Grumman American Cougar

[Mark Elen]

I have been thinking long and hard today, regarding the comments above, re the weight of wood, versus fibreglass nacelles.

Dennis Tapsfield’s article says that the original build weighed in at 9 and a half pounds, and it needed 4 oz of lead in the nose, along with a nicad for the retract servos. (I’m not going to have the luxury of a heavy nicad to use for balance)

Would this be classed as heavy for a 80 4 stroke?

Considering that the nacelles are pretty much all forward of the CG, is it going to be wise to loose roughly a pound from them, only to have to lug around half a pound of lead in the nose to compensate?

The other worries are that:

a) the nacelles will take a lot longer to produce, due to it being something I need to learn how to do.

b) I will need to fix them securely to the wing, considering the forces involved - this is my biggest worry.

c) It will be easier to cover wood with the same covering as the wing, than paint and match with the covering - no, this is my biggest worry, by far! 😂😂😂

Decisions....decisions.

Cheers

Mark

[Danny Fenton]

Hi Mark, personally i would keep going as you are. Possibly not embark on the molding route. I would be more interested int weight you may be able to save behind the C of G. A gram saved on the tail is 5g avoided on the nose.

I would think about removing perhaps the centre of those rear bulkheads for a start. I don't know the plan, how are the tail surfaces built?

But at the end of the day, a heavier model just flies a little quicker, so don't worry about it, do what you instinct tells you to do

I wish I hadnt said anything, but just thought it was interesting comparing modern design methods to how this is done.

Cheers

Danny

[John Minchell]

Mark

Mine is going to be set up for twin electric and one 1000kv motor, ESC, 12" prop and a 3s LiPo weigh 400 grammes. Twice that is 800 grammes for the power system. Are your 40FS motors, throttle servo and tanks lighter or heavier?

I will also be running 2x 2s LiPos (one for the Rx/servos and one for the retracts) located each side of the nose gear leg to acheive CG balance. I will just vary the pack size and location of the rudder, elevator and flap servos so as not to need lead nose weight.

The bare fuselage including tailplane and fin is 802 grames (but without the bottom sheeting). Wing bare structure, without sheeing yet, is 358 grammes. Hope that may be of help.

I will hold off posting the nacelle till you let me know you definately want it.

John M

 

Edited By John Minchell on 16/05/2020 00:41:01

[Mark Elen]

Hi Gents,

I had considered removing part of those Danny. The plan on outerzone calls for 1/8 x 1/4 uprights with 3/16 square cross braces. My plan from sarik, calls for 1/8 x 1/2 uprights but doesn't specify the cross members.

To be honest, if I was to hack some away, I don't think I would save much, as I remember thinking how light they felt when I was making them up.

Thanks for the info John. I have had a weigh of the engine bits this morning:

439 grams. It looks like I'm going to need that bit of extra weight at the back to balance these out.

I have pretty much decided to carry on 'per the plan' and see how it turns out. If I can get somewhere near 9 1/2 pounds I will be happy. Many thanks once again John for your kind offer.

I got a little done last night and this morning:

Cheers

Mark

[John Minchell]

No problem Mark - I will continue to monitor your posts as they will encourage me to get on with mine and get it finished for next year's BMFA "light scale" competitions. I still think you are a braver man than I am (or an ex pylon racer) for putting two 40 FS engines in something which the designer said two HB 25's made it ballistic!

John M

[Mark Elen]

Hi John,

Either brave or stupid. My money is on the latter rather than the former😂😂😂

I was worried that it would be underpowered.

Cheers

Mark

[Mark Elen]

I'm nearly there with the rear formers now. But i'm not happy with the 2 laminated ones. In my excitement of a new technique, I rushed them a little and the 1/32 ply strips didn't quite go together properly. Also, I took them out of the jigs after a couple of hours. - they have both bowed inwards at the bottom and now need forcing out to sit in the correct location on the fuselage. The front one has a slight twist in it too.

Nothing for it but to make up a couple of new ones.

These new ones will sit in the jig for a couple of days before they get touched.

Cheers

Mark

[Gary Clark 1]

Hi Mark,

I used that technique of laminating strips on my 1/4 scale moth minor. Jerry bates designed it in a way that you use 1/16 balsa strips around a former cut to the shape of the fin, rudder, horizontal stab and elevators and this formed the leading and trailing edges of all these parts. It creates a very strong part with few joins and is easy to shape but is incredibly strong even though it's only made up of 1/16 balsa. Just have to put the ribs in and shape the leading edge and it's done. It should retain a lot of flex so might be worth bending beyond where you want it and it will spring back a bit.

