Dumas Curtiss F9C - 2 Sparrowhawk 30"

[ZK-AUD]

I recently acquired one of these quite nice and well-sized kits for the right kind of price.  (The kind of price that overcomes the usual objections and prejudices against buying kits I guess).

The truth is I like rubber powered biplanes and this one has a lot going for it - plenty of wing,  plenty of dihedral, a big round engine and bags of character.

The model is quite well designed apart from the undercarriage,  which I think is asking for trouble and which I will both lighten and re-engineer.

The kit seems to have been packed by 2 people with very different ideas on acceptable wood selection - the bulk of the sheet wood comes in two sets of 3 x 1/16 sheets.  Each set of 3 includes top wing,  lower wing and a fuselage half,  One set is pretty decent light quarter grain and the other is that hard shiny heavy wood that would go better on the hull of the HMS Victory.

So my first job is to scan and re-cut the parts in better wood, but I have a question for you.

The plan shows zero zero incident on all flying surfaces which probably reflects that most of these kits are now converted to electric RC where that would be  OK.  What do we think about whacking in some decalage?

[OZPAF]

I would think the camber and thickness of the airfoil needs consideration here. If it is a typical kit 10-12% thick flat bottomed Clark Yish airfoil and you are referring to the incidence as measured at  the airfoil lower surface then the aerodynamic incidence is likely to already be around 3-3 1/2 deg more than that - ie around 2deg higher than that of the chordline.

However using more incidence - preferably on the top wing may be best if adjustable tail incidence is allowed for.

John

[ZK-AUD]

Thanks John. The wing is flat with just a rounded LE

What I should have asked is whether anyone else has built one and whether they fly.

Anyone done one?

[Rekitus]

oh welp...  I have the kit...
I am hoping your build and fly thread would
give me enough confidence to built it.

I have informed opinions about degalage...
If it were a Guillows kit I'd state
built the CG into the correct place and ADD 2 degrees
of stabilizer pushing down the tail.

I've not built a Dumas kit so I don't know.
there is a fellow with a vid of an indoor flight
(oh what a flight, I held my breath the whole run)
he would have an answer...
If you don't want to ask, I will
found the vid
https://www.youtube.com/watch?v=hycyPRhEE80
and held my breath...

my very small aerodynamics hat says
flat bottom main wing will have positive decalage
to have a stable flight.
the solution I would use is to make the elevator adjustable.

victor

[ZK-AUD]

Thanks Victor that was a good callout.  I emailed the gentleman concerned - John Cooper.  John says he has 2 degrees on the top wing as measured on the flat underside and has about 3 degrees up on a moveable elevator.

That's great intel that gels with my initial instincts, I'll now be confident building in some longitudinal dihedral.

In the meantime my Laser guru has done a beautiful job of re-cutting the fuselage formers in nice Sig quarter grain.  I have save 1.5 grams on those parts alone.   

[ZK-AUD]

Time for some proof of (modelling) life!

Here is progress on the fuselage to date.  

I started by getting the fuselage parts re-cut in lighter wood - so far I have saved 2 grams just in the formers and relieving some of the excess keel framing in the back end.

I bought a metre of aluminium right angle and made these very useful little support - the base has 2 pin holes for clamp pins and the upright has firm balsa cyano'd to it for pinning the former into.

I'll call out my new building board as one of the best I've ever used. (see last photo) The caretaker at our local school saw me one day and asked whether I wanted some light wood from a crate of photocopier paper they had just had delivered.  To my surprise it was not balsa but something very like it though firmer.  

After a couple of days in my hot shed it proved it was unstable however so I had the bright idea of gluing it to a firm base.  The base I use is those old cupboard doors with ply on either side and solid laminated pine in between - very strong and very flat.

Once glued down I just ran it through the deWalt planer / thicknesser and there you have it - perfect building board.

I have not run the stringers right to the rear.  It gets very busy in there and you can get problems with covering sometimes - I will carve and hollow those sections from my lightest deadest 1/4 sheet
 

[Rekitus]

ah dingit.   You are going to have me looking at the plans.

And I know the box is under... things... so do I try to slide it out
and risk things going awry...  or move things.

your methods have my attention.

back soon.

victor  

a later post but no one added  I'll go on.

the landing carriage is an interesting idea.

It should be serviceable for indoor landing.
I have not figured how to assemble it however.

I would likely learn to fly it on that big skid.
I would think a balsa keel inside would make
for a stout landing rig.

the upper gull-wing was the item I spent a long time sorting out.
I decided to build as described ...  then see if I like it.
with ply and balsa laminated to make spars as a back-up option.

victor

[OZPAF]

Very tempting - nice work .

happy building.
John

[ZK-AUD]

Crappy weather meant a productive modelling weekend so here is further proof of life.  I had re-cut the formers in very good wood but the 1/16 stringers supplied in the kit were ideal so I used those.  

You'll recall that I was planning to carve and hollow shells for the tail section,  which I did.  These are under 1/32 in thickness and made out of my lightest stock.  I haven't weighed them but would be surprised if they were any heavier than the sections of 1/16 square would have been - and I think the effect is so much better.

