Hart Trainer upper wing change?

[Graham Boak]

The Hart Trainer had the upper wing sweep reduced from 5degs to 2½degs, for c.g. reasons without the gun in the back seat.  Does anyone know how this was done?  Was there a section taken from the front spar, and the rear spar simply straightened?  (Please accept the use of the term "simply", for modelling purposes.)  Or was the front spar straightened and an additional section placed in the rear?  I suspect, with the curved wingtips, neither would have made a significant difference to the span - not  that I've seen such a change noted.

 

I see Hannants are offering the Hart Trainer at a reduced, if still high, price.  But have Kora changed the wing - I believe not but additional confirmation would be good.  I'd prefer modifying a Hart kit, but need to know which route to take.  Has anyone else tried it?

[europapete]

This is one for John Adams I think.

Will have a look through my refs when I get home to see if there is any mention of "how".

Edited Saturday at 04:14 PM by europapete

[Graham Boak]

I agree, but I have discussed it with John, and he didn't have an answer either.   Thanks for looking.

[dogsbody]

You could read this, to see what it might say: https://boxartden.com/reference/gallery/index.php/Aircraft-Profiles/Britain/Between-Wars/Hawker-Hart

 

 

 

Chris

[Graham Boak]

I've got that, thanks, and it has corrected my change of angle, but is no help on how it was done.  I also have the MM book, the Putnams Hawker Aircraft, and a number of other fairly obvious options.  However, I might have missed something.

[Work In Progress]

I don't know either, and barring detailed examination of K4972 it's a subject on which I have a not-very-Cunning Plan and several largely evidence-free assumptions. My plan is to reduce the sweep at the front, because it's easier and I'm not sure any other reason matters much in 1/72, at least bearing in mind my rather average modelling skills. 

 

By calculation of the resulting triangles, and measurement of the upper wing chord off the 1/72 drawings in the Crawford book, if you do it by the cutting-down method you are needing to take 1.06mm off the leading edge of each swept panel in 1/72, so call it 1mm in the real world. That is of course as opposed to adding a wedge which lengthens the trailing edge of each swept panel if you do it the other way. I don't think I would notice such a difference on a 1/72 model either way because even assuming the model is displayed alongside other Hart variants the eye will be drawn to the more prominent difference of the reduced sweep rather than any other variation (and likely the model being yellow rather than aluminium-doped or camouflaged), and different kit makers have in any case taken slightly different views of the span of a 1/72 Hart variant.

 

This also ties in with my Evidence-Free Assumption 01: given that standard Hart components were abundantly available, and it's a lot easier to shorten something than lengthen something, if I had to guess, which hitherto I've had to, I would bet on shortening the mainspar.

 

What would draw the eye more than the root adjustment itself a consequence of my Assumption 02*: that they built the wings on new jigs adjusting the angle of the ribs versus the spars by the same angle as the change in sweep (unlike dH with the Tiger Moth). If we follow that logic then whichever way we adjust the root, all the ribs on the kit upper wing outer panels would incorrectly no longer be parallel to the airflow, having a toed-in appearance on the top wing which would be quite prominent from above. For that reason I've envisaged the use of a surplus Airfix Demon wing, given that one would wish to file and sand the exaggerated rib peaks off those anyway, reinstating the ribs with some subtle surface treatment. I experimented some years ago with using plumbers aluminium foil tape for that but it was pretty tough to do. What worked better was thin strips of thin self-adhesive signwriting vinyl, the 'weeds' from which can usually be talked out of people who cut such things for a living, or similar film sold for car body panel protection. It's more repositionable and can be cut much more easily.

 

Another consideration falls out of my Assumption 03: is that if we assume the dihedral angles are unchanged from standard (again that's an assumption, I don't actually know) then the interplane struts must necessarily be a bit longer as they are slightly more heavily raked forward from the lower wing.

 

I see that K4972 is, according to the RAFM website, now on display at Cosford, having disappeared from Hendon a while ago. Last time I was at Cosford the staff were very accommodating in providing access to a couple of airframes for some detail photography so perhaps they will do so again, and allow some measurement too. 

 

*now no longer on my assumptions list given that the Aviation News drawing shows the top wing ribs toed in, so there is no change in the angles of ribs meeting leading and trailing edges. 

Edited yesterday at 07:55 AM by Work In Progress

[Graham Boak]

I'm sure that you are probably right about the easier method for modelling, although it may not be quite as significantly different when it comes to making new spars in the real thing.  Disregarding the dihedral, which for other reasons is most likely unchanged, I agree about the need to modify (build new) interplane struts.  It may be worth checking to see if any of the kits provide struts that are too long anyway... I can hope.  I have the attitude that a freshly doped wing shouldn't show ridges and valleys anyway, and also that kit wings would end up too shallow were the ridges removed.  Filling would be better - if more tedious.  I tried it on a gash wing, but then when I sanded it smooth the ridges turned into roads.  I think that I have enough wings to try again using Mr Surfacers to build up the valleys more slowly.

 

You are probably right that not doing something would make the fix too obvious, but there is a more visible related problem and that is the angle of the ailerons.  Cutting and filling required.

[Patrik]

Kora has not taken care of the wing sweepback change. Picture below shows the Kora Hart Trainer wing compared to AMG Hart wing. By the way, Air Publication says top plane span 37 ft 3 in. Kora wing is 36 ft 9 in, AMG 37 ft 3 in.

