1/5 Scale Slingsby T51 Dart 17.

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B Sharp
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Location: Perthshire

1/5 Scale Slingsby T51 Dart 17.

Post by B Sharp » 04 Jul 2019, 13:00

Following my recent foray into 1/5 scale with the Mike Trew Skylark 4 I decided to follow up with another build to the same scale. Back in 2012 I built a Dart 17R from the CW plans so I felt that a scale model of a scale model was in order. I remembered that Pat Teakle used to make a Dart 15 kit so I contacted Cliff Evans to see if I could purchase a glass fuselage which could save me some time and effort. At Middle Wallop in June Cliff turned up with the fuselage, but also a nicely drawn plan for a built up wing and packets of pre-cut wing ribs. Better and better, thanks Cliff!
The fuselage is fabricated with glass mat and polyester resin but it has a couple of issues for me. First of all it is a bit short for true scale as the rear fuselage should be 130mm longer for true scale. This will mean that I have to redesign a tailplane which is a bit larger to maintain stability. Secondly there is a front skid moulded into the fuselage which will have to be removed.
The supplied wing plan is fine although I will make changes to suit my own construction methods (Sorry, I cant leave things alone and have to insert my individual preferences). I have also added 3 rib bays to the tip to recreate the 17 metre wing.
Attachments
Dart 01.jpg
The PTM fiberglass Dart 15 fuselage.
Dart 02.jpg
The printed wing plan with my extension.
Dart 03.jpg
Moulded front skid to be removed.

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Peter Balcombe
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Location: Clevedon, North Somerset, U.K.

Re: 1/5 Scale Slingsby T51 Dart 17.

Post by Peter Balcombe » 05 Jul 2019, 14:06

Brian,
If Cliff has passed on the alternative built up wing design that I did, you may be interested to know that the wing section is different from the original foam wings.
I used HQ3.5/13 at the root, changing to HQ3.5/12 at the tip.
Consequently, I jacked up the wing root TE by 1/4” at the rear of the fuselage aperture to maintain a longitudinal dihedral of approx. 2 degrees & try to reduce maiden flight nerves - especially with an AMT back end ;)

Other changes I made to improve flight performance (after speaking to Mr Teakle) was to cut off the top of the tail fin (top of the straight LE) & extend this upwards 2”? before sticking the top back on (original fin then extended rearwards 1/2” to line up). My rudder post length is 11.5”.
I also made a built-up tailplane (open structure behind joiners) + built up rudder to reduce ballast weight requirements.

The resultant model flies very nicely & doesn’t seem to be prone to vicious tip stalling like the original I built many years ago.

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B Sharp
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Location: Perthshire

Re: 1/5 Scale Slingsby T51 Dart 17.

Post by B Sharp » 05 Jul 2019, 17:35

Thanks Peter. I had already worked out the HQ 3.5/13 - 12 change and that was precisely what I had intended to use when I initially planned this build. However I had not checked the fin height against my scale drawing so thanks for the heads- up on that. I had already planned to increase all tail surfaces by 10% due to the shorter moment arm.
I have not used your form of trailing edge construction before but I will give it a go as it would be a shame not to use all these nicely cut ribs. I will however be altering the centre section attachment to the fuselage as I don't fancy four bolts down through the wing. I will also be using servos out in the wing tips for the ailerons as I hate long pushrods and bellcranks.
I hope you don't mind me altering your work as I'm sure that there will be lots of changes as the build progresses.
Brian. :)

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Peter Balcombe
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Location: Clevedon, North Somerset, U.K.

Re: 1/5 Scale Slingsby T51 Dart 17.

Post by Peter Balcombe » 05 Jul 2019, 18:07

Hi Brian,
No problem, do whatever works for you.

I’m not sure how the enlarged (or original) tail compares with scale, but I made the alterations for flight characteristics only on Pat’s suggestion, plus using built-up construction on the suggestion of another West Mendip club modeller.
(My Dart came out even lighter than his).

Not sure if Cliff sent you my Mk1 version design which had a flat centre section & dihedral starting where the wing halves plugged on (non scale), or the Mk2 version where the centre section was angled (after I worked out that I could just about get a single straight joiner through everything).
I built the Mk1 version.
In both cases, I worked on the basis that the centre section would stay attached to the fuselage, so attachment can be more solid here. My Dart uses MPX Unilocs to secure the wing halves to the centre section & steel screws to attach the centre section. My wing cover is removeable but only after removing a couple of screws at the rear of the cockpit.
I work on the principle that if something can go wrong, then sooner or later it will & I will need access to fix it!

