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Top Flite P51 - 65" span

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Peter Jenkins13/06/2020 14:22:03
1624 forum posts
305 photos

Life's been getting in the way far too much of late! However, I have made some progress and have glued the tail plane and fin in position. Having assembled a long spirit level and a try square, I set about adjusting the tail plane and fin to ensure they were at right angles to each other and the fuselage/wing. This required a relatively small amount of sanding on one of the tail plane supports on the fuselage.

The instructions tell you to glue the tail plane into place and then the fin. I felt this was wrong as there was no mention of drawing together the fuselage at the aft end where the fin post fitted. So, after clamping the fin in position with 3 clamps, I found that every thing was still squareAfter that, and liberal use of my magic piece of string, it transpired that I needed to adjust the rear of the tail plane a touch after which everything lined up.

I decided to use 30 min epoxy to attach the tail plane to the fuselage with a small dob of epoxy to gjue the front spar of the fin into the fuselage former and the elevator snake. This anchors the snake outer almost at its end. The fuselage/fin and tailplane/fin junctions I trusted to aliphatic.

The final job to do before gluing the tail plane in place was to connect the elevator joiner to the elevator pushrod clevis. This required the elevators to be put in place, the joiner holes to be marked and drilled and then the whole lot dry assembled to check everything still worked smoothly which it did.

So, having removed the elevators and mixed the epoxy, I applied it carefully to all relevant parts and placed the tail plane into its saddle. The fin was next up with careful application of aliphatic and slid into position. I pressed everything down and then clamped the fuselage end with the fin post inside. This allowed the fuselage sides to pull in under the tail plane with out causing any issue as the epoxy was still very wet. Then out with the magic string to check wing tip to tail plane tip distance was the same both sides and rush round to the front to check that the tail plane was level with the fuselage top and the fin was at right angles.

mustang 123.jpg

mustang 124.jpg

At this point, I heaved a huge sigh of relief and left the lot to dry.

Several hours later, I took the rear fuselage bottom sheeting and cut the opening for the tailwheel.

mustang 125.jpg

I then fabricated the undercarriage doors out of 1.5 mm ply. As these were a bit flimsy, I decided to reinforce them with 3/16" balsa. The balsa would also act to trap the hinges in position. I used fast cyano to tack glue the hinges to the ply and then aliphatic to glue the balsa to the ply and trap the hinges between them.

I also glued some scrap fuselage bottom sheet to each side of the wheel opening to provide a footing to which the hinges could be glued. Once the aliphatic was dry on the doors, I hinged them to this footing as you can see below.

mustang 126.jpg

However, all is not well. I don't have any aluminium strip that I can bend to the required shape so used some aluminium tubing squashed flat. That does not have the required stiffness. So, my search for a suitable spreader arrangement goes on. Incidentally, you can just see the V shaped piece of ply that I glued into the lower u/c arm and then drilled to take a long servo screw. That's what Top Flite recommended in their Stagger Wing instructions. That bit seems to work fine.

Limobob17/06/2020 12:07:11
60 forum posts

I have an almost completely constructed Mustang from, it must be at least 2 1/2 decades ago. I dragged it out during the lockdown and was quite suprised at how little needs to be done for completion. It has moved house twice and picked up a few dings but not really too bad and needs just some tlc to get it going again. I have a Supre Tigre 90, unrun and with the necessary in cowl silencer and manifold. It would not be my choice these days but will be used as i can’t justify extra expense. Also the correct Robart air retract system complete with the required Robostruts. All very expensive stuff and again if i were starting from scratch would use Robart electric units. Having read comments on the forum regarding weight, I thought I would put it on the scales. The wing c/w retract units, wheels and three standard servo came out at 3lb 1oz. The fuselage fin and stab plus elevators, again with three standard and one small servo for the air system, the engine, soft mount and silencer plus spinner came to 4lbs 14ozs. A total then of 7lbs 15ozs. My intention years ago was to glass the whole model but now i shall use Oracover Silver all over which is far superior to the Solarfilm silver i used on my DB Tiger Moth. I have tried a small piece and it does ot mark like the Solarfilm did. Two rolls of Oracover plus other bits and pieces of hardware will surely shove the finished weight well above 10lbs, so getting pretty weighty. The aluminium spinner alone weighs 8ozs! Any thoughts from anyone as to whether all this will be a viable proposition? Such a shame to have it just slowly deteroirating as it is a handsome looking beast!.

Peter Jenkins17/06/2020 12:36:33
1624 forum posts
305 photos

Limobob, that sounds great! It's definitely worth proceeding to the ready for flying stage and as far as I can determine those who have an 11 lb Mustang are still OK provided you use the flaps and keep power on until you have rounded out. You didn't say if you had made the flaps functional - did you? If not, it's not too much hassle to cut them out and then all you need is the leading edge and to install the servo hatch/mount. If you didn't include the servo mounting rails you can add those once you've cut away the servo hatch in the wing skin.

Let's see what it looks like. I've got some Oracover Chrome to finish my beast but I'm debating whether Oracover silver might not be better.

