Prophet 941 - RC Groups
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Dec 05, 2012, 10:01 PM
Lapsus calami
Undercarter's Avatar
Build Log

Prophet 941

The madness has returned after many years. It’s time for first principles - and a cutting-edge design from 1984. Let’s build an old classic out of balsa ’n glue and get forgotten skills going again. Why not? There’s nowt on telly and there’s been a fire at the exchange - the phones are down and the internet ist kaput.

Bought this old bird on ebay couple of years ago from an old timer whose eyesight was on the wane.

Where else but in the land of the free and home of the brave would an aircraft be named Prophet? Was this model born of epiphanous vision in Salt Lake City one dark and storm night? According to the instructions:

“Word origin: PROPHET (Greek pro- before + phanai to speak) from Greek prophetics, a divinely inspired predictor. One who foretells the future.
And that, fellow sailplaners, is Webster's definition of the word PROPHET! The word PROPHET also describes your new sailplane kit, which is a true foreteller of your sailplane future.”

Strewth! Is that snake oil I smell arising from the old and battered box?
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Dec 06, 2012, 12:39 AM
Registered User
prodjx's Avatar
I scratch built a Prophet4 from plan's and I have to tell you I've had alot of great flight's with it off the winch high start and slope. I should repair it from damage on the slope. I once let Brian Agnew fly it and from a handlaunch he specked it out and then he said mine flew better than several that him and his dad built and flew. I've thought about making a mold for the fuse.
Dec 06, 2012, 07:17 AM
Thermal Wrangler
DrFragnasty's Avatar
Any changes planned? I only ask this as I have a completed 941 in storage (mint condition) and given the chance I'd run a different section on it and use composites for the spars and joiner.

Good to have another on the board, looking forward to updates.
Dec 06, 2012, 09:23 PM
Registered User
I foretell that if you have the talent of Tom Keasling, you'll be able to do very well with this glider in contests. That's not very much in the prophecy department as I saw him do it when he was younger. Before that, he was beating us all up with a Gentle Lady!
Dec 06, 2012, 10:54 PM
Lapsus calami
Undercarter's Avatar
Laying out and getting to know the model commenced one evening t'other week - after cleaning up the mess stacked high and spread wide across the bench (what great sage once observed that to us blokes, tables are shelves?). It was so hot and sticky - massive invertebrate hatch bringing tree frogs out to hunt on the windows - that I not only poured over the plans, I poured on them.

First impressions of contents (after knee-jerk ‘What the Dickens are all these bits for?’) are that there is plenty of hard and heavy balsa, but it’s all here and in pretty good nick, so who cares? Some of the hardware is ordinaire, but some is superb - control cable outers are silky and cables are top quality. The alloy tow-hook slider is a treat.

Reassuring advice from instructions: “This kit can easily be built with any of the new super glues now available.”

A few options pondered:

We’ll go with spoilers . . . though I am tempted to try flaps for those tight LZs when slermalling in volcano country. (Who am I kidding? I’ve got my hands full completing the model as specified in competent fashion without experimenting with fancy ideas and additional complexities. In addition, I want to keep this bird as period authentic as possible. Get your head out of the clouds, sport!)

Rudder pull-pull would be nice. I can use the good quality spoiler strings for cables, as wing servos will be used for spoiler actuation (what was that about period authenticity?).

Wing attachment will be dowel and screw, rather than lacker bands (linguistic giveaway, that).

Note: the dowel and screw option is shown on the powered fuselage version on the second plan sheet. Some profile dimensions differ considerably (eg. depth of fuselage) from the first sheet, so care will have to be taken. Closer inspection of the plan suggests the paper has stretched over the years as most dimensions are slightly bigger than the cut parts. Or has the balsa shrunk? (Your brain more like.) Ah well - there’s going to be some fudging as we go along with this one (just like the good old days).

Some time ago, I downloaded a copy of RC Soaring Digest dated February 1986. The Currington Capers section contains plans for improving pivot bearing for stabilators that also enables them to be detachable. Thanks Ernie - I like this a lot, though I might dispense with the detachable feature.
Dec 07, 2012, 05:07 PM
Registered User
ieyasu's Avatar
Oh boy, a Prophet 941 build I stared at the Prophet series through middle school and finally put together an Ariel (the HLG) in high school. Now I'm trying to get started on a Prophet IV scratch build, just gotta stop getting sick. I've heard mixed reports on how well the 941 flew, but I always liked the look and looks are as important to me as anything so I'll be interested to see how this goes for you.

