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Old Aug 01, 2014, 02:33 AM
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Zenair 701 Robin Fowler 'after-build' log.

Hi folks,
I was attending a vintage fly-in over on the east coast of UK hoping to research the Hornet moth (which I did) but a tiny aircraft had been pulled out of the hangar. I took a number of photos of it with the thought that they might come in useful one day.

It was only as I began drawing up my desired Hornet Moth that I realised that it was too big at 1:4 scale to fit on my drawing board. Suddenly I found myself looking up the smaller model, which I went ahead with. (I now find that it fits this forum very tightly on two counts... it is both 1:4 scale and spans exactly 80" - made for the job!)

Here are the photos I took...

Robin
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Old Aug 01, 2014, 03:11 AM
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Researching on the web I found some more photos of the same aircraft (and others) PLUS, because it is a home-build design some fairly detailed drawings are downloadable from the Zenith Aircraft website - which is a big bonus.
Also on YouTube are a number of videos showing the short field performance of the type... more than enough to get me enthused about building a model.

Robin
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Old Aug 01, 2014, 05:45 AM
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Old Aug 01, 2014, 07:15 AM
I eat glue
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Hee hee, mine has begun!
Got the fuselage side sheets joined, and the longeron stock all cut out. This weekend just might see the fuse sides all built up!
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Old Aug 01, 2014, 11:01 AM
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Welcome raife- and Craig!

This post should keep me just ahead (for the moment!)

The tail end is nice and straight forward. No tricky bits... except perhaps for the cutting of the flaperon arcs in the side.
Set the pivot point and mark it then make up a couple of activating arms from mild steel or brass - whatever is available. Three holes need to be drilled in these, one for the pivot point sized to the wire pin so that it just taps in place, one about two thirds to three quarters the way along sized to the actuating wire (double bent to connect) and the third to take a short length of plastic tube so the flaperon end pins will have a softish connection.

Swinging these arms around their pivot points will give the arc required.

The tail itself needs keeping light so preferably no hard - or even medium-hard balsa used here. Once the sides have been made up it is an easy job to put a pencil mark where the various stiffening members go then the longerons can be fitted and all the other members on top and cut over them. Left and right hand sides complete the top sheet is added with grain running across the fuselage and the cross bracing added. The bottom of the fuselage is held meanwhile by a single strut from 6mm medium balsa.

Now we come to the joining sections that form the bottom frame of the door openings. I was pretty sure the aircraft I was modelling had a cabin that narrowed slightly towards the front rather than the parallel sided cabing shown on the Zenith 3-views, so that was how I did mine. Whichever way your example was built it is probably going to help if you cut out F2 at this point to act as a template.

The sheeting at this point goes on the inside face with the framing glued on to that. Outer sheeting will be added later.
Now IF I got my geometry right (no guarantees and no claim of infallibility!) the cabin should be parallel sided. If not, adjust to suit.

I am expecting some scratching of heads at this point - probably one being mine as I try to remember what really happened three years ago! The main joiner not only has to be the right shape to set the sides to the correct angle from the top but from the side - and to add to that it is itself sloping down towards the front... Not really that simple!

Still having fun?
Robin
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Old Aug 01, 2014, 11:10 AM
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Just as an aside... the range of undercarriage patterns is quite wide for a home-built design and the one I happened upon is not the best. Sure, those spatted wheels look nice - but they didn't last more than three of four landings - and the dural cross bar I used proved too soft to take the shocks, bending up out of shape far too easily. I soon re-bent it to the pattern shown on the Zenair 3-views, in which form it is far less liable to deforming.

Just thought you ought to be warned at the outset... and it is another reason for building light!
Robin
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Old Aug 01, 2014, 11:39 AM
I eat glue
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The tail end on mine is probably going to be heavier than it should be, had a hard time getting good wood here so I used what I had. I will be putting lightning holes in the tailfeather ribs though to compensate a bit. Might save some weight on the finish as well since I will probably use poly span and finishing resin. That works out pretty light, but strong. I've seen a lot of the 701's that had the slightly sloping forward fuse, but I will build mine as per the plans. I might end up finishing mine off as I would if I had built the full size. That way I won't have to worry about being picky about scale fidelity! I assume I use the same flap linkage even when using a computer radio with mixing rather than the mechanical mixing?
Still having fun!
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Old Aug 01, 2014, 12:15 PM
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Well Craig I guess you could have the ends of the flaperon pins locate directly into a hole on the servo output arm if you get the arm length between pivot and the operating point exactly the same as the output arm.

There are two possible problems with that,
1. There is no scale-down effect except for the option of reducing the throw at the Tx and,
2, By the time you get the movement small enough (+/-1/8" for aileron) for the model not to cavort about like a mad thing at the least aileron stick movement I would be wondering about servo resolution.

At least with indirect operation I was free to use any hole on the servo arm.
Better in my view to have the servo movement high for the resolution and gear it down mechanically.
I had enough work to do getting the controls so they were reasonably well harmonised as it was.

The only downside of that is the more connections the more slop in the system.
Robin
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Old Aug 01, 2014, 01:33 PM
I eat glue
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Aah, so you're saying sticking with the full mechanical version as designed is probably better. Then that's what I will do.
Went to get my plans copied so I could build on one set this weekend, and the dang print shop is closed for a long weekend!
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Old Aug 01, 2014, 04:04 PM
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Ah, patience is a virtue.... and sometimes it is forced on us!

I think it's a matter of choice. Either a six or two threes.
The two add up to the same and it may be a case of what you gain on the roundabouts you'll lose on the swings.... How am I doing for trite and cliche?

Actually you know your Tx and what it's capable of. My choice was forced on me by having only two mixes available to start with. The aileron movement needs to be small with full servo movement whereas the flap movement is much more. It's a bit of a mismatch in that way and I can only say that the system I worked out worked - but it took a bit of effort.

Seems like you have the weekend to think it through.
Robin
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Old Aug 01, 2014, 04:56 PM
I eat glue
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You just had to rub that in, didn't you! Yes, I have the weekend to think it through.
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Old Aug 02, 2014, 10:58 AM
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eye4wings's Avatar
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Meanwhile you could make up the metal operating arms?...
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Old Aug 02, 2014, 12:10 PM
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Gotta check my bin of bits first.
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Old Aug 02, 2014, 01:58 PM
I eat glue
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Did some digging in the scrap box and I think I found enough relatively light 1/16 to get started on the tail feathers. Think I'll do the rudder/fin first.
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Old Aug 02, 2014, 04:28 PM
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Well okay, I see the logic.

Barring hangar rash it's the longest-lived part of any model.
Softest grade 1.5mm available...

I have just 3 shots of the rudder...

Robin
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