 

 

Then the Aviation News drawings show this. Looks more like extension of the rear spar.

 

 

I know, drawings are drawings, but they seem to be confirmed by the photo below. Unfortunately, I have not found better picture of the Hart Trainer upper wing in the short time, but some of you may have more luck.

 

[Work In Progress]

2 hours ago, Graham Boak said:

I have the attitude that a freshly doped wing shouldn't show ridges and valleys anyway, and also that kit wings would end up too shallow were the ridges removed.

With you on the look of a decently good order Hart wing: I am not trying to do ridges and valleys, because I don't like them. Really all I am trying to do is provide the impression of the rib tapes, and thereby the line of the ribs.

Ailerons: I might have misconstrued your meaning, but note that on the Trainer as on the other variants the aileron ribs are all perpendicular to the trailing edge

[Work In Progress]

So, just realised that on the basis of those Av News drawings they actually didn't re-jig the ribs, but did a half-hearted job a la Tiger Moth, so the upper wing ribs DO look toed-in. Evidence-Free Assumption 02 now discarded.

[JWM]

Hi Graham,

 

Unfortunately the Kora Trainer  kit has "regular" Hart wing. I cannot find it now, but when I was considering construction of Hart Trainer I have found in one of BM thread that someone even contact Kora to tell about their mistake and to ask if they do not will to correct it, and the answer was "who cares about such small difference". The BM fellow who was telling this story answered "well , I do"... There are at least couple of us there who do care about this 2.5 degree difference...

Therefor I gave up with thinking on Kora kit and I did such conversion by scratch modifications few years ago, it is here;

I have only changed the angle of out parts of top wing following the drawings.  Of course it implied some modification in "N" shaped struts, which have to be a bit extended. 

Regards

Jerzy-Wojtek

[Patrik]

8 hours ago, Work In Progress said:

So, just realised that on the basis of those Av News drawings they actually didn't re-jig the ribs, but did a half-hearted job a la Tiger Moth, so the upper wing ribs DO look toed-in. Evidence-Free Assumption 02 now discarded.

I am afraid we would need a good photo of the Hart Trainer wing from above to cofirm that without any doubt.

By the way, @JWM, I am the guy who asked Kora many years ago and got the mentioned answer accompanied by rised eyebrows.

[Simon]

6 minutes ago, Patrik said:

I am afraid we would need a good photo of the Hart Trainer wing from above to confirm that without any doubt.

 

 

This one from the Flight archive, labelled as a Hart Trainer is currently on Ebay:

 

https://www.ebay.co.uk/itm/204243857946

 

Not the clearest of photos, though...

 

[JWM]

6 minutes ago, Patrik said:

I am the guy who asked Kora many years ago and got the mentioned answer accompanied by rised eyebrows.

That is great that you confirms thet, I am sorry that I lost who it was... This is a part of a general rule - there was a sentence in a book on general system theory which i've read when I was a student - "it is much easier to remember something, that to remember from where or from who you remember it....

Regards

J-W

[Graham Boak]

11 minutes ago, Simon said:

 

This one from the Flight archive, labelled as a Hart Trainer is currently on Ebay:

 

https://www.ebay.co.uk/itm/204243857946

 

Not the clearest of photos, though...

 

Clear enough to show that the ribs on the upper wing do not run parallel to the lower wing, confirming the toe-in.  Also that the first rib out has been moved outboard showing that the pivot in the design has been around the main spar not the rear spar.  Which might be considered as the easiest,  in manufacturing terms.  I think that I'll go with that.  Thanks to all contributors.

[KRK4m]

20 hours ago, Patrik said:

Then the Aviation News drawings show this. Looks more like extension of the rear spar.

 

I think this remark from the drawing above may be decisive here: Note 1" wider wing span than Hart.

If the pivot point was the rear spar, the span would have to decrease.

Cheers

Michael

[Graham Boak]

I find the photo more convincing than the Av. News drawing.  I've found that the Av.News drawings can be unreliable for details.  In this case it looks convincingly a match to the photo.  As to the span - one inch sound about right but I've not seen this quoted in any of my references.

 

Do we know who drew this?

[Patrik]

17 minutes ago, Graham Boak said:

Do we know who drew this?

Ian Stair.

[malpaso]

Would they need to do anything to the spars?   From photos the Harts are "traditional" single bay biplane, with separate left mainplanes, centre (cabane) section, and right mainplane;  so there are no continuous spars.   A change of sweep "might" have been achieved by adding a simple chordwise wedge at the joint, either separate, or semi-permanently attached to the mainplanes or centre section.  This would avoid having to alter any of the wing parts themselves, no need to recalculate or retest maybe(?) And any resultant change of geometry sorted in the rigging.

[Work In Progress]

That would be structurally poor. The upper wing spar joints, from the outer panel to centre section, need not have any bending strength, as the rigging wires cater for that. But they do have to work in tension in landing loads and negative G. You need structural integrity in tension all the way from the carry-through of the centre-section into the outer panel spars, so that has to be spar joint to spar joint, not simple filler sections

[malpaso]

Most biplane upper spars do not carry through, there is a bolted connection to provide any tension requirement from centre section to mainplanes.  There may be a socket to confirm positioning.  None of this precludes a spacer or shim to adjust angles.