Limobob18/06/2020 06:32:10
60 forum posts

It certainly will have working flaps! Theze have already been hinged with 3/16” dia. Robart hinge points. As far as Oracover is concerned, the silver as I mentioned before is dead easy to apply, the only thing I would say is the underlying finish has to be perfect S the bit I tried did show up the balsa grain. Sanding sealer may not be recommended but maybe a thin wash over with Balsaloc or Oracovers own product would sort this out. Must have come up dozens of times in the past, a quick mail to the company may come up trumps! I wont be rushing i to anything with this model as we hope to get away in our camping car very soon and other things tend to get in the way of modelling this time of the year!

Peter Jenkins19/06/2020 23:43:19
1624 forum posts
305 photos

It's been a bit frustrating waiting for some 64 thou thick aluminium sheet. It arrived last Friday and I was able to cut a strip and form it into the type of yoke I had seen used in the Top Flite instruction manual for the tail wheel doors on the Stagger Wing. This is what I made.

mustang 127.jpg

Unfortunately, the force needed to open the doors during the tail wheel extension has proved to be too much for the servo and indeed my fingers when I turned the servo arm! I therefore considered that the effort of lifting the doors against the force of the rubber bands was too great so made an additional door pusher that would push from the middle of the doors. When I finally got it to work the next problem was that the door pusher was not too high to allow the gear doors to close as you can see below.

mustang 128.jpg

So, I will have to think up another solution that will not over come the servo's ability to open the doors while allowing the doors to close fully on retraction! This is now the biggest problem that I've had to deal with on this build! If I can't get the doors to open I shall have to consider having a separate door servo but I don't really want to do that. I need to think how best to arrange the door opening to require less force while the way the door closes needs to allow the wheel to retract fully before closing up. Currently, the wheel tends to get stuck between the 2 doors as they close with the wheel not fully retracted.

I'm open to any bright ideas on how to solve this problem!

Ron Gray20/06/2020 07:05:15
2235 forum posts
978 photos

On one of my Freewing models, I think it may have been the Scorpion, the nose leg doors were joined by a coil spring which held them apart (open) then when the wheel retracted it pushed against the spring which then pulled the doors closed. A very simple, if not that elegant solution which required very little force to work. In fact their Avanti uses something similar and you can buy the parts for it Motion RC Would be easy for a man of your calibre to adapt for your use.

Peter Jenkins20/06/2020 12:25:24
1624 forum posts
305 photos

Thanks for that link Ron. I have one more idea that I'll check on today. That is to bring the contact point from the inner edge of the doors to the centre where there will be more leverage over the rubber band to push the doors open. I could try a larger rubber band to reduce the tension when they are retracted but I'm worried about the doors starting to flutter in a dive.

Looks like the Freewing solution is to spring the doors open whereas mine is to spring the doors closed. Might also look at that change in design. That would also help the tail wheel extension action by providing the tail wheel with a push to overcome the near over centre condition I have engineered!

I should just add that the mini digital servo I'm using has around 5 Kg cm torque so it's no weakling!  It's my design that's the problem.

Edited By Peter Jenkins on 20/06/2020 12:26:50

Ron Gray20/06/2020 15:09:52
2235 forum posts
978 photos

Defo get the spring to do the work to open the doors!

Peter Jenkins21/06/2020 13:32:45
1624 forum posts
305 photos

Well, my second solution didn't work as the power of the servo is negated by the fact that with the geometric arrangement of the tail wheel mechanism, very little power is transmitted when the tail wheel is retracted - enough to move the tail wheel but not enough to overcome 2 elastic bands apparently!

So, I looked at Ron's suggestion and that left me with the concern that perhaps there wouldn't be enough power to drop the tail wheel completely as it would be pulling the doors shut.

So, I bit the bullet and have ordered some micro servos. There was a package of 5 available for £11.99 so even if there is a duff one in the pack I can find at least 3 that would work and I'll use two for the main u/c doors secondary door operation rather than my 2 mini digitals as I have currently planned.

I did manage to position the Bowden cable to operate the throttle but the throttle arm has been drilled out too much for the use of either a plastic or metal clevis. I'll need to see if I can get a new one that has the standard size holes.

Richard Clark 221/06/2020 14:53:15
418 forum posts

Peter, a suggestion re throttle.

I always use a 'helicopter' type ball link on the engine throttle arm rather than a clevis. They have zero play, don't wear the hole in the throttle arm loose with use, and you can pop the link off the ball with needle-nose pliers to wind it in and out for adjustment (they come with the same 2 or 3mm thread you find on a kwiklink).

And in your case, as you fix the ball to the throttle arm with a 2mm nut and bolt (I advise using blue thread locker on the nut) the enlarged hole in the throttle arm being too big for a kwiklink won't matter.

Peter Jenkins22/06/2020 00:24:31
1624 forum posts
305 photos

Good call Richard.

Richard Clark 222/06/2020 06:56:28
418 forum posts
Posted by Peter Jenkins on 22/06/2020 00:24:31:

Good call Richard.

One small point. Being an American kit all the kwiklinks etc. use US threads. On my Spitfire I replaced the whole lot with European metric ones to be compatible with what's in my odds and ends box. You should do it at least on the throttle ball link if you use one as a SLEC, Hirobo heli, or similar one from your local shop or wherever will have a 'tight' metric thread on the plastic socket part.

