DH Sparrow glider for bungee launch

[Pete Fardell]

I want to take part in the Oxford Club's scale glider contest in July and have been looking for an appealing subject. I think I've settled on this one; the DH Sparrow, designed by Polish born Waclaw Czerwiński in Canada during the war and handed over to the De Havilland gliding club there for training purposes. There is this 1947 59" span plan by Bruce Lester on Outerzone which gives me a good starting point. Mine will be a good bit smaller than 59" to comply with the 36" limit for the Peterborough Flying Aces HiStart contest thereby giving me another potential opportunity to fly it. So, the first question is, how much should I change the plan as I reduce the size? For instance, should I use those fattish, undercambered ribs? And should I sheet so much of the wing? Also, the Lester plan is 77 years old, so are there things about it which should probably be updated or discarded to make a better flying model?
What do all you glider experts think?

[AndyB]

Not an expert but have observed several 36" bungee-launched gliders, scale and non-scale. My two penn'orth:

I don't think it much matters what the airfoil is on a 36" glider, as long as it's less than about 10% thick. A lot of people seem to use flat-bottomed sections and it's really difficult to identify any substantial difference between those and more cambered sections. I note that Tom Hallman's SGU 2-22 has a 10% thick flat-bottomed section on about a 4 1/4" chord and that has performance to spare.

> And should I sheet so much of the wing?

I wouldn't, personally. You're probably looking for an overall weight of about 40-45 grams including a substantial amount of noseweight, which will require some reasonably light construction. And there's always the possibility that a forward spar might offer some free turbulation.

> Also, the Lester plan is 77 years old, so are there things about it which should probably be updated or discarded to make a better flying model?

I'd say have a look at some of Lurk's efforts (e.g. Airspeed Tern), and change the structure to match. Make sure it still has 2-3 degrees longitudinal dihedral (measured from the airfoil entry to the T.E., not the bottom of the section) if you're changing the wing section. And I'd be tempted to use a bit of spruce for the top and bottom members of the booms...

A.

[Pete Fardell]

Thanks Andy, that’s really helpful. I was already thinking along spruce lines for the booms, as building them hollow seemed a bit ludicrous at my proposed size.
Another thing I was wondering about was the tow hook location. It looks quite forward on the plan, but it would be easy enough to give myself a few alternative hooks on the skid.

[Squirrelnet]

 Interesting choice of design I look forward to seeing it progress

I agree with Andy that a thinner section is the way to go with it and probably don't need the sheeting, though a small strip on the top near the LE may help the wing keep it's shape.

With tow hooks - I did an article for the Oxford mfc newsletter and its seems having the tow hook around 3/4" in front of the CG for a 36" should give a good starting point. Not sure why the hook is so far forward on the plan but I would have thought that rule would still apply with a pod and boom glider

As an alternative too heavy spruce for the tail booms how about balsa with a carbon tube core ?

[TheLurker]

Ahh we can always rely on you to pick something a little bit out of the ordinary.  I thoroughly approve of your choice.

Emphatically not an expert, but you're welcome to my opinions.

> aerofoil. 
Concur with AB & CB.  And I think I'd go for a flat bottom section rather than anything concave because it's such "fun" making the small tip ribs concave and you'll want a decent depth to take the spars. Shame that because concave wings are sooo pretty.

> Flying surface sheeting
I wouldn't bother with any at all, unless you intend making the wing demountable*, in which case I would sheet the the centre section to about the width of the pod. 

> Tow hook position.
I'd be tempted to leave it where it is.  The designer put it there for a good reason.   Joking aside I found that I had to put the hook for my Horsa right forward like that to get a controlled and stable launch.

> Boom
I think CB's suggestion may prove the most robust solution, but I don't much like C fibre and would (were it mine) prefer some wood laminate. Balsa/thin ply/ balsa perhaps.

I'd also think about building in a degree or two of washout into the wings.  I know they're not *very* tapered and they are mounted high, but I know of other (roughly) contemporary high wing types without markedly tapered wings where the prototype exhibited tip stall.  The one that comes to mind immediately is the Willow Wren .

Other odds and ends?  I'd lose the sheet outlines for the tail assemblies and use formed outlines instead and I know CB doesn't like them but I'd use a double offset hook.

You'll have your own ideas about the pod construction, but I'd probably use a 1/16" strip framework with soft 1/32 sheeting and I would build in ballast box. For details send a postal order for 3/6d to .... or you could just crib what I did for my take on the Tern & Slingsby Prefect.

*Not sure how you'd fit an unobtrusive DT, but if the wing is demountable a tip wing might be a solution.

[Pete Fardell]

Thanks Chris and Lurk,
Added to AB's advice I now have a clear way forward now and lots of great intel. I will build in a little washout, and will probably leave the hook where it is with provision made for moving it back if necessary. The booms will probably be either carbon-cored or ply-cored balsa as suggested for lightness (and also to fit in with my current complete lack of spruceness).
As to a DT, I found it a bit of a faff to set on the only model I've ever put one in (my Frog Petrel) and its best flights were when I just banded on the tail firmly and forgot all about it. So this time around I think I might just use the completely hassle free BAMITOFA* system instead.


