John Cobb’s Crusader

[Colin BishopModerator @colinbishop34627]

Pretty amazing. I don't think we will see anything like that at Bushy.

Colin

[Paul TParticipant @pault84577]

Hi Colin

Don't underestimate Ashley

[redpmgParticipant @redpmg]

Problem might be cheapest Gas Turbine costs over 1,000 GBP last time I looked – saw a model of a B52 with four of them – imagine how the owner must have felt as it had the usual end to model aircraft – why most gave up the hobby.

Ernies article I posted about earlier is a pretty good description of the Gas Turbine pro & cons.

Edited By redpmg on 03/04/2020 13:55:03

[ashley needhamParticipant @ashleyneedham69188]

Going that fast would be an issue to be honest. If nothing else our pond is not that big, and, like the Bluebird, I hope to get ‘the look’ at the minimum speed possible.

So saying, the Bluebird has not been actually tested yet and by the way things are going, this one will have been finished without even touching the water!

Started on the”chassis” this afternoon and it seems like I am building a second BB, as the hull will be constructed almost the same way..

Ashley

[Ray Wood 3Participant @raywood3]

Hi All,

When the gas turbines became reliable with electric starters built in was when I started to lose the urge to fly every week with a club , I just couldn't justify flying a model costing 5k !! I still know guys how do 😀I

Rather gutted not to have flown my planes this year so far after registering with the CAA 😕

Regards Ray

Edited By Ray Wood 2 on 03/04/2020 21:49:55

[ashley needhamParticipant @ashleyneedham69188]

Construction continues apace. The hull is basically in three sections: A centre section that has parallel sides, the rear which is tapered but also changes angle from outwards leaning to inwards leaning at the rear, and the bow.

Fortunately Vosper has the good sense to make the plan view of the bow straight (edges), and the bow can therefore have one piece of 2mm lite ply either side, with a foam nose.

At the rear the "twist" has only used three planks, and having sanded them today looks really nice.

As usual, despite the apparent amount of room, everything it a tight fit and my plan to have just a centre removable roof (thus the parallel section) may spill over into a longer double taper section which I had hoped to avoid.

Similarly, having found out the rudder was at the front, just aft of the wedge, the linkage gets in the way of battery insertion somewhat. Cant have a rudder at the rear as the hull centre section is (or will be hopefully) clear of the water when at speed.

Lovely day for fiddling in the garden with a workmate!

Ashley

[Colin BishopModerator @colinbishop34627]

I thought you were retired Ashley?

Colin

[ashley needhamParticipant @ashleyneedham69188]

Its sad, I have swapped work for building boats!! (and its hard work!)

Ashley

[Bob Abell 2Participant @bobabell2]

Ashley……A progress picture is worth a thousand words

Bob

[ashley needhamParticipant @ashleyneedham69188]

Just for you Bob. Snapped with the mini iPad as I am on it.

grey wedge with clamps will be the forward section top. Also showing highly detailed top secret plans.

I know it doesn’t look like much, yet…

Ashley

[Bob Abell 2Participant @bobabell2]

Thank you, Ashley

To be honest, I can't make head nor tail of it!

Bob

[Paul TParticipant @pault84577]

Hi Ashley

I do remember the little rudder on the back of the wedge was next to useless on a model, if fact I couldn't see it being much use on the full size version.

I resorted to putting twin rudders, one at the rear of each sponson.

Paul

[ashley needhamParticipant @ashleyneedham69188]

Bob. The pointy end is to the left, the hole is at the rear.

It is essentially the bottom half of the boat, and as such will make no sense. Even when the front foam bit is on it STILL won’t look like anything!

Paul. I am not afraid to over-scale the rudder to get it to work. Having twin rudders on my sponsons would be dead fiddly at this size (500mm long). I can well believe that low speed manoeuvrability will be poor, but I won’t be going slow very much…👍

it would appear that it was handy to put the rudder where it was on the original as it made the linkage from the steering wheel easy and direct and much can be said for mine as well.

Had to make another foam wedge as I have just coked up the first one on the bandsaw ho hum

Ashley

[Bob Abell 2Participant @bobabell2]

Thank you Ashley

That explains it now

Should have built a larger model?

500 mm?

My ruler doesn`t start till about 750 !

Bob

[ashley needhamParticipant @ashleyneedham69188]

Unfortunately Bob, the bigger the model, the more power is required. Bigger 4~6s Lipos, massive edf….big cost…

PLUS, a bigger model would have to be `better`, and not simplified as mine is.

Storage is also an issue.

Ashley

[ashley needhamParticipant @ashleyneedham69188]

Just read an interesting thread on the American RC groups forum regarding a whopping 46 inch Crusader built by a chap.

EDF powered, it looked like his was too heavy to perform on an EDF and he couldn't get the c of g far enough rearwards so converted to conventional prop. His nose was being pushed into the water despite various mods to move the weight aft, however hopefully mine will be light enough not to do this, and I am making the lower planing strip (for want of a better word) wider than scale to assist. I have allowed that the battery can slide under the edf from the rudder position (about 1/3 back from bow) to the very rear.

