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Top Flite P51 - 65" span

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Peter Jenkins12/04/2020 19:00:06
1602 forum posts
266 photos

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.

mustang 23.jpg

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.

mustang 24.jpg

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.

mustang 25.jpg

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 A12/04/2020 20:04:32
230 forum posts
13 photos

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 Jenkins12/04/2020 22:59:55
1602 forum posts
266 photos

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 A12/04/2020 23:05:38
230 forum posts
13 photos

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

Peter Jenkins14/04/2020 00:55:30
1602 forum posts
266 photos

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.

mustang 26.jpg

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

mustang 27.jpg

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 Gray14/04/2020 07:23:58
2175 forum posts
941 photos

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.

cymaz14/04/2020 08:39:25
9254 forum posts
1196 photos

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 Jenkins14/04/2020 09:52:07
1602 forum posts
266 photos

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 Jenkins14/04/2020 09:56:02
1602 forum posts
266 photos

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

mustang 28.jpg

Peter Jenkins14/04/2020 16:09:29
1602 forum posts
266 photos

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.

mustang 29.jpg

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:

mustang 30.jpg

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.

mustang 31.jpg

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!

mustang 32.jpg

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

mustang 33.jpg

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

Nigel R15/04/2020 12:12:30
3916 forum posts
678 photos

" In theory, therefore, tilting the retract support rails by 10 deg should produce the required effect."

All looks good, and the value of the mockup is clear to see, but I'm sure adjusting the tilt will mean the wheels are further back (1/2" or so?), when they are extended and down. Are they still a good distance in front of the CG when extended?

Bruce Collinson15/04/2020 12:50:04
537 forum posts


A post “Anyone use these” this am reminds us that sheets of epoxy glass suitable for horns, servo arm extensions and ... u/c doors? ... are readily available with a helpful link. Of course it may be that you welcome an opportunity to laik about with carbon mouldings custom made for the profile.

Good build.


Peter Jenkins16/04/2020 00:02:38
1602 forum posts
266 photos

Ron and Nigel R you have both raised the issue I had been pondering about bringing the main wheels back when reducing the tilt by 10 deg. I measured the distance from the CG point marked on the wing to the centre line of the retract unit. That came to 5.8 cm. I did some trig and based on the length from the top of the strut to the bottom of the wheel (21 cm), tilting the retracts back by 10 deg moves the bottom of the wheel backward by 3.64 cm.

So, would changing the wheel contact position (albeit on a fully extended oleo) from 9.44 cm in front of the CG to 5.8 cm in front of the CG make the aircraft more likely to tip onto its nose during a landing run especially on grass? Since pilots report that the aircraft can tip onto its nose during landing then clearly this might not be such a good idea after all.

I thought the best way to analyse the options was to do a scale drawing of the wing box at the centre line of the retracted struts. I then drew on the measured position of the wheel within the wing under the tipping back by 10 deg. By judicious use of the real ply ribs attached to the mock up, I was able to determine that removing 2 mm from the rib nearer the wheel would produce a 2 deg droop of the leg and allow the now tilted wheel to sit low enough to provide the required clearance for the door above the wheel. The leg mounted door is also able to sit on top of the wing skin without fouling the leg. This, of course means that when the legs are extended, they will not sit perfectly vertically downwards but we are only talking a degree or two.

By this stage of the day, I only had time to glue up the main wing strut and reinforcing piece before time ran out. So, while I've technically started the wing build it's hardly worth putting up the picture.

Peter Jenkins16/04/2020 00:07:28
1602 forum posts
266 photos
Posted by Bruce Collinson on 15/04/2020 12:50:04:


A post “Anyone use these” this am reminds us that sheets of epoxy glass suitable for horns, servo arm extensions and ... u/c doors? ... are readily available with a helpful link. Of course it may be that you welcome an opportunity to laik about with carbon mouldings custom made for the profile.

Good build.


Bruce, thank you for that suggestion. However, as I am just as likely to use thin ply and make up the detail inside the door with balsa and bend the ply by steaming it, there is little point in putting out a request as you suggest. It will depend on the final wing profile exactly how difficult a task this will be. If I do need to use some of my carbon cloth, I have a plan to sandwich it between two sheets of plastic and weight the whole lot down onto the wing with a pile of mags on the top of the wing. That way, the resultant curved sheet will match the wing contour exactly. That is not a big investment in time and will cost nothing more as I've already got the stuff to hand.