Gary

[Mark Elen]

Hi Gary,

Many thanks for your comments. This is 8 laminations of 1/32 x 1/4 ply. They have tightened up rather than springing back. Here are the two ends, the bottom one is still in the jig:

They have tightened up about 1/4", I can only assume this is down to me removing them whilst they weren't really set properly.

Whilst these are still sat in the jigs, I got on today with making a tool to pull the front end of the fuselage together:

That is going to get left for a couple of days to go off properly.

Cheers

Mark

[Gary Clark 1]

I'm surprised to see it tighten up as much as that Mark, never seen that before. Now the glue has set, would a soak and then put it back in the mould until it dries help?

It all looks great though, I've downloaded the plan so I can follow along.

Gary

[Mark Elen]

Hi Gary,

Thanks for your kind comments. Soaking and putting back in the formers might do it, but, I have got another set done and they are staying in the formers for another day or two.
I didn’t need to wet the ply to get it to bend into the formers. I’m hoping this set work, as they are a pain to cut up and sand down.
They look much better though, it was my own fault for being impatient.

Cheers

Mark

[Mark Elen]

Bit more done today. I got the fuselage out of the bending tool this morning, it didn't move at all. So I got started on this planking lark. I have been both looking forward to it and dreading it in equal measure. To be honest, I'm really enjoying it, although, I don't know if i'm doing it right:

I did the centre with straight 6mm planks, then whilst this was drying, I went flying:

An enjoyable couple of hours. On my own the whole time, it was good to do something 'normal' for a change. I haven't flown for about a year, so I had a couple of interesting 'arrivals' with the trainer before I got my eye back in. I then flew the repaired Ballerina, if anything, it is easier to fly than the Trainer.

Back home and more planks done:

Cheers

Mark

[Rich too]

Great skills Mark

[Nigel R]

Interesting build - don't know how I missed this one until now! It's a great looking airplane.

Hope you've invested in nice light wood If you can have light wood it should come out ok for weight. There are definitely places where you could use thinner wood, or build up. Solid 3/8" for the tail for instance, screams sheeted framework to me. Same for tips, ailerons... but I think a lot of the thicker sheet will end up being carved and sanded away? I might forego the torque rod controls on rudder and elevator and go for lighter external horns...

At 9lbs this will be a docile sport flyer - lots of wing area (I make it 825sq in) to carry the weight.

Power wise I can't see it being ballistic on a pair of old crossflow HB 25s though. I think I must have a very different definition of ballistic to the designer. That is a really large fuselage to drag through the air. Nacelles are not small neither. I would be thinking about a pair of lightweight 40 size (FP, LA, Irvine 39s, SC36), for that weight, to have something in reserve. Your twin four strokes would be around the same power I would think.

How smooth is your runway?

[Nigel R]

A couple of thoughts -

I don't know that I'd use the HS85s on flaps. The install shown on the plan is decidedly busy at the wing root. Looks a bit of a faff, to be honest. But, you won't have that - with air retracts, outboard aileron servos, and throttle servos in the nacelles, you're only left with flaps to deal with in the centre. Leaving a simple torque rod install that one cheap standard servo could happily service. Maybe that's just me thinking about where money is better spent but also the single standard will put less load on the RX battery.

I'd think about running some really thin glass out to where the wheel well is. I really regret not doing that on my twin. Just outside the nacelle is the natural place to pick it up when the engines are going. I find the built up wing is a bit fragile for that sort of manhandling without the surface reinforcement that thin glass could provide. Or, if you don't fancy glassing, think about putting in a couple of extra riblets in that area to provide the extra support for the wing sheeting near the LE.

[Mark Elen]

Hi Nigel,

Thanks for your comments. I’m going to take on board the slimming down ideas from both yourself and Danny earlier. I will make built up ailerons and flaps, and use a single servo on torque rods for the flaps.
I am also going to attack those rear formers with the dremel.

As it’s going to be semi-scale, and I have picked out a lovely scheme for it, I would like to keep the internal linkages at the back to hide them.

I’m going to have a play with some glass cloth and poly C, with the view to how it subsequently covers with film. If it goes well, I may well follow your idea.

