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Sep 19, 2016, 06:04 AM
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Sopwith Mike's Avatar
Build Log

Ivan's Druine Turbulent - a BIMBO build

The plans for Ivan's latest arrived a couple of days ago and four of us, Tim Calvert, Nick Chudleigh, Scott Wallis and me are going to make up the display team for our BIMBO meet in 2017.

Lots more information as the builds start, and of course anyone else building the model is very welcome to chime in.

5 Oct 16

Ivan has responded to requests for more detail with photos and these comments:

Someone asked me if I had more details on the full scale Turbulents so I was looking around for what I had. Quite a bit is on line, and I found that the NZ one I flew, ZK-CAH, was once decked out in yellow. I know it was rebuilt following an accident somewhere along the way during its 21 year life, so maybe the colours were changed during the rebuild. I was living overseas by that time. Some photos of it in yellow are included with the pictures attached. Martin Hardy sent for a plan of the Turbulent and I sent it off to him last week. I think he is going to ZK-CAH.

Most of the pictures I had of the Swiss HB-SVB were from an article about a model of it in “Radio Control Modeler” magazine back in 1975. It was a gas model of course in that era, and the cylinder of the horizontally mounted engine came out the right side of the cowling, with a dummy VW engine on the left. Notice that all of the photos in the attachment were taken on the left side.

After the affair with my Mosquito 480 where you noticed that one letter was wrong in decals I used, I am surprised you didn’t let me know that I made a similar error with registration of the new Turbulent I built. Furthermore, when I checked back to the previous 57 inch span one I built in 1995, I noticed that I had one letter wrong in that one also. Usually I keep a bunch of letters on hand, and when I don’t have enough of a letter that I need, I substitute with something else. I guess that happened with both of those models. You well know that I am not a “scale purist.” The rather small fuzzy pictures attached show the 1995 model. At least I had a canopy on that one.

Greetings, Ivan
Last edited by Sopwith Mike; Oct 05, 2016 at 05:03 AM.
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Sep 19, 2016, 11:31 AM
Wanted for breaking Ohm's Law
Dennis Sumner's Avatar
Did Ivan build ZK-CAH? I am assuming yes....

Cool build I'll follow along if you don't mind....
Latest blog entry: RC Throw Gauge
Sep 19, 2016, 01:13 PM
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Sopwith Mike's Avatar
Originally Posted by Dennis Sumner
Did Ivan build ZK-CAH? I am assuming yes....

Cool build I'll follow along if you don't mind....
No he didn't, he was at pains to point that out!

Glad to have you along Dennis.
Sep 19, 2016, 03:11 PM
Complete the Illusion!
35Mhz's Avatar
The Ivan plans were collected today from Mike, Thanks Mike.

Examining the plans (and the 8 Pages of Ivan's Build notes) show hat build should be fairly straightforward with no undue complexities. The build notes and Ivan annotated plans provide a wealth of information to help the builder. It great to see a hand drawn plan rather a change from the usual CAD plans, with useful sketches and comments.

A rummage in the workshop unearthed most of the hardware that will be needed.

4 x Servos (12g MG units)
1 x ORX RX
2 x 2 1/4 Wheels
1x Tailwheel
30amp ESC
and a Park 450 Sized motor.

The motors a tad overpowered with a 10x7, it was used to power my Phoenix Glider at over 300 w, its 1120KV compared with Ivans 880KV version, (a bit like Twin Turbo power on an enlarged 2000cc VW engine against the 1200 cc 35bhp unit!!, ) I think a 9x6 prop will tame it to something more realistic, if that is still too much than RC timer have some suitable motors (about £9 posted)
Sep 19, 2016, 04:05 PM
build like there is no 2moz
wallis_100's Avatar
Collected my plan too. it made it back to the flat dry (despite the rain)
Looking forward to the build, and the eventual mass fly :-)
Sep 20, 2016, 03:34 AM
Complete the Illusion!
35Mhz's Avatar
My build will be based on G-ARGZ. Looking at the history this was built in 1961 and is fitted with a 1600cc VW engine, so may be my choice of power is not so bad, I also discovered that the plane was involved in an incident in 2003.