Peter Jenkins16/04/2020 20:45:02
1602 forum posts
266 photos

Today was a pretty simple sort of day. Construct the port wing. Just needed to follow the very good building instructions. It started with making the spars which require two parts to be glued together.

mustang 33.jpg

A total of 4 spars are needed. While they were drying, I remembered I needed to make the access holes for the servo lead to the aileron servos. While the plan shows a single centrally mounted aileron servo with a bell crank in each wing (rather shows the designs age) I decided to go for two wing mounted aileron servos. The simple way to do this was to put all the port wing ribs inboard of the aileron servo together, aligned with the spar slots and use my pillar drill to drill a 10 mm hole through all of them. The hole needed enlargement as it wasn't quite large enough to allow a servo lead lock though. Once that was done, it was just a case of pinning down the spar and then gluing the ribs into place making sure they were truly vertical and right on top of their required positions.

mustang 34.jpg

Suffice to say that the accuracy of the die cutting meant this was a pretty painless task. I made sure that the rib tabs were pinned to the board and also pinned the ribs in front of the spar. Once that was dry, I glued on the rest of the wing parts. Some, like the spar for the aileron hinging needed to be glued together and roughly sanded to shape and others, like the moulded LE, needed to be cut a joined to create the distinctive kink in the Mustang wing. The flap trailing and side edges followed and then the whole lot has been left to dry overnight.

mustang 35.jpg

The clamps you can see in the photo above are holding the u/c ply formers that will hold the u/c mounting spars. I also meant to add the servo hatch rails for the aileron servos but decided that I'd leave that operation till the glue was quite dry. Talking of glue, I'm using aliphatic for the first time. While it's nothing like as fast as Cyano, it's a huge improvement on PVA, my previous glue of choice.

So, tomorrow, it will be a case of remove the port wing panel, turn the plan over, and build the starboard wing panel. That will no doubt take a lot less time to construct as I shall be utilising today's knowledge. The washout on the wing is quite noticeable when you look at the construction as a whole. No tip stalling if I can maintain the design washout angles!

Peter Jenkins16/04/2020 20:56:56
1602 forum posts
266 photos

Incidentally, I've now read through about half of a very long thread on RCU on building this Mustang with contributions from a wide range of folk. One poster was particularly helpful in his description of how he'd constructed the inner u/c doors and another provided a link to the TF Beech Staggerwing in which they provide very helpful detail on making tailwheel doors using two rubber bands to close them and an aluminium spreader attached to the tail gear to push the doors open. That's solved that problem so things are looking up as regards the things I need to research. The other interesting bits were the AUW of the model. The guy with the detailed u/c doors, covered his with glass and then went to town on weathering. His model tipped the scales at 11.5 lbs. He had a OS 120 FS in it and reported that the power was adequate. He found he needed to go to full throttle, allow the aircraft to accelerate, and then it would loop well. Otherwise, there was the danger of the loop degenerating as speed decayed. Interesting as I am going to power mine with an OS 120 FS!

Mike Rieder18/04/2020 11:02:16
41 forum posts
18 photos

Looking good Peter, re strengthening the landing gear support area, i would consider glass clothing the inside of the wing area. I can give you some sutable cloth at some point or pos even drop off if needed.

I had a similar issue with the spit as i had to remove most of the rib to get the wheels in.

Peter Jenkins18/04/2020 16:54:44
1602 forum posts
266 photos

Hi Mike, my plan was to install a ply doubler to both ribs supporting the U/C blocks and extend them back towards the rear of the rib to provide a path to transfer the impact forces of landing. I have also decided to use my supply of carbon cloth to reinforce the whole of the bay from around the LE, along the upper skin line, down the face of the spar and then back up via the bottom skin. Only problem is that I think the instructions call for skinning the bottom of the wing first. That might do as the torque on landing that the wing has to take will be to place the upper skin in torsion and lower one in compression. Need to think that one through a bit more.

I'm also going to build some structure around the wheel well both to reinforce and area where most of the rib has been cut away and also to box in the opening probably just inside the door opening to reduce the ingress of oil and dirt while allowing easy cleaning of the area.

Peter Jenkins18/04/2020 23:53:49
1602 forum posts
266 photos

I haven't managed to do as much as I wanted today but I have carried out the first phase of reinforcing the wing to soak up the landing loads. The first part was to cut out and glue in a 3/32" ply rib to W4 and W5 . Here are the reinforcing ribs:

mustang 36.jpg

and here they are glued to their respective wing ribs. It was only after I had glued the W5 support rib in place that I noticed I'd forgotten to drill a hole for the aileron servo lead! Ah well, another awkward little job that will take up valuable building time!

mustang 37.jpg

You can also just about see the u/c rails which I glued in yesterday evening using Hysol.

Tomorrow, I shall reinforce the ply ribs with light weight carbon fibre cloth.

Peter Jenkins19/04/2020 00:00:26
1602 forum posts
266 photos

I have been wanting to draw out the tail wheel retract and doors so that I had a good idea of how I was going to position and build them. The link to the TF Beech Stagger Wing manual (that I got from the RCU thread) shows how to use the retract to open and rubber bands to close the u/c doors. So, the drawing below, is how I'm going to implement this on the Mustang.

mustang 38.jpg

I will need to check how long the tail wheel doors need to be by reference to the photos I have of the Polish Mustangs. The other interesting thing about the retract is that the tail wheel is steered by a horn that is controlled by pull-pull wires. As the leg retracts, the wires go slack and a spring centres the tail wheel. There isn't much movement of the tail wheel to steer the aircraft and this is down to the tiller contacting the support structure. I'm going to investigate filing the plastic support structure a tad to see how much that improves the steering angle that can be achieved.

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