The fin is built up, and, it has been giving me headaches as to how to build it square. I don’t know how much it would save to build up the horizontal stab and elevators, but I may well have a go at both and see how they compare.

Once again, thanks for the pointers.

No photos today. Still planking.

Cheers

Mark

 

Edited By Mark Elen on 20/05/2020 20:58:38

Edited By Mark Elen on 20/05/2020 21:00:49

[Mark Elen]

A bit more done today.

I wasn't going to post any more planking photos, but I spoke to a friend of mine today and he is following along, although he isn't currently a modeller. I promised to post up a couple of photos. Hi Brian!!!

The last piece is cut and sanded for one side, just need to get it in and the other side to do.

I'm very happy with how that turned out for a first attempt. Next job is to get the ends cut down and sanded to shape.

I'm waiting on a wood order for some 1/32 ply to make up the side window pieces, I'm hoping it will turn up tomorrow, but I'm not holding my breath. It has been nearly 3 weeks now.

Cheers

Mark

[Gary Clark 1]

I hope the planking on my P-47 comes out like that Mark. Lovely stuff

Gary

[Mark Elen]

Hi Gary,

Thanks for your kind comments. I hope you get on as well as I did. I found that only chamfering one side worked well, just don’t go too mad, they only need a little chamfer to get them to sit right.
Take it slow and steady, this is one job that needs patience. I worked on the basis that closer joints need less glue, so less weight. I put too much glue on the plank, then placed it, then cleaned up the excess. The last couple of days were a struggle, as even using aliphatic, it was going off in seconds in the heat.

I hit it tonight with a permagrit block, the ugly duckling is turning into a thing of beauty....slowly.

Cheers

Mark

[Mark Elen]

The next job on my list is to sort out the noseleg including a means of steering it.

Whilst in the middle of my last build, I was thinking about this and found this on YouTube: **LINK**

I made this up a while ago, though, I am going to re-make the blocks out of aluminium to save a bit of weight:

There is still a bit more to do on that, but the principle works and I'm going to carry on with the concept. I wanted to see how much slop it would have in it, surprisingly, there is very little.

The gear legs didn't come with any axles, and I don't fancy bending 4mm piano wire, and, getting the bend in the correct place, so this morning I have made an axle for the noseleg, out of a piece of 8mm silver steel that I had kicking about:

6 grams with a M3 grubscrew and a M3 nyloc. I'll take that.

Cheers

Mark

[Danny Fenton]

Very neat indeed sir, you clearly have some engineering skills, watch out you will be getting asked to make things for others

The E-Flite electric retracts also do a similar system for steering the retracting nose wheel. same idea.

The one on the left did not work, the pivot was not on the retract pivot, would never work. E-Flite "sold" me a replacement with altered geometry that "Does" work......

Cheers

Danny

[Mark Elen]

Hi Danny,

Many thanks for your kind comments. I did think about doing something similar to that, but as it retracts forward and the servo is forward of it, it didn't work mechanically.

I got the servo bracket made up and in, then had a couple of hours with the mill and got the final bits made and they are temporarily in, I just need to make a small bracket with a slot to hold the pushrod in the correct orientation while it moves. Nearly there.

Cheers

Mark

[Mark Elen]

A bit more done the last couple of days,

I have got the top sheet on to the nose, I didn't have any 3/4" soft sheet, so I raided the scrap box for the softest sheet that I had. I ended up laminating the front end. Its a bit agricultural, but nobody is going to see it.

Don't look too closely at the above photo. I did most of this after I had had a pint or two.....

One of the upsides of laminating that top sheet, is that it has allowed me a good view of getting both sides somewhere near symmetrical when it came to sanding down the curves.

There is a bit more to come off this, but I will leave this now for just before covering, if it gets dinged during the build, it wont matter so much.

I also put up a couple of photos for some inspiration:

The above will be my chosen scheme.

I also made a start on the bracket for the nosegear steering pushrod. I have had to order a 2mm slot drill, so here is the embryo bracket without the slot:

Cheers

Mark

[Mark Elen]

Today I have got the rear formers slimmed down and in. Here are the ones shown on the plan as solid that can be cut down on the scales before:

20 grams.

Here they are after:

7 grams.

That's 13 grams or just over half an ounce saved.

Cheers

Mark

[Danny Fenton]

Way to go Mark, it all adds up, and the skins carry much of the structural load.

Enjoying your build