Report reads

"The pilot reported that he had carried out an uneventful landing on the grass runway surface. The aircraft was then taxied clear and whilst moving on the grass taxi area the pilot heard a 'ping' like noise from under the inboard starboard wing. The starboard undercarriage leg folded forwards, the wing sank down and the aircraft rotated gently through 90 degrees about the wingtip. The pilot reported that he cut the magnetos as this occurred and the propeller did not strike the ground. "

Sounds all too familiar!!!
Sep 20, 2016, 11:02 AM
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Sopwith Mike's Avatar
Here's my guess at a materials list for the build. All dimensions imperial as per the plan, all sheets 3" x 36": adjust if necessary

1/16th rib stock 1 sheet
1/16th medium for D bpx, fuz sides etc. 8 sheets
1/8 sq hardwood strips for spars 4
1/8 sheet for LE and various items 1
11/4 x 1/4 TE stock for TE and ailerons 2
1/8 sq balsa for longerons, uprights and cross-pieces 8 (at least!)
1/8 ply (or laminate from 1/16th ply (easier to cut)
1/16th piano wire 1
3/32 piano wire 1
Wheels of choice (I'm going to use 3" which are scale for my version)
Park 450 or similar with SC
10 x 7 prop
Spinner to suit (some aircraft did not have them)
Covering...about 3 metres (sorry, 10 ft)

I've made some pdfs from a decent 3-view that show where the plan deviates from scale. If this doesn't bother you (and Ivan's model looks just fine to me) then you can ignore the next post.
Sep 20, 2016, 11:07 AM
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Sopwith Mike's Avatar
Here's the pdfs of areas where the plan deviates from the 3-view I'm using as datum (from the Richard Ferriere website).

I know that there were a number of different builders as well as all the homebuilds, so I'm not saying any one plan is right or wrong, but I prefer to take my dimensions from just one source as close to the plane I'm going to model as possible. The nose area in particular seems to be different for just about every plane. But I'm not going to build the fixed slats!

Enlarge the pdfs by 7 for a pretty close match to the Ivan plan.
Sep 20, 2016, 11:17 AM
What could possibly go wrong?
nickchud's Avatar
Wahoo. Thanks for all your work Mike and of course to Ivan for another super job.

Here's mine, if I can get the decals from

Sep 21, 2016, 10:06 AM
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Sopwith Mike's Avatar

Ivan's design philosophy

I wrote to Ivan asking why he had made certain design choices with the model. I knew I would get a fascinating reply, and I'm not disappointed!

Hi Mike and gang.

It has been great to follow news about your group build. I just heard last night from Luke regarding the thread you started about the project. So interesting. It is quite amazing all the things that different ones have come up with about the various home builds over the years. In the early sixties there was a team of three Turbulents that often performed at air shows in our area. They were based in the New Zealand city of Christchurch which you likely have heard of in recent years because of the earthquakes. That team was quite entertaining. Your UK foursome reminded me of it.

One thing I was intending to write about is the width of the fuselage. Traditionally, all my models have a narrow fuselage. It is probably a hang over from the early nicad/brushed motor era when every bit of drag was sqeezed out so that we could get the model to fly. When I went to fit a pilot in the present Turbulent model I realised the problem and had to trim the arms. I should have made the fuselage wider and it is really obvious to me now when I look at the model. For years I have had a couple of quarter scale pilots here waiting for me to build a quarter scale model. When I doodled recently on making a ¼ scale Turbulent I pulled these pilots out and checked them for width. The Williams pilot was far over width (like me) but the other one of some unknown make actually fits with room to spare. But a scale width fuselage would look better and I am going to go in that direction with any future plans.

I’m not answering your questions in sequence, but as they come to mind, the easy ones first. Regarding strip ailerons, it was just an effort to make for an easy build. I have never liked the look of them as much as scale ailerons. Strip ailerons do not make for more resistance to tip stall. What make the difference to tip stall is where the aileron ends. In most of my models the aileron ends short of the wing tip, and an extension of the trailing edge a little beyond the end of the aileron is what is claimed to make for less tip stall tendency. You will notice that the end of the scale aileron on the Turbulent forms part of the wing tip, like a Tiger Moth. It is actually easier to build that way, but who knows if it is really more prone to tip stall as claimed by some. Tiger Moths and Turbulents were both prone to tip stalls, but there were likely other factors. I think the nasty stall of the Turbulent could have been because the airfoil section was very thin. I wonder if the designer, Roger Druine, (himself an aero modeller who tried a full scale project) used a thin airfoil so he could get a good cruise speed from such a small engine. In that regard he really succeeded. Go ahead and make scale ailerons if you wish, but you know that the width has to be almost double that of the strip ailerons. They sure look better and are really more aerodynamic. I often think that the inboard section of strip ailerons does little more than add drag.

The wing spar construction was considered to be easier than the one I traditionally used when ribs were cut into two parts. The only ribs that are cut in two sections are the ones in the centre section, but I had difficulty writing the notes about this. I should have numbered the ribs, but hadn’t because they are all basically the same.

There is one problem I have had with my traditional method of cutting the ribs in the past, and that is that it is very difficult to get the short nose section of the wing at exactly the right angle so that the points of the ribs are exactly lined up with the leading edge. Sometimes I have found the leading edge of one wing is not lined up with the leading edge of the other. I felt that this method of constructing the wing gives a better chance of maintaining the true airfoil shape, and is easier to assemble, much as I used to hate cutting web plates and gluing them in place.