Peter Jenkins22/06/2020 08:16:14
1624 forum posts
305 photos

Hi Richard, yes, I'm very aware that US sourced stuff is still Imperial. I already have a set of metric threaded ends to use with my metric links. That having been said, I've used the supplied threaded couplets with the supplied plastic links for both ends of the elevator, rudder and tail wheel pushrods. The throttle run will be metric though.

Peter Jenkins25/06/2020 00:24:28
1624 forum posts
305 photos

Sorry about the delay in posting but I've been waiting for the 9g servos as I cannot progress till I've sorted out the tail wheel doors. Also waiting for the ball links to arrive to sort the throttle cable out. Very frustrating!

However, the servos arrived today and all of them work. There was a deal for 5 so I thought it would be worth having some spares. Still working out how best to mount the servo. Sadly, it seems that aft of the tail wheel is the only option. I am also keen to find a way of mounting the servo so that it can be replaced easily should it fail!

Mike Mueller 101/07/2020 14:26:55
4 forum posts

A friend of mine found this for me, and am going to be starting the same plane in about 3 months. I do have a question: I know the plane is rated for a 120 4 stroke, but I want to put in a Saito 180 gas engine. Is there any reason why I can't put that in? Thanks!!!!

Peter Jenkins02/07/2020 00:04:19
1624 forum posts
305 photos

Mike

I suppose my first question is why do you want to go for that big increase in power? I have yet to fly this aircraft but from what I have read about it it should perform well with a 120 four stroke or 90 2 stroke. My reason for wanting to fit a 120 rather than a 90/100 FS is so that I will have the power for large loops. I fly aerobatic aircraft mostly where I am at half power for all level flight manoeuvres and only use more power to fly big vertical manoeuvres so that the speed remains constant. I very rarely use full power. The full size Mustang does not have the excess power that allows unlimited vertical! Remember that the full size P51D had a max speed of 437 mph. A 1/7 scale speed flat out would therefore be a whisker over 60 mph!

Second, my experience of Saito 180 glows is that they are very rough - and yes that was with balanced props - and prone to losing their exhausts. Don't know if this is a problem with the petrol Saito 180 but be warned. The vibration from my Saito at full throttle was shocking. With a petrol you will also have to factor in the weight of the ignition system although the fuel tank can be half the size of a glow one.

I know that the Saito 120/150/180 are all based on the same crankcase and get lighter as capacity increases so I don't see a problem with fitting one. You will need to pay attention to cooling as petrols run hotter than glows.

Apart from that, there is no reason not to put a Saito 180 in it but make sure that you check the integrity of all the bits in the aircraft regularly as vibration will cause a lot of damage.

Are you going to fit retracts and flaps?

Nigel R02/07/2020 08:54:08
avatar
3981 forum posts
714 photos

30cc on a 65" plane with about 700sq in area... wow.

Denis Watkins02/07/2020 09:09:24
4536 forum posts
123 photos
Posted by Mike Mueller 1 on 01/07/2020 14:26:55:

A friend of mine found this for me, and am going to be starting the same plane in about 3 months. I do have a question: I know the plane is rated for a 120 4 stroke, but I want to put in a Saito 180 gas engine. Is there any reason why I can't put that in? Thanks!!!!

Mike, Nigel has already suggested that the answer to your question is " No "

A 120 is sufficient and you should plan along those lines.

Nigel R02/07/2020 10:10:11
avatar
3981 forum posts
714 photos

Many moons ago we would have fitted a 10cc two stroke glow to this size airframe.

Just my gut feel, but a 90FS in this P51, a 1/7 scale, would be plenty.

More all round performance is to be gained by making sure the build is as light as possible, than by overpowering.

Peter Jenkins02/07/2020 14:41:12
1624 forum posts
305 photos

Nigel, you are right provided that the wood in the kit is designed for a light build. In practice, in these older kits, my experience has been that they were built to survive a crash landing so, those of us who learned how to land, pay the penalty in weight. Today's light weight models with formers fretted out to save weight do not survive a robust arrival and tend to have the u/c plate ripped out early on.

So, you have to be realistic about the issue of what size engine to fit. I flew a friends TF Spitfire with an OS 90. It was grossly underpowered given its weight. You had to dive at full throttle to work up the speed required for anything other than a tight loop. It was not fun flying it as I was always on tenterhooks lest it stall. Even a pull out had to be gentle lest it tip stalled.

On that basis, I decided that the OS 120 was a better bet as I want to be able to fly the Mustang like the full size and carry out large sweeping manoeuvres.. It doesn't need all the power to be used all the time. I fly F3A competitively and learning how to use the throttle properly is something that took me 2 years. OK, I'm a slow learner but when I watch newcomers to aerobatics they struggle just as I did with both throttle, and the other essential control, the rudder.

This is not to say that I think it's a good idea to fit a 30 cc petrol in this model but if proper throttle control is used then it shouldn't be a problem. The problem is, what is proper throttle control! Back to my earlier point.

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