I have infilled a number of areas for localised strength - the plan shows no infill and you could get away with it but I plan to modify the undercarriage from what is shown on the plan.  The plan design is far too weak in my experience and also includes a hell of a lot of iron that I'd rather not be flying with.

The real aircraft has a v strut to each wheel that hangs off the big fairing that sits underneath the fuselage, and a compression strut up to a point on the side of the fuselage.  I plan to replicate that system, hinging the V struts off a central keel mounted under the fuselage and hidden in the fairing.  The compressions strut will enter the fuselage sides and be internally sprung.

I'm planning to use 1mm carbon rod encased in the balsa fairings to avoid all the ironmongery.

You might notice the diamond pattern of 1/16 bracing that I glued to the back of Former 7.  There was relatively little strength in that area but that's all it took to make it rock solid.

AUW so far is 13g which I'm very happy with for a 30" model with this sort of construction.

[Arnold S]

That looks lovely, looking forward to seeing how this beauty progresses…

Andrew

[MKelly]

Very nice Mike.

Mike

[Crabby]

Covering that neat and tidy framework is going to go smoothly.

[packardpursuit]

According to excellent (IMHO) scale drawings by Pete Westburg,  the Sparrowhawk had 0 deg. incidence @ both wings . However, it is my understanding that a Clark Y produces zero lift at approx. -5 deg.  Interesting by at this date Curtiss appears to have started using a more modern incidence line, instead of measuring from the flat bottom. IIRC, Curtiss P-1 used latter reference and a much steeper angle of negative incidence.  The more I model, the more I appreciate full-scale dynamic force arrangements, for successful miniature flight.

https://www.rcgroups.com/forums/showpost.php?p=35715445&postcount=7

[packardpursuit]

I had to check wing incidence and airfoil reference line, again according to Westberg's P-1B:

https://www.rcgroups.com/forums/showpost.php?p=35670157&postcount=1

[ZK-AUD]

Most of the basic fuselage is now complete and sanded - I have relieved the formers between the stringers to improve the covered look.  I've roughly fettled the keel fairing and that seems to fit nicely.  The undercarriage will pivot from a reinforced structure inside this.

The third photo is the corrected shape and size of the tailplane courtesy of some really good drawings that PackardPursuit has posted on RCGroups.

Don't know where they got their shape from on the plan,  but I have to change it.

The kit is a very good design and from what I have seen online can fly very well even inside,  however it is simplified and even arguably a little cartoonish in a couple of areas so after seeing Mike Stuart's one (I blame Mike!) I will have to make a few changes.

[Rekitus]

yow!   Nice work!

surface outlines for any aircraft that
found to need a bigger area, rudder is most common,
usually have all directions moved. easiest was make tubes longer and open the curves a bit.
so I was surprised the rudder trailing edge got stretched.

The outline is going to vary a bit because these 6 fc9 planes
are a style of building that is hand made.
and I gotta admit the outlines shown vary a lot.

any idea which source you will trust?

victor

[ZK-AUD]

Got the tailplane built over the weekend.  This is the correct scale size and shape.  Weight is 2.8g

I also re-engineered the structure,  which lent itself nicely to a laminated outline.

[knapster]

That's purty!!

[Crane]

Awesome! Relieving the area between the stringers must have taken some time.

[ZK-AUD]

Actually it’s one of those satisfying little jobs that doesn’t take long. I use various sized round things (dowel, brass tube, marker pens etc with fine sandpaper stuck to them with double sided tape. Have a go - also adds lightness

[packardpursuit]

ZK
Following with great interest. Seems as you're trying to "gild the kit lily" to be more scale like and I like that. I goofed in assuming the Sparrowhawk featured a Clark Y , when it was actually a Clark YH. Don't know if matters all that much on a rubber scale ship, even at Jumbo Scale? I did note Westberg shows considerable neg.  incidence.   Decisions, decisions... However, looks to me you've got things well sorted out and don't need my "help"(!!).
All the best
PP

[ZK-AUD]

Thanks PP.  With this one the basic kit design appears to be very sound and I know they fly well.  All I'm doing is to correct the more obvious deviations from scale and a bit of re-engineering to add lightness and in the case of the undercarriage, strength.  It's a rabbit hole and I'm fighting my OCD not to go too far down it!  I want to get flying with this one ASAP.

I noticed the negative stab on your splendid Westberg drawings.  The note said it was 3 degrees an later reduced to 2.5.  I'm guessing that was to facilitate the slowest possible nose up approach when docking with the airship.  Would have been a handful otherwise!

[ZK-AUD]

More progress over the weekend.

I built the fin and rudder - corrected the shape and went with a laminated outline.

I also built the wing centre section incorporating the gull effect.  I decided to use the kit structure here as a frame and then add sheet infil and some light block to improve the curves.  That still to be done.

The last 2 pictures are how I set it all up for gluing - another really great use for those aluminium angles I made for setting up the fuselage formers.

If you look carefully in those photos you'll see that the wing root ribs have little construction tabs that are cut away after the unit is glued.

These little tabs proved to be a very convenient feature.  I was able to trim the rear ones to accurately impart the 2 degrees incidence we talked about earlier in the thread.  This was particularly convenient because the other way to achieve it would have been to trim the fuselage seat but that would have meant making all kinds of modifications to the wing centre section to take the angle into account - this was far easier.