Below are scans from the catalog that introduced me to these beauties. Hope you don't mind me sharing them in your thread here.
Dec 08, 2012, 03:11 AM
Lapsus calami
Undercarter's Avatar
I am a slow, slow builder. Many hours indoors sheltering from the big heat in these days, listening to the Third Test from the WACA against the Proteas (heads up for the Great Unwashed - it’s cricket) - a couple of long sessions stretching into the wee early hours - all the while feeling utterly isolated in my quiet rural corner waiting . . . waiting . . . waiting for the phone and internet to come back on line - 10 days since the big fire at the exchange and no service.

Fuselage is largely complete without too many problems. Some small mistakes and one misreading of the instructions. It was good to start with the fuse to get the plan-reading eye back in, making sense of instructions, and getting some building skills going again.

Couple of mods. Doubled the floor just behind the nose block to permit vigorous sanding to shape nose while providing additional strength for those inevitable heavy landings. Extended ply doubling (0.8mm) on the sides under the whole wing saddle area. This was largely to provide strength for the wing bolt-down option. Hopefully, this will spread stress load and not simply create a weak point aft of the anchoring frame.

When released from the building board the strength and rigidity of the structure was immediately apparent. To celebrate, I selected a suitably dramatic film theme and swung it around like chunky Australian opener Davey Warner’s bat; “Global forty-five Lincoln Tower. Clear to land runway two-niner. Wind one-fife gusting two-fife” - cue the strings and brass to bring her in. (Settle down Ace - it’s only the ruddy fuselage.)

Intense pouring over the plans and instructions to make sense of the stabilator fabrication. The next bit is going to take some careful crafting.
Dec 08, 2012, 03:23 AM
Thermal Wrangler
DrFragnasty's Avatar
Nice workbench.
Dec 08, 2012, 06:39 PM
Lapsus calami
Undercarter's Avatar
Stabilator and rudder fabrication is complete - a straight forward process in the end. It remains to trim and sand them to shape, a much bigger job that will test my skills.

Two small gussets were added to the stabilator trailing edge framework to strengthen the joins, and packing added to improve the connector rod installation.

The instructions sound a warning worth bearing in mind - after all, most mods add weight to the structure: “Cut lightening holes in the keel if you desire. Remember, 1/4oz. removed in the tail will remove 1oz of lead in the nose.” A drill and Dremel session is scheduled.

The rudder employs the same construction and structural principles as the stabilator, relying on laminated balsa spars for torsional strength and 1/8th square stock for ribs. This design is simple and robust, providing impressive rigidity, though it cannot be the lightest of structures.

Another note of warning from the instructions: “This aircraft can fly very fast, so glue each joint securely.”

And now an historical note: second pic shows Ricky Ponting on the telly, walking out onto the WACA for his last innings in Test Cricket. Time to hang em up with pride, Punter.
Dec 09, 2012, 01:40 PM
Lapsus calami
Undercarter's Avatar
Finding the sanding process - sculpting the raw balsa into near final aerodynamic form - a slow but very satisfying and sensual experience - feeling the grit bite and the balsa resist - changing with the paper and timber grade and grain direction. Nice new abrasive materials, shaped blocks and a pane of glass help with accuracy and promote a technique that is comfortable, consistent and efficient. Diamond files, emery sticks and abrasive tapes help with finesse. Oh how I struggled with this when I was a whippersnapper - hence my initial disquiet.

Tapering the stabilator spars span-wise from 1/8th inch at the root to zero at the tip was the biggest challenge. Started by sanding a flat ramp using scrap balsa as prop at the root to establish the approximate angle, using short strokes across the sandpaper. When a flat was sufficiently established on the spar, used this to feel and guide freehand sanding, using long even strokes across the entire width of the paper and figures-of-eight. Carefully monitored progress for accuracy and adjusted as required.

Soon paused to clip thumbnails. Robust thumbnails, normally a useful modelling tool, got in the way and caused some minor nicking.

I collected some of the balsa dust in a jar to mix with glue to make filler (ala micro balloons).

Suitable soundtrack to induce working rhythm; Moby's 'Extreme Ways' (would have gone down a treat in the old triremes).