*Build Another Model If This One Flies Away

[AndyB]

> The booms will probably be either carbon-cored or ply-cored balsa as suggested for lightness (and also to fit in with my current complete lack of spruceness).

Can I suggest balsa-cored (thin) ply, rather than ply-cored balsa? Always better to have the stronger stuff on the outside. Unless that's what you meant to write, in which case please ignore my drivel...

A.

[SP250]

Just a thought on the tow hook position. 

Might be that far forward as it was for catapult launching. 
If you want to drag it up on a tow line whilst running, then it may need the hook position (and shape of hook) changing to 10* to 15* angled down from the CG on the wing.  Which would put it on the skid at about where the arrow for the "balsa skid" points to.

Just my tuppence worth over morning coffee.
Nice and obscure choice Pete as we have come to expect from you.
John M

[cvasecuk]

I agree with John on hook position and as for the airfoil I would go for a Clark Y thinned to about 8%.
Ron

[Slowmatch]

Everyone seems to have covered the major points nicely but I would definitely use a thinner airfoil. 10% is getting there but still a bit thick. Ron's 8% would be better. I used AG37 on my 36" non-scale glider, but a flatter bottom, ie a bit more camber wouldn't hurt. GOE795 wouldn't be a bad shout (an Andy Sephton favourite) or I can send you the 9% Sweet Pea airfoil I use on E20's.

If you wanted to go under-cambered but thinner, the attached BE50_Mod is one that works nicely, perhaps not the best over all but it would give a nice slow glide. The Mini-E foil is another nice E20 one but that might be getting a bit too thin for easy structures.

Not particularly enamoured of that tail airfoil to be honest, a flat section or thin flat bottomed Clark-Y style would be fine.

Can I suggest balsa-cored (thin) ply, rather than ply-cored balsa? Always better to have the stronger stuff on the outside. Unless that's what you meant to write, in which case please ignore my drivel...

Very much agree. 1/64" ply skins over 1/8" or 3/16" balsa is really quite strong. I use it CLG fuselages sometimes. You end up with an I-beam sort of thing that is stiffer in one direction. (This model might need that extra stiffness to be laterally?)


I would expect a bit of washout would be helpful too.


+1 on John M's hook position, but some adjustability is always nice.


The DT (not that you need one) could be done by hinging the elevators and having a Z spring vs line arrangement. This works very well on some of my fixed tail models. The spring arrangement can be seen here in the radio context but works as a DT method too: https://peterboroughmfc.org/technical-articles2016/3-PullSpringControl%20.htm

[Pete Fardell]

Thanks very much, chaps! I’ll take all that on board too and will use one of those aerofoils Jon suggests or similar.

If anyone’s interested, there’s some information about this glider here:
https://en.m.wikipedia.org/wiki/Czerwiński_Sparrow

And even this colour (or colourised?) photo showing it with some other Canadian aircraft. The Sparrow is the one registered CF-ZAI with the green and white wings.
https://www.mediastorehouse.co.uk/mary-evans-prints-online/new-images-july-2023/canadian-registered-light-aircraft-gliders-32254424.html

[cvasecuk]

Here is the Clark Y at 8%.
Ron

[Pete Fardell]

Thanks Ron- I may well just go with that.

[LASTWOODSMAN]

     Hi Pete!   You guys have got me printing out Glider plans now ...   and studying them up.   All day yesterday.     Even the different rib outlines.    Your build threads are great to learn about new, rare, and obscure aircraft on HPA.   

     I am definitely following this build,  and I am very curious to see how you incorporate the suggestions.

     For the competition,   is this tow line a maximum of twenty feet long,  with an additional seven foot max section,   of bungee/rubber ?

     That "Canadian Glider" has a very interesting history.

Lastwoodsman
Richard

[Squirrelnet]

Hi Richard

 The competition at Oxford MFC will be using a Peterborough rules bungee so 7.5m of 1/8” rubber + 22.5m of line so 30m in total or  98.4'

There's more info on the class here https://oxfordmfc.bmfa.club/omfc-scale-glider-duration/

[Slowmatch]

There's more info on the class here https://oxfordmfc.bmfa.club/omfc-scale-glider-duration/

Chris, is there a span limit for the scale gliders?

[Squirrelnet]

Hi Jon

There's no limit on span, the limit is the P/Boro rules bungee which we've found will handle an A1 sized model with a bit of breeze.
(Thanks i will add that info to the web page)

 I and few others have gone for staying under 36" as then the model would also be eligible for P/Boro comps too 

[Slowmatch]

Thanks Chris, I think that's sensible. Some of the higher aspect ratio scale models could handle the bungee at bigger spans.