(I don’t have this problem because I always pin down the bottom of the D Box sheet and other lower sheeting before laying on the spar, ribs and front ribs, false LE etc. I know there are builders who don’t do this TrevorH for example, but can’t understand how they get the wing built. – Mike)

Undercarriage. Yes. On the full scale Turbulent the UC is well back. Because it does not have brakes it is possible to put the wheels further back. There is no fear of going on its nose with application of brakes. Any tail dragger is easier to keep straight if the wheels are further aft. The scale position would be OK in a model if you only flew on pavement or very short grass with little drag. I put the UC further forward on most of my models and it allows operation on typical grass that is not extremely long. You did not mention the angled struts. To make the scale UC it would be necessary to attach the legs to the wing and I don’t like that. I prefer to attach the UC to the fuselage and it makes it easier for storing the wing.

Tail Surfaces. As a general rule the thing I change most in my models from scale is the area of the horizontal tail surfaces. It is usually increased to at least 18% of wing area. A larger tail surface in relation to the wing area means that the C of G can be a little further aft without making the model dynamically unstable. The further back the C of G, the more efficient the wing, and in tail draggers it also means reducing the risk of nosing over in grass. The tail surface I thought I enlarged most is the vertical fin. If you noticed that the vertical fin is quite a bit larger than the original, it is out of consideration for strength. The fin of the homebuilt is very narrow, so I increased it quite a bit.

Wing chord. I am a great disciple of Andy Lennon. He had a thing about tail moment. It should be at least 2.5 times the wing chord, preferably about 2.7, and can be up to 3.0 for a really smooth flying model. The way to estimate the tail moment is to mark the average Center of Pressure point of the wing at 25% chord. Then mark a similar 25% point for the horizontal tail surfaces, meaning the combined horizontal stab and elevator. Now measure the distance between these 25% points on the wing and horizontal tail surfaces. Divide this distance by the wing chord. It should be somewhere between 2.5 and 3.0. (On Ivan's design it is 2.9, on the full size, about 2.56 - Mike) In the small drawing I had for the Turbulent, I believe it came up short at something like 2.4. One way to improve longitudinal stability in this case is to increase the length of the fuselage, but I prefer not to do this. I do it by moving the trailing edge of the wing forward a little. This decreases chord and increases the aspect ratio and there are a lot of advantages to that, one being that it reduces the induced drag of the wing tips considerably. (Note – he has also moved the tailplane back an inch or two, presumably with the same aim of increasing the tail moment – Mike)

For years designers of small planes built tapered wings if they wanted to reduce the wing tip drag. Then I believe some of them, knowing how much more expensive it is to build a tapered wing than a straight non tapered one, started making non tapered wings that have more span and less chord.

In my early years of designing I didn’t know about the effect of changing chord, and because we were fighting a weight war with heavy nicads and brush motors, I increased the chord on some models to increase the wing area. There was a noticeable deterioration in handling. To compensate for that added wing area I had to increase the moment arm and the tail surfaces, until the penny dropped and I realized I should just scale everything up and build larger. When I designed my third Catalina, the MiniCat, I actually decreased the chord from scale and it turned out to be a good decision.

It is actually a year since I did the layout for the Turbulent plan and I am not sure how much it was influenced by the plan of the earlier electric model I had built about 20 years ago. That very old plan could have been far from accurate. I’m sure you will come up with a good mix and have a lot of fun.

Last edited by Sopwith Mike; Sep 22, 2016 at 04:18 AM.
Sep 22, 2016, 04:42 PM
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Mike Smart's Avatar
I have a few pictures of this one, if they are of any help.

Sep 22, 2016, 05:00 PM
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Sopwith Mike's Avatar
Nice Mike. Was the tractor yours as well?
Sep 22, 2016, 05:06 PM
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Mike Smart's Avatar
Originally Posted by Sopwith Mike
Nice Mike. Was the tractor yours as well?
Neither are mine , the Turbulent belongs to a mate.

Sep 23, 2016, 08:32 AM
Registered User
I really like Gordon Whitehead's old version; if you want better scale fidelity, yet good flying, his would be a good choice. Much more scale, and with the construction article he wrote one of his usual delightful pieces about flying it. Span is 48 inch, but of course it could be scaled up, and nowadays you would want to lighten it too.

Sep 23, 2016, 09:35 AM
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Sopwith Mike's Avatar
I'm a big GW fan. His articles and plans were always first class and the old magazine "Radio Modeller (UK) carried his Hawker Demon plan and superb write-up. He had a very good book on scale modelling too. But to completely misquote the pirates in Gilbert and Sullivan "for all our faults we love our King". Anyway, it wouldn't be an Ivan build if we'd chosen GW's plan!

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