Regarding my Bluebird, chat on the topic appeared that ALL Bluebirds built want to lift the nose up. I have a completely free run to position the battery from almost at the front , to almost at the rear, to cover all eventualities.

I am ordering some epoxy resin, in order to make a glass fibre top and air intake scoops…or at least, if not using glass cloth, then reinforcing card (or whatever). Eze-Kote does not stiffen card or paper up like resin.

Ashley

[ashley needhamParticipant @ashleyneedham69188]

Ah yes….his forward mounted rudder was also useless at low speeds, but as far as I can see he used quite a deep thin one. Forewarned I will use a much longer rudder to try and get some low speed manouverability.

a

[ashley needhamParticipant @ashleyneedham69188]

foam top, not completed, fan housing mounted

[ashley needhamParticipant @ashleyneedham69188]

Have ordered some epoxy resin from “easy composites” and the plan is to turn a master out of foam on my wood lathe and wrap this in cling film to make a thin two thickness top for the hull.

Used thin ply steamed -to-shape for Bluebird top, but the cabin area on Crusader is perforated by the cabin cutout and the side air scoops, and I think this will weaken a ply top too much to hold shape.

Faced with a big filling job to get rid of the grain on the lite ply, which is actually quite course.

Next big decision is how fat to make the sponsons and the front Skeg? Scale or fatter? The fatter the more buoyancy and more lift generated at speed due to the wider planing area, but obviously a bit more drag……tricky.

Side sponsons are angled quite a bit, unlike Bluebird which were flat. BB was easy, body and sponsons were simply held down flat and glued in place, but I may have to make some sort of template to hold the Crusader ones.

Ashley

[ashley needhamParticipant @ashleyneedham69188]

Resin to hand, made the foam master, covered it in glass woven cloth and resin. Went for a bike ride and expected it to be dry but still wet.

I ordered slow cure hardener, but mis-read the blurb…it says 90-115 mins pot life…ie use before then, but I think it takes about 8 hrs to harden (like last time I used it). Not a problem. Tomorrow morning it will be dry and I can hack it about and see that I have made it to the wrong size!!!!

The great thing about epoxy resin is that it does dry, I mean….not sticky and `orrid, but bone dry to the touch.

Ashley

[ashley needhamParticipant @ashleyneedham69188]

Hull top seems to be taking its time going rock hard, but should be ok this morning.

Looks like the top is about 5mm too narrow, but this may bea good thing at it should then ‘clip’ onto the hull when I put a suitable combing on, and I will have leeway to slap some filler on. If it was exactly the right size it might be more difficult, or at least that’s what I think!

Made sponsons and glued the forward planing wedge in place which needs a strip of 0.8mm ply on top.

Tricky bit now will be mounting the sponsons as they droop at an angle and although the struts meet the hull square on, the hull has sloping sides…with differing slopes for the fr.ont and rear strut position.

I think the way forward will be to make a jig with MDF and a hot glue gun to hold the hull and sponsons in position and then make struts to fit, individually as there is bound to be some variation either side…

Ashley

[Paul TParticipant @pault84577]

Good Morning Ashley

The sponsons were a nightmare to draw because of all the changing angles, I had to establish a datum from the vertical centre line to provide a reference point and I ended up using the theoretical water line (the orange line)

The connecting struts were difficult to detail but to provide extra strength I designed them to pass through the hull/sponson where internal gussets could be fitted.

Paul

[ashley needhamParticipant @ashleyneedham69188]

I have made a jig to hold the hull and sponsons together. I will then mark the hull as to where the struts fit, THEN make 4 over-length strut bits and "trim to suit" and araldite in place.

I cant see any other way of doing it, as you say, the hull changes shape so its difficult. Bluebird was dead easy!

As a working model, the sponsons are perhaps a bit deep, and the dimensions have been tweaked slightly as I am wont to do. Some aspects of the model need to be …optimised.. as I dont want to fire it up and find the back wont lift/front wont lift / its unstable/waves from the bow/sponsons flood the intakes….

When the struts have been glued on, I will be able to use a long series drill and drill through the hull/strut/sponson and araldite in a carbon fibr

Who knows eh???

Ashley snap….jig

[ashley needhamParticipant @ashleyneedham69188]

The new top is wonderfully thin and light but although a “clip-on” fit initially has expanded a bit when filler was applied.

i still have the top half of the foam core, so might make another top, using heavier woven cloth to maintain shape. If this is successful then I will be more sparing with the filler to prevent loss of shape. As I know the former is a touch narrow, I can afford a thicker construction.

Clingfilm worked well to stop the resin adhering to the foam core.

Ashley

[ashley needhamParticipant @ashleyneedham69188]

Made a new top, or at least, its now drying. Two layers of heavier woven glass cloth.

Gave up trying to make in-situ sponson supports. EVERYTHING is at an angle!

Went for drilling holes and setting carbon fibre tubes to hold the sponsons, which I will cover with either balsa of foam.

The top bits can be flat and easily sanded to fit. PLUS I can lay these bits on and hold in place with a flat strip of wood to ensure they line up nicely.

A whole morning and not much to see! want to give the epoxy time toi go nice and hard before moving on.

Ashley

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