of using the steamer to get the skin to bend worked well today and it only took 40 mins to: finalise the sheet shape, apply aliphatic glue to all the contact points, glue in the leading edge, pin that down, glue the rest of the skin down using the steamer to get the balsa to bend to shape, finishing off with another ration of pins. Then left it to dry for several hours.

The only other job was to cut and glue in 1/16" sheet for the front edge of the wing. I would have liked to have finished off the rear gaps as well but time ran out.

Once all the sheeting is complete and the wing tips glued on, I will weigh the wing with the ailerons.

[Richard Clark 2]

Posted by Peter Jenkins on 13/05/2020 23:28:28:

Once all the sheeting is complete and the wing tips glued on, I will weigh the wing with the ailerons.

That'll be interesting

I just weighed my Spitfire wing. Finished of course (doped on lightweight tissue, lightly sprayed on satin Solarlac colours) not bare wood. Six servos, all low-cost 'standard size' JR 519 ones. Old BVM ex-glow ducted fan mechanical retracts with a servo for each leg (necessary to get a straight pull due to the Spitfire leg angles), and operating only one leg a 'standard' servo is more than adequate, flaps, ailerons.

SEVEN POUNDS!!!

That's considerably more than my complete tricycle retracts, mini 'snap' flaps, tuned pipe equipped , OS AX61 Prettner Curare.

I think from your earlier post we just have a different 'philosophy'. Neither of us is 'wrong'.

Flaps, retracting tail wheel, etc. are nice an' all and make the plane less 'toy like' but ALL weight impairs the performance. It's not a matter of power (the OS91 FS pumped is fine on my Spitfire), as you suggested in your reply to me, it's a matter of inertia. The heavier it is the more it is reluctant to change direction and you get high speed stalls. Any given plane of two different weights the lighter one ALWAYS flies better. Faster too, as the AoA, required for level flight is less.

Re the servo load you mentioned .That was all stuff from a guy who ran a big model shop/importer and pushed the then very expensive JR digital servos he sold because of their 'holding power'. It's all nonsense. The gears, even metal ones, will strip long before the 'holding power' runs out. And anyway load is much less than he would have us believe. It's not like hanging a weight off the servo arm as the tail or whatever moves in the opposite direction thus instantly relieving the load.

And a tip. After landing and before taxying put the flaps up. You will be the only modeller on the planet except for myself, no matter how 'expert', that knows that all flyers of low wing full size aircraft always do this as it avoids possible flap damage from stones etc thrown up by the prop

[Peter Jenkins]

Hi Richard,

Having completed the wing sheeting I weighed the wing and then all the bits that will go into the wing. The outcome was as follows:

• fully built and sheeted wing 24 oz
• Wing tips and ailerons 1.5 oz
• 12 Robart 3/16" pin hinges 0.2 oz
• 4 standard servo 5.6 oz
• 2 mini servos (doors) 2.0 oz
• 2 Robart electric retracts inc wheels 12.6 oz
• Retract sequencer and Robart contol 1.7 oz
• Servo lead extentions (estimated) 2.0 oz
• 2S LiPo for gear operation 2.0 oz
• GRAND TOTAL 51.6 oz or 3 lb 3.6 oz

I will need to cover it obviously and put on the decals but I can't see that adding more than say 8 oz so say 3 Lb 11.6 oz.

I don't understand your wing weighing 7 lb. The only guess I've made is the weight added by Oracover the rest are actual weights. Also, the u/c doors will be made out of 1 mm ply reinforced with balsa. As I'll have to cut the same amount of wing skin out the additional weight will not be very much.

[Peter Jenkins]

Oh, and as an ex full size pilot, I was always taught to clean up the flaps once I'd turned off the runway so that will be my normal practice.

As for digital holding power, I think you might be wrong. A digitial will always resist any force up to the limiting torque and that's why you always hear them chuntering away. A standard servo will drive to a commanded position but can be pushed back and does not have the necessary circuitry to drive back. A bit academic as most servos are perfectly adequate for our needs.

The reason I want flaps and fully enclosed undercarriage is that it looks right from the scale point of view. A Spitfire never had fully enclosed wheels, till after the war, and also grew a retracting tailwheel. It's not that it makes it less of a toy aeroplane in my opinion.