$100 Scratch build: Frog Interceptor

[jungmann1]

Back in pre-WWII England, Frog model aircraft were king. The Interceptor was one of the coolest ARF's you ever saw. Pressed aluminum fuselage, paper wings, geared rubber power free flight "semi-scale" fighters that were guaranteed to fly great right out of the box. Prolific English R/C designer Gordon Whitehead decided it was a perfect candidate for one of his early electric designs. This design appeared in RCM in January of 1993. It spans 46 1/2 inches, exactly 4 times the span of the original. His flying weight came out at one ounce shy of 3 pounds, powered by an old 05/540 brushed motor with gear reduction and a 6 or 7 cell, 1200 mah nicad. I bet we can improve on that a bit nowadays. With the price of balsa these days, I might have a hard time coming in under budget, but will give it the old college try. Wish me luck! John




Here is my material list. I am sure I am forgetting something obvious. Just yell it out when you figure out what it is.

Motor : Hobby King part # SK-2830-750 $13.97
ESC : Hobby King part # HKz30A $10.47
Battery: Hobby King part #N1600.3s.25 $13.29
RX : Hobby King Orange orng6 $7.95
Wood order from Lone Star $29.76
One piece 1/8 music wire $1.89
One half ounce each of thin and thick CA $4.00
One half ounce Ambroid $1.00
Servos 1 each DS919MG $5.90 ea
2 each HXT900 $2.69 ea
Doculam 500 foot roll $22.96 Used 6 feet $0.27
Paint and primer $17.52 (ouch!)
Total $111.40

[hillbillynamedpossum]

Looks pretty cool!

Possum

[Stevephoon]

I'm liking this contest already! Very cool plane! Watching and learning...

Steve

[jungmann1]

Got my balsa order in to Lone Star last night. Haven't dealt with them since they came back from the fire. I imagine the quality will still be there. Need to go tomorrow and get a copy or two of the plans made so I don't tear up the originals. Hope that doesn't go against my budget; that stuff can be expensive!

[lucas2696]

Subscribed!

[jungmann1]

Couple more pics of the inspiration for this build. Wish I could afford one of these originals. They go for about $400.00 on ebay in good condition.

[kenchiroalpha]

Originally Posted by jungmann1

Back in pre-WWII England, Frog model aircraft were king. The Interceptor was one of the coolest ARF's you ever saw. Pressed aluminum fuselage, paper wings, geared rubber power free flight "semi-scale" fighters that were guaranteed to fly great right out of the box. Prolific English R/C designer Gordon Whitehead decided it was a perfect candidate for one of his early electric designs. This design appeared in RCM in January of 1993. It spans 46 1/2 inches, exactly 4 times the span of the original. His flying weight came out at one ounce shy of 3 pounds, powered by an old 05/540 brushed motor with gear reduction and a 6 or 7 cell, 1200 mah nicad. I bet we can improve on that a bit nowadays. With the price of balsa these days, I might have a hard time coming in under budget, but will give it the old college try. Wish me luck! John

Attachment 144646

Hi
Im pleased to meet you
This is going to be good
Do enjoy
http://www.thestuarts.eclipse.co.uk/senior_plans.htm
For some reason the home page doesnt work but you can access all the pages from here
Take care
Yours Hank

[jungmann1]

Howdy Hank! Thanks for the link; I had that one at one time but had lost track of it. Frog had some great designs, didn't they? Hope you like what I do with this one. John.

[Dereck]

Hi John
Yes, another who could never afford an Interceptor! No way when I was a kid and they were still on the market, and I'll let others squabble over them on EBay

Gordon Whitehead - I knew him for years through aeromodelling, though we only crossed paths occasionally. We were in the RAF together for much of my 22 years, though in very different branches, and got together at the RAF model aircraft ass'n a few times a year.

I got to see him fly his 'super large scale' Interceptor once or twice - like all Gordon's designs, it looked the part and flew well.

What you'll be looking out for is how much lighter motor, battery and RC gear we have now are used to maintain the model's CG. That 7 cell nicad alone was around 14 or 15 ounces for 1200 or 1700 mA, plus your motor is likely to be around half or better the weight of that geared 540.

Ideal circumstances for your CG to migrate backwards...

