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Nov 02, 2010, 11:45 AM
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Trevorh's Avatar
Build Log

Ivan Pettigrew Chipmunk

Well, the glue is setting on the first joint, so I guess it is time to start this thread.

A bit of background: I have built two of Ivan's designs, the Sealand 480 and the Rapide, and been impressed with both. I was also lucky enough to get over to Chilliwack this year and saw Luke's and Al's Chipmunks perform. Prior to that I had also seen Pentaxman's flying at the BIMBO meetings.

So, in many ways, I feel that I know this model already.

However, it is always nice to do something a little bit different so I am contemplating a few changes to Ivan's plan:

i) Oleos: This will be a first for me. I have ordered a pair from Unitracts and will post pictures when they arrive.

ii) Flaps: I have enjoyed playing with the flaps on the Astro Hog although it really is too light to need them. The Chipmunk will hopefully be on the light side too but we shall see.

iii) Hatch: The plan shows a long hatch from motor bay to wing trailing edge which certainly gives great access but is it really necessary? The only access needed when the wing is in position is for battery changing so a hatch in the anti glare panel will hopefully suffice. Everything else should be accessible through the wing aperture.

iv) Power: Even though the three examples mentioned all fly perfectly well on 3 cells, it still looks to me like a 4 cell model (and I've got too many three cell models already!). I have ordered a motor which hopefully will be suitable. Again, more details when it arrives.

So, here's the symbolic first picture. The mainspar is from 1/8in balsa sheet, spliced to accommodate the dihedral. Don't panic - there are hardwood reinforcing strips still to be added!

That's all for now.

Last edited by Trevorh; Nov 20, 2010 at 12:54 PM. Reason: Updated web links
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Nov 02, 2010, 11:51 AM
Slip the surly bonds...
Sopwith Mike's Avatar
That looks familiar! I usually start with the ladder frames of the fuselage, or even the tailplane, to get my "building eye" back, but wherever you start, it's a really interesting and involving journey.

Those hardwood strips should be on now!

Nov 02, 2010, 12:02 PM
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vonJaerschky's Avatar
Great to see the thread started. Please ensure all errors are well detailed, so I don't repeat them! Which oleos did you order, Trevor? I had a look at Robart struts but they are ridiculously expensive. Hobby King has some, but they appear too large and heavy. Unitract seems to have several different flavours that would fit the bill.
Last edited by vonJaerschky; Nov 02, 2010 at 12:38 PM.
Nov 02, 2010, 01:06 PM
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Going to love following another great Ivan build!

Good on ya!
Nov 02, 2010, 02:05 PM
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pentaxman's Avatar
Good luck Trevor.

Toughest part is the lack of documentation for the lower fuselage right behind the wing.
I know you will, but study early and work out a solution before you get there.

As to flaps, I doubt you need them with this bird. She flies slowly anyway.
Nov 02, 2010, 03:15 PM
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Trevorh's Avatar
Thanks for the encouragement folks.

Mike - Yes, I've just glued the basswood strips on - the glue is drying as I type.

VonJ - I went for Unitract sport oleos, but I have only the vaguest notion what they might look like! They should arrive in a week or so.

Adrian - Have you an Ivan project underway yourself by any chance?

PT - Thanks for the early warning about the fus/ TE joint - just the sort of timely info I need. The fit of the fuselage around that notched LE seems a bit vague too. I have got a canopy from the Sid King mould via Sarik Vacform. It is spot on for height and length but a touch on the wide side. I suspect that the canopy is to scale and Ivan has thinned the fuselage a bit. I intend to widen the fuselage 1/4in or so to avoid having to squeeze the canopy too much so will have to take extra care over the wing / fuselage joint anyway.

I agree about the flaps being unnecessary, but I do like the look of an aircraft on finals with the flaps dangling. And besides, I've already bought the hinges and servo!

Nov 02, 2010, 04:08 PM
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LukeZ's Avatar
Signing on Trevor, and it will be good to see how another modeler does things. You are indeed making several changes which I will be quite interested to watch. I agree with PT about the lower fuselage, though you will have no problem in the end.

The other nice thing is that we will now have not only a second Chipmunk thread (the more the better), but a Chipmunk thread over here on RCGroups. I added a link to this in the Ivan list.

Nov 02, 2010, 04:29 PM
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paulbw's Avatar
Hi Trevor,
I'll be watching too - another favourite aeroplane of mine!
Paul W.
Nov 04, 2010, 09:58 AM
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Trevorh's Avatar


Well, I now have the Unitract Oleos. They look like straightforward, workmanlike units that should be well up to the task and the good news is that my interpretation of the sizing seems to be about right.

As a complete newcomer to the world of oleos though, I have a couple of queries:

i) Damping: The units are damped by means of an O ring and a special grease. They are supplied pre-greased and I found that I had to lean on them quite hard before they slowly compressed then they would take 2 -3 seconds to recoil. Since the main idea is to absorb bumps in the ground whilst taxiing, this level of damping seemed excessive. I have taken one of the units apart and wiped off quite a bit of the grease and this has improved things but they still seem a bit slow.