I would have liked to fit aileron servos at the horn location, but chickened out due to the reduced wing thickness with the HQ section. However it may well be possible with slim wing servos, so await the outcome of your build with interest.

N.B. If you find any issues with the drawings, please let Cliff/me know so that they can be improved.
Peter

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B Sharp
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Location: Perthshire

Re: 1/5 Scale Slingsby T51 Dart 17.

Post by B Sharp » 05 Jul 2019, 18:12

I have reinforced the fuselage floor with a patch of glass cloth and polyester resin. Once that had dried I cut and ground off the forward skid. I will now require to add a little P38 to the bottom of the fuselage before sanding it all back to profile. While I was in dust creating mode I cut the hole for a fixed undercarriage (I don't want to mess about with retracts at this scale) as although the model will be flown mostly off the slope I still want to do the occasional aerotow launch. I also trimmed down the sides of the cockpit and wing openings to get a neater finish.
Brian. :)
Attachments
Dart 06.jpg
Glass patch in place.
Dart 07.jpg
The re-profiled fuselage with U/C hole.
Dart 08.jpg
Fuselage openings now neat and tidy.

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B Sharp
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Location: Perthshire

Re: 1/5 Scale Slingsby T51 Dart 17.

Post by B Sharp » 06 Jul 2019, 15:16

I was trying to resolve in my mind the whole “tail volume” problem that Peter mentioned in his post. I resorted to drawing a complete fuselage/fin/rudder at 1/5 scale as measured from my 3-view drawings. I checked this with the CW drawings as well and found that there were only very minor differences (well done Chris). When I superimposed the fuselage moulding on top of the scaled drawing the difference became instantly apparent as you will see from the attached image. Pat and Peters approach of adding 2 inches to the top of the fin is not really the answer as it actually only needs to be elongated by just over an inch (30mm). The rear of the fin needs to be stretched backwards by nearly an inch and the hinge line rake increased.
Ok, how am I going to achieve this?
Don’t know as yet! It’s time for a little thinking and perhaps a little glass of Scottish falling down liquid.
Brian. :?
Attachments
Dart 09.jpg

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chris williams
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Location: Blandford Dorset

Re: 1/5 Scale Slingsby T51 Dart 17.

Post by chris williams » 06 Jul 2019, 20:15

Believe me, Brian, when I built my 3.5 Dart, I had many sleepless nights over the size of the tiny tailplane. It turned out to be no problem at all: that, and the experience of losing one half of an Algebra tailplane with no noticeable effect, has led me to believe that the tailplanes on a lot of models were much larger than they needed to be.

When it comes to the smaller models, though, I have become a great advocate of increasing the chord of the wing by at least 5%. Wing loading becomes increasingly relevant the smaller the size of the glider, and any unnecessary weight has a greater significance...

BTW If you'd kept quiet about the shape of the fin, I can guarantee that nobody would have noticed!

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B Sharp
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Joined: 19 Mar 2015, 17:18
Location: Perthshire

Re: 1/5 Scale Slingsby T51 Dart 17.

Post by B Sharp » 06 Jul 2019, 21:26

Yes Chris, all very true. However, I would know and it would annoy me.
Brian. :)

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chris williams
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Re: 1/5 Scale Slingsby T51 Dart 17.

Post by chris williams » 06 Jul 2019, 21:43

I have always found that the secret of a happy life is learning how to keep secrets from yourself... :D

Jolly Roger
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Location: Sutton Bank, North Yorkshire

Re: 1/5 Scale Slingsby T51 Dart 17.

Post by Jolly Roger » 07 Jul 2019, 00:18

Really excited about your build Brian.

A question about the fuselage. To me the fin looks pretty close to scale outline, but did you say the rear fus was 130mm too short? This would bother me more than the fin discrepancy.

And a very fine point of detail (cos I know you like this stuff Brian!) I spent this afternoon crawling over a full-size Dart 17R and noticed that even the retracting u/c version has a very slight nose skid. Who knew?

The skid protrudes about 1/4" below the fuselage, is maybe 3" wide and runs from about a foot back from the nose tip to the wheel bay. It's slightly filleted so that you would only notice it if you looked for it. See photo.

I asked the owner if this was the vestige of it originally having a fixed u/c and skid but he confirmed it came out the factory like this, as did all Dart 17Rs.

I only mention this now as I know you'll be filling and sanding your fuselage about now and may wish to include this feature with a strip of 1/16" ply or whatever.

Cheers,
Rog
Attachments
P1040291.JPG

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