1/4 Scale ASK21**

[Peter Balcombe]

This the photo that I was trying to add.
It was taken on an iPad held in portrait mode, so image needed to be rotated 90 to have fin vertical.
Image appears on its side in preview but will maybe come out ok when you click on it!

[Peter Balcombe]

Canopy area blanks

Current plug progress

A little bit more progress over the past couple of months, but a bit of one step forward & two back in places

The plug is now nice & smooth and I have added a front wheel coaming, plus made smaller canopy blanks in order to avoid having a very deep valley between the 2 canopies in the mould, which would no doubt cause problems when I tried to lay up this area.

After studying more photos of the full size, I noticed that the wing root fillets were too big, so that has been sorted.
I have also noticed after finding a good rear view shot that the rudder horn fairings should be much more prominent, so that is the next job.
The main wheel fairing also looks wrong as this should in reality, reflect a bolt-on fairing below the normal lines of the lower fuselage rather than being fully faired in as I have at present. (I did have a problem relating the 3 view profiles to the actual 3D shape in this area).

So... a bit more work before applying the pattern coat & buffing up to a finished plug

[GeeW]

Would you like some close up shots with measurements of the rudder cable fairings? (or any other bits of a K21 come to that) As I have one in the workshop this weekend.
Regards

Gordon

[Peter Balcombe]

Thanks Gordon,
I think I have the rudder cable fairings sorted now, as I was able to scale the rear opening from a photo I found on the web.
I fitted some more prominent fairings today, which I hope are much more representative of the full size.

However, I would appreciate a side shot of both the mainwheel and tailwheel fairing areas, plus the nosewheel if you could manage it. I found the tyre sizes in a manual on the web, so think I have scaled the wheels ok.
I think that the mainwheel is a sprung rear swinging arm type, so if you do have the mainwheel fairings off at any time, a view of this would also be interesting in case I think about using other than a hard mounted wheel. I assume the mainwheel doesn’t move all that much, to avoid grounding the fairing on a hard landing.

Some photos on the web seem to show the mainwheel fairing & others seem to show what I assume is the attachment moulding.
The photo below seems to show the fairing in place, wheel at lowest extent.

A further query;
The picture below shows a fin mounted pitot (KDP photo) whereas I have also seen a vertical installation on the top of the fuselage, just aft of the wing (3rd pic). Is the upper fuselage installation also likely to be a pitot, or is it more likely to be a radio antenna?

[GeeW]

I will take some images for you of wheel fairings.
Main wheel fairing is normally attached with half a dozen screws, but is often seen removed.
In winter it is removed as it fills up with mud and on a hard landing tears off and is then promptly run over! In the late summer it fills up with dried grass which has an annoying habit of catching fire if the brake is used in a energetic manner! That is also why the Air Cadets don't fit their main wheel fairings either.

The factory standard T/E probe is the one fitted half way down the fuselage. It is an optional extra to have it fitted on the fin. The pitot lives in the hole at the very nose of the glider. For aerobatics including inverted a 100mm extension tube is fitted to the pitot as the ASI doesn't work inverted, which can become an issue if things go in anyway awry.

The radio antennae is inside the fin so never seen on nearly all non-metal gliders.

If this weekend goes to plan I will be re-installing the whole undercarriage assembly so can get images of that as well. The undercarriage frame is pivoted at the front edge on a trailing link with big rubber blocks above to take the (sub-optimal landing) shocks.
The amount the wheel hangs down varies for each glider as its maximum extension is dictated by how tightly you do up 2 pieces of string....I kid you not. As you rightly observe there is not much travel, about 75 to 100 mm max.

Regards

Gordon

[GeeW]

Just looked it up. Incidence is +2 degrees on full size.
Dihederal is 3.6 degrees measured from the inboard top wing skins.

See page 20 for drawing of undercarriage
See page 27 for T/E probe location
See page 29 and 30 for rigging and incidence information


Knew there had to be a copy on-line of the Maintenance Manual somewhere!

http://www.aeroclubrieti.it/w/wp-conten ... I-IVWO.pdf

[Peter Balcombe]

Thanks again Gordon.
If you do get some pictures of the mainwheel fairing, it would also be useful for a fore/aft view as well to show the profile of the fairing itself, relative to the main fuselage (including a scaling object if possible)
The photos show the fairing as a distinct “add-on” to the underside, particularly around the sides, and it is that change in profile that I want to try to recreate if possible.
My current plug has a smooth profile from fuselage to wheel aperture which is very different from real life!

It has just occurred to me that if the fairing comes off as one piece, then it might be possible to take a photo of the fairing sitting on the ground or flat to a wall, looking directly in line with its centre (either top or bottom) and scaled if at all possible using an object of known length. (Top and bottom views would be even better).
I should then be able to see both top & bottom apertures in the same image & can scale these from the photo using CAD.

It may actually be worth thinking of cutting off my existing fairing, re-profiling the lower fuselage to ignore the fairing, then making a fairing add-on to fit in place to complete the plug, much like the canopy blanks.
Peter

[GeeW]

Peter
Don't cut off anything just yet! The fuselage does not follow a fair line under the fairing. There is a bit of a lip moulded into the belly of the fuselage to accommodate the main wheel fairing.
The fairing is on the workbench so no problem to grab some scaled images of that whilst I'm waving the camera about. The fairing is a strange shape as it is symmetrical on the fuselage join and is asymmetric at the bottom to allow space for the brake caliper.

Gordon

[Peter Balcombe]

Thanks again Gordon.
I found a copy of the manual on the web when looking for the wheel sizes.
I had previously guessed that the incidence would be about 2 degrees, so that is good although the tailplane seat could be tweaked if necessary.
The wing dihedral is a bit more than I expected but that is easily dialled into the wing design design program when I finalise the joiner arrangement.

Whilst waving the camera about, could you please also point it at the tow release.
Although I think I will install the aerotow release in the nose pitot aperture, it would be good to see how prominent the real release is.
Peter

[Peter Balcombe]

Gordon,
PM sent with contact details in case you wish to send full resolution pics.