Any advice on what sort of recoil time to aim for and how to achieve it would be appreciated. I do have some Teflon grease which I use for lubricating gearboxes and I wonder whether that would be better. However, I'm a bit reluctant to clean the legs off and try it, since there is no spare grease supplied. (Ambersil ejector pin lubricant is the recommended product)

ii) Spring rating: The springs do move slightly under a load of 2lb or so but, as you can see from the photo, even at 4lb the legs are only compressed about 1/4in. Are these springs too stiff for a 5lb aeroplane do you think? I have no idea how many g the u/c sees on a typical landing but again, considering the taxiing scenario, it would seem to me that the legs ought to be compressed a certain amount when the model is just sitting on the ground - but I have no idea by what proportion of the available travel.

The instructions do say that stronger springs are available - I haven't yet enquired about weaker ones.

Meanwhile, the spar is built and rib cutting has started. . .

Nov 04, 2010, 10:17 AM
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vonJaerschky's Avatar
Earlier this summer I watched in amazement as an electric 1/8 scale 4WD vehicle raced across our grass field at what must have been at least 60 kph. The wheels were going up and down like mad, almost a blur, and the chassis/body of the truck remained totally flat and level, seemingly floating across the grass. It would be fantastic to be able to put that sort of suspension in an oleo. The travel of the truck's suspension was quite huge and probably could not be duplicated in the confines of the Chipmunk's landing gear dimensions, but I wonder if somehow something could be easily adapted from the R/C car community.
Nov 04, 2010, 11:21 AM
Slip the surly bonds...
Sopwith Mike's Avatar
Originally Posted by vonJaerschky of the truck's suspension was quite huge and probably could not be duplicated in the confines of the Chipmunk's landing gear dimensions, but I wonder if somehow something could be easily adapted from the R/C car community.
I was thinking of that too. The bit of plan we can see in Trevor's photo shows that there is a trouser round the UC leg. Some sort of parallel suspension system within it, with fore and aft legs linked by a couple of torsion bars...but he's bought the oleos now!

How about a chat with Darrell at Lesro Models Trevor? He's the RC cars king round here and will know what's available.
Nov 04, 2010, 11:54 AM
Complete the Illusion!
35Mhz's Avatar
I can provide a bit of insight on Monster Truck suspension. Basically just like full size auto suspension in miniture. Coil spring to provide the "sping", and again different spring rates for tuning suspension, and oil filled damper to provide the damping, changing the oil viscosity to vary the damping rate (or changing the internals with bigger or smaller hole in which the oil is forced through.)

For a plane the main issue is the spring rate to absorb the shock on landing (several G i would guess), the damping is provided by friction to avoid the pogo effect. Sounds like a bit too much friction if spring is not returning. Your not racing a plane round like a monster truck!
Nov 04, 2010, 12:01 PM
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Trevorh's Avatar
Interesting. As part of the slippery slope which saw me resume my aeromodelling 23 years ago, I did build a Tamiya Boomerang. It came with a ball race conversion set and in fact, I plan to use some of the nylon bearings liberated from it for the flap rod bearings on the Chipmunk.

The Boomerang has oil damped suspension which works incredibly well, just as vonJ describes - and what's more, it came with a bottle of damper oil which I still have. Who says hoarding doesn't pay!?

I'm sure it is just a question of finding the right lubricant to suit the weight of the plane.

Nov 04, 2010, 01:24 PM
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pentaxman's Avatar

I can't imagine springs which at 4lb are only compressed about 1/4in being good on a 5lb plane.
That would mean that each unit would only move about 1/8" at rest.
I would have expected the leg to move about 1/4 - 1/3 of the travel with the weight of the plane on it.

Sounds like the spring rate is way too high, so the springs will need weakening or replacing.

I also can't imagine the rate of reaction that you are getting being suitable for the plane.
That leg needs to be moving far quicker but not as fast as a RC Car. Maybe use oil instead of grease on the shafts & O rings?

May as well use piano wire legs if that is as soft as you can get the springs.
Nov 05, 2010, 04:16 AM
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paulbw's Avatar
Hi Trevor,
We are probably not comparing like with like, as the RC car units can and do actually have an oil-based damper, whereas the a/c units are simply a spring whose movement in both compression and return is impeded by the friction of a heavy grease. I haven’t as yet found a properly realised ‘damper’ for model aircraft use.
I think the damping, however achieved, is to avoid that kangaroo feeling on landing, as against to provide a long travel when taxying – full-size light aircraft tend to avoid ruts and ramps like the plague anyway – or creep up on them at 45 degrees!
If the model weighs 4 lbs and the spring rate allows full compression at between 2 and 3g that should be somewhere near the springs you are looking for? (Just off the stops at 2g) Most of our landings shouldn’t exceed 2g..?! You’ll recall my saying that the Reeves units I have are designed for an 11 lb airframe (Spitfire) and it sounds as if yours are similar units?
I too would like to know more about damper design...
Paul W.

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