Gordon probably used British made 'Solarfilm'. It's lighter than Monokote by a fair amount as I recall. If your budget allows, go for Solarfilm Sales' lightweight 'SoLite'. If you want cheap, 'Towerkote' from Tower Hobbies is lighter than Monokote etc, and pushing half the price. As most of my models are to my design and have a certain degree of uncertainty about them, I use it a lot

Good luck

Dereck

[jungmann1]

Hi Derek,
Your last name must be Woodward. I have seen your name pop up in the modelling press quite often over the years, too. I really appreciate your input on this project; what you've stated already is pretty insightful, considering you haven't, I assume, seen the plans yet. I got lucky on one of your points in that Gordon placed his battery tray right on the C of G. But the motor is a real consideration. I have no idea what the old brushed motor with gear reduction weighed but I am sure you are right that whatever I hang on the nose will be much lighter. In Gordon's construction article, he mentions that there is a difference in tail construction between his model and what is shown on the plans. He had stick built his tail and came out nose heavy so he changed the tail construction to solid sheet to help alleviate that. I think I should probably go back to the light weight tail on my bird. Hope you stick around to keep an eye on me while I build. One of the great benefits of this type of forum is the great feedback and advice you can recieve from guys who have already "been there, done that". Later, John.

[jungmann1]

"Gordon probably used British made 'Solarfilm'. It's lighter than Monokote by a fair amount as I recall. If your budget allows, go for Solarfilm Sales' lightweight 'SoLite'. If you want cheap, 'Towerkote' from Tower Hobbies is lighter than Monokote etc, and pushing half the price. As most of my models are to my design and have a certain degree of uncertainty about them, I use it a lot " Unquote

I think I will probably use doculam for a covering. Not sure it you are familiar with it but it is one of the lighter films I have used and takes paint quite well. As elaborate as the paint schemes were on this little bird, I think I need to paint it to do it justice. John.

[Dereck]

Hi John
The somewhat unusual spelling of my first name is a big clue! Okay, momentarily into a 'commercial break' - you see it atop 'Over Here' in Traplet's 'Quiet & Electric Flight International' magazine. I 'cover' the US electric scene. There is massive bias towards electric models that folk have built and put effort into - having personally signed every credit card slip for the most expensive model this month is usually ignored ...

Something built in the US from an OFB's English plan would be straight in the next column...

I've been writing a monthly for Traplet since 1988, so could be doing something right.

Just weighed the nearest I have to Gordon's Interceptor drivetrain, a greared Astro 035G cobalt, albeit minus the motor brushes - it came out at 6.8oz. Okay, I keep it out of sentimentality and intend to replace the brushes and put it in something appropriate one day.

If you work on seven ounces for a typical geared ferrite as he used, you won't be far off, they were a little heftier than the alloy cased Astro rigs. I've found that most models that have spanned the brushed/nimh to outrunner/Lipo trade-over need the battery shifted forwards some. Just keep an eye on the hatch location and if it would affect your battery access.

The built up stabs look to be a real good idea. Use a typical stick built frame of 1/4" thick sheet and strip and you shouldn't go far wrong. Even cooler, and much more complex, would be a built up with a symmetrical wing-type section that showed off ribs through the covering. 3/8" deep at maximum and a lot more work spring to mind.

Doculam - great stuff. I've only tinkered with it, but know plenty of folk who've done real good jobs with it, and it's tought as you need. Recall spraying test pieces with some 'rattlecan' spray paint and being impressed with how well it stuck. All willing, will be re-covering an Electrifly Focker DR1 in Doculam sometime soon, finishing with brushed water soluble acrylic hobby paint - spraying and stinky stuff is not good in a condo.

The Interceptor deserves a real good looking 'period' paint job. Must be something in the 1930 US Army Air Corps that can be adapted - many Interceptors were done in various European schemes, so a US variant would be good in your case.

You need any suggestions, just holler. Aeromodellers will be watching your progress with interest.

Regards

Dereck

[jungmann1]

Got a little start on the parts "kit" for the Interceptor. Still waiting to hear from Lone Star on my balsa order so I broke out a rather rough looking sheet of lite ply to make the fuse formers. Kind of a neat system Gordon worked out for the cowl attachment. I was going to sub some magnets but figured, might as well follow the plans and see how it works. At least it is only one screw and there is a generous size access hole in the nose.

[jungmann1]

Couple more pics of my progress so far.

[jungmann1]

Here is a shot of the cowl/firewall area. I thought it would be easier to do this bit of assembly now rather than after the fuse was all together. Sure wish that balsa order would get here; I am jonesing for some balsa to work with.

[Dereck]

Formers are looking good - only Gordon could build a sports model to be more complicated than his scale designs! Good move doing the cowling formers first. Too many rush into the big wooden bits and suffer later trying to fit small stuff like that into a finished model where they can't really see what they're working on.

Of course, the tiny and incredibly strong rare earth magnet hadn't been marketed when Gordon built his Interceptor. Knowing him, he'd have hated just screwing the cowling on if he could figure out how to hide its fastening some.

Have you thought of using the top latch as per plan and a magnet for the bottom fastening?

As long as that doesn't ruin the $100 thing

D

[jungmann1]

Hi Derek,
Yes, I immediately thought about a magnetic fastening for the cowl and may yet go that way. I am sure you are right thinking that Gordon would have used such if available at the time. And, I don't think it will be a problem, budget wise. Got my Lone Star order in yesterday. Nice wood and super fast shipping! And made a bit of progress on the fuselage. Gordon was sure thinking of this as an electric right from the start. He designed the battery compartment as a key element, almost a jig for the fuselage construction.