The pic shows pre (24.5g) and post (20.5g) shaped stabilator, and the rudder before shaping (15g).
Dec 11, 2012, 01:48 PM
Lapsus calami
Undercarter's Avatar
Well, I made an elementary blunder. I kept sanding stabilator sheeting to shape - tapered from 1/8th at the spar to zero at leading and trailing edges - for too long without a break and got jaded and impatient. No permanent damage, but I did take wood off several places I shouldn’t have. Soon fixed by discrete application of surface filler paste made of balsa dust and aliphatic wood glue, skim-coat body filler and replacing a small panel. I’m cross with myself for being cavalier though. This kind of crafting requires the calm and considered touch and feel of a healer, not a Jack the Ripper let loose wielding 80 grit garnet rushing to make an appointment with his psychotherapist lunacy.

I compared the rudder (14g after shaping) with a similar sized scratch built rudder (7g) I made last year. The Prophet design is heavier but much stronger. A twist test on the scratch built rudder led to failure at a joint, prompting plans for gussets and a laminated spar. The Prophet empennage will undergo a careful lightening program.
Dec 12, 2012, 07:55 AM
Lapsus calami
Undercarter's Avatar
Back to the fuselage. Fabricated servo tray and trial fitted in cockpit, along with receiver and battery pack. What at first appeared a cavernous cockpit soon filled with equipment and space became tight. There’s still sufficient room for lead at the front, some padding around the battery pack and fore-and-aft packing around the receiver. Provision will be made to secure a small altimeter and lost plane alarm.

The location of the rudder servo is designed to provide pull-pull cables a straight line-of-sight run to the tail, clearing ballast blocks in the next compartment and wing bolt-down structure in the compartment aft of the ballast. Stabilator cable will run along the port side until aft of the wing bolt-down frame. Spoiler servo loom will run starboard side under the servo tray. A switch may be installed on the first frame (I don't really like switches - extraneous complexity).

Looks OK so far . . . fingers crossed.
Dec 14, 2012, 07:09 PM
Lapsus calami
Undercarter's Avatar
Progress slow but steady - mainly fiddly bits in recent days.

Running the stabilator control cable involved some delicate work with twist-drill and file. In the rear fuselage, the outer is held in place by two blocks of Styrofoam so that it locates centrally and cannot flap around during flight, but can still slide fore-and-aft to aid final fitting when the fuselage is sealed.

This process not only informed pull-pull cable run, but also location of servo tray, which will now be moved forward of the RX. This is better weight distribution and places the pull-pull servo where first planned (servo tray back-to-front in pic).

Installed a flash toggle switch I’ve had laying around for years (very Rolls Royce).

Drilled some lightening holes in the servo tray (overkill and ineffective, but good practice) and rudder. I don’t have a pedestal drill, so round I went to my good neighbour Farmer Jack's. His shed can only be described as a shambles, his pedestal drill a wreck. I was questioning my course of action as I installed a shiny new bit in the scarred and rusty chuck. Flicking the on-switch provoked a cacophony of grinding and screeching as the machine shook and wobbled. I mentally grimaced as I gingerly placed a carefully crafted component on the work table and gently pulled the lever. I had no choice. My neighbour, curious about my eccentric purpose, was leaning on the back of my chair looking proudly over my shoulder to see how I would handle his favourite precision tool. Somehow, I managed to do a passable job, but after thirty tense sweaty minutes, I went home exhausted. After a rest and a drink, I cleaned up the holes with Dremel and grit, and was pleased with the result.

I fabricated the rudder pull-pull horn out of two plywood horns from Paul Daniels at the now closed and sorely missed North Queensland Radio Control.
Dec 15, 2012, 02:57 AM
Lapsus calami
Undercarter's Avatar
That's more like it.

Had a rummage through the scrap box and came up with a piece of aviation-grade 2mm 5-ply. So used the first effort (1.5mm 3 ply) as a pattern and crafted this one. Certainly stronger and suspect it is lighter, though my daggy old postage scale isn't up to weighing such light components (might have to improvise a beam balance - I'm enjoying this :-)).
Dec 15, 2012, 12:13 PM
Balsa breaks better
Thermaler's Avatar
Hey Undercarter, would you happen to have the entire 1993 catalog scanned?
Sure would be nice if you could post it to the R/C Soaring History thread ;<)
I would like to buy the plans from you when you are finished if you would be willing to part with them ;<)

Thank You,


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