[Dereck]

Ah! the smell of a fresh box of balsa, and the puzzled look on the Spousal Unit's face...

Interesting battery compartment indeed. How is it for getting the battery out for swapping or recharging? When Gordon designed this model, the English standard battery hatch was the wing!

It was known as the 'Seven Cell Trap' and meant that most electrics had seven sub-C sized nicads, looked like gliders, used the wing as a battery hatch and refering to what they did as 'flying' was being kind.

Gordon's Interceptor was one of a rare breed of electrics that actually flew well in the usual 'sports model' style. It was a good trick considering the weight and available power of its drivetrain. Idly thinking that this is too good a model to scrimp and save on - if it runs too close to the magic number, throw that idea out the window, carry on and do a good job on your Interceptor.

If you go with a magnetic cowl fastening, watch out. I just built a new design low winger with a top hatch. As it's a bigger model, by my standards, I used some magnets I already had, around 3/8" diameter. The durn things are that strong, I didn't even make a hole for the second one I placed, it's holding fine through the 1/16" balsa hatch bottom.

The other is placed face to face with its fuselage mounted element and I can barely get the hatch off

Keep it up, she's looking good

D

[jungmann1]

Hi Derek,
The battery compartment is open and accessed via a hatch on the fuse bottom. The aft end has a ply tab, the front is held in place by rubber bands that also retain the plug-in landing gear. Not sure about retaining that particular feature, though.

[Dereck]

After a fair few electric models, I've always found it better to make the hatch as a non-structural cover. The battery is supported by a built-in structural tray. In the case of the likes of yours, where gravity is doing you no favours, velcro on the top of the battery and a single velcro strap around it to help out should do fine with that size of pack.

Have two electrics behind me in the last century where the battery went in through the bottom and was held in place by a couple of rubber bands. Not only do I pre-date LiPo, I was around BV (Before Velcro).

Just be a little paranoid over how a battery falling out of your model could ruin a good Sunday at the patch...

Best reason to build off plans is that they make a good starting point Suspect Gordon built the plug-in UC to make it easier to toss the model into his car - that was usually one of his design aims!

D

[jungmann1]

Got a bit more done in the past day. Fuse is coming along pretty well. The aft most former was too narrow so I had to cut out a new one to make sure the fuselage sides flowed right. I also had a problem with the match of the wing cut outs in the fuselage side and the wing saddle doubler. I used the seperate template for the doubler, and while the curve of the doubler matched the airfoil of the wing ribs well, the fuse sides I cut out were pretty seriously undersize to the doublers so new ones were in order. The second set I just left the wing saddle area of the fuse sides oversize and trimmed them down once the doublers were glued on. Worked much better.

[jungmann1]

Hey Derek,
Yes, my battery hatch will definitely be non-structual. I, too, have lost aircraft due to failure of battery hatch latches. I am in the habit of using velcro seat belts to restrain my batteries and plan to do that here also. Thanks for keeping track of my build, Derek. Stay in touch.

[Dereck]

The joys of building from OP (Other People's) plans!

This one was done in time honoured fashion, using such museum artifacts as pencils, drawing board and strange pieces of sheet plastic cut to exact angles. Which meant it was much harder to move a line than if it was 'drawn' in CAD.

This suggests sneakily checking the fit of parts before cutting them out

Which, as has been proved by a recently digitally produced plan, applies as much to modern plans as to ones from the last century

FROG holds a fond spot in British aeromodellers' hearts. The Interceptor is unique - it was ahead of its time and little ever got close to it. They also made many conventional balsa kits, some of which have been 'fudged' into ultra largescale electric RC models of late

[jungmann1]

Yeah, I hear you, Derek! I have been plans building for a lot of years and have yet to find one that didn't have some degree of fudge factor built in. My previous post wasn't a complaint, just an observation to help out anyone else who might give this design a shot. In the constructioin article with these plans, Gordon mentions that he drew them up on his kitchen table so who knows what implements he used to create the lines and angles that define his final design! I have been away from building for the last few years so I might be a bit rusty on the finer points. Later, John.

[Dereck]

Gordon likely used pretty much what I did. Some of my squares dated to my late 1960's days as an apprentice coppersmith - we had a day and an evening at a trade school then. My first plans were drawn on sheet of 1/4" ply around three feet by two feet, brush painted in white.

However, model mags then had very good pro draftsmen who could sort out designer level drawings into mag formats. Know one mag now where the editor's son, who's in college, sorts out their CAD plans into their standard format. Most CAD plans are drawn up by 'enthusiastic amateurs' at best

Flexibility and pragmatism are still very needed

D