Levanter's Missus' Isabella

[Levanter]

The problem is I am not always the first one to flick through the latest copy of RCM&E. This means from time to time the missus spots something that she would like me to build for her. This time around it was Isabella by Marco Penk in the September issue but previously it had been Jemima by Jim Newberry. There some similarities in style.

So it comes to be that Isabella is on the building board having taken a Summer break from the Grumpy Tigercub.

Perhaps it is the sit up and beg looks that appealed and I had an OS LA 10 hiding in a box, second-hand but in good nick and a runner. The plan shows and Enya 09 and I have one of those but in bits.

Apologies first to Marco Penk as I wasn't really that keen on the wing construction which I have redesigned and a few detail changes. I am sure it retains the spirit of Isabella and hope that Marco won't mind.

This thread will just deal with the significant changes and snapshots of the build progress. Actually I am having a lot of fun building it.

Levanter

[Levanter]

Here are a couple of pictures reproduced from Marco's website.





I built the fuselage first and had some slight misgivings about the stick at the rear end just butting up against the solid fuselage sides. There are some large gussets but even so the connection relies on gluing on end grain. To get around this the longerons were extended as far as they could at the drawn angle effectively creating a splice and some continuity especially as this is where the fuselage is bent around to start the taper.



Shown here



And here.

The gussets were not made separate and are a continuation of the fuselage sides. The V has a generous radius to help avoid splitting but anyway the V is prevented from splitting by the former and wing supports inside the fuselage.

My little OS engine came with a glassed filled nylon mount which I decided I should use. This meant widening the nose a little from the plan. The sides came around in the fuselage jig but needed quite some force. Probably the sides would need wetting to get to the narrow nose on the plan which is all that's needed for the electric version. I did choose some medium hard balsa which was fairly crossed grain. I thought this might help prevent splitting as no doublers are used.



This is what will be installed and goes in very nicely inverted. The spinner is a fraction bigger and helps disguise the slightly wider nose.



Here is the nose former made from Liteply to give some strength in both directions. The slot will need to be widened slightly to fully uncover the carb.



This is the underside of the nose and you can just see one of the triangular fillets behind the nose former on the right of the picture. The holes are drilled in the 3mm birch ply firewall for the engine mount. These will have bolts as supposed to blind nuts as I can get to both sides. 3mm is to thin for my M3 blind nuts without using doublers.

The corner stringers were cut at 15mm centres to get the bend. I had a quick go first without the cuts but something would have let go for sure. I always put centrelines on every former as this makes everything so much easier lining up in the fuselage jig. To mark the line on the opposite face I drill through with a 0.3mm drill and this gives me a permanent reference if the line gets rubbed out.

The squared off hole in the former is dimensioned to retain a 2 oz Sullivan tank. I got the dimensions from a web page and hope it works as I still have to order the tank (SMC have them in stock)

This describes the significant changes to the fuselage.

Levanter

[Levanter]

A few other details.

I decided to sheet the top deck aft of the wing. This made some considerable difference to the stiffness and will help a bit with the framing for the glazing at the rear of the cockpit.




I had a bit of grooved beech which will recess the piano wire of the undercarriage flush with the fuselage. I moved this to the forward side of the former to make it clear of the vertical wing supports. This meant I could glue it to the former as well as the fuselage sides.
This will not affect the position of the wheels in relation to the balance point. It just means the piano wire will have a bit less rake.




Marco suggests the model will look good with struts and I agree. This meant it would be useful to fit hard points in the fuselage and the wing so that's what happened. It was a piece of hardwood from the scrap box, not as hard as beech but plenty good enough to take a screw.




At the tail end the post was made a bit wider to match that on the rudder which follows the profile of the fuselage. Instead of the little former with the piano wire for the tailwheel stitched in place I glued a triangular piece of ply between the lower longerons. This means that I will be able to fit the tailwheel after covering and it will be dismountable. Wherever possible I like to be able to unscrew fittings and equipment rather than build them in.



Despite the decking and the ply insert, an early test balance with the OS LA 10, suggests that the model is heading to be quite nose heavy even with the servos rear mounted. I will have to keep an eye on this.

Levanter

[Levanter]

As I mentioned on my original post I was not that keen on the wing. The main spar is very much to the rear over most of the wing so it is in the right place only at the wingtips. The spar is not as deep as it could be in this position. This also means that the covering has big dips between the ribs which I think looks a bit strange. The other thing I felt was that there was quite a lot of dihedral but the ailerons were mounted inboard where they are less effective. I thought the two features would be in slight conflict. So again with apologies to Marco I re-designed the wing.



The pink dotted outline is the original plan view with a double taper. I decided to make the leading edge straight and have all the taper on the trailing edge. This meant that the front spar could be continuous and therefore a bit stronger. The green lines show the new shape with the spar moved forward and the ailerons moved outboard to the tips. At this point I decided to do away with the polyhedral and just have a small amount of simple dihedral to prevent the "droopy" look. This also meant that I could increase the span of the aileron by one bay to make it look more in proportion.

I kept the wing area very close to the original by using the original wing tip rib but just moved forward. Actually there is a very small reduction of area due to the aileron extending one bay inboard but I reckoned the projected area of the wing would slightly bigger due to the reduced dihedral and the two things would cancel out to give the same wing loading as designed. The other consideration of having the straight leading edge was to move the centre of lift slightly forward and this would help with the nose heavy concerns.



Changing the wing planform meant three new ribs required each side. I don't have any design software so it was back to basic drawing principles. The wing tip rib and the rib at the inboard end of the aileron are unchanged. These were cut out and used as templates to draw the smallest and the largest rib. The datums are the straight leading edge on the left and the straight baseline of the aerofoil. On the right hand side of the drawing the rear ends of the intermediate ribs were marked equidistant on the baseline. The space between the small and large ribs was then divided into three spaces at a number of points along the chord. This is easy to do by eye and the small dots can just be seen in the photo. The intermediate ribs were then drawn in using the larger rib like a French curve. On a detail this small the academic errors are too small to be noticed and the approximation works perfectly once the ribs are spaced out on the wing.



There was not much change on the inboard section of the wing. The simple dihedral is about 10mm each side and as the wing fastening now goes through the main spar there was no need for the ply ribs on the centerline. This has save me a few grams.
My lower sheeting is continuous and is not fitted between the ribs. I find it easier to build by reducing the depth of the rib to account for the sheeting thickness. It is also stronger. The sheet can also be made to extend slightly outside of the fuselage so it still acts as a seating if the wing gets knocked a bit out of alignment.
The aileron servo is now of course much further outboard.

On this model the wing is mounted quite over the built up cabin. The centre of gravity is therefore quite low in relation to the lift with a pendulum effect. I though it should have ample stability without any dihedral because of this but as a join was necessary on the centerline anyway it has a bit of dihedral for aesthetics.

One further note on the wing, the ailerons were from stock material which saved me a bit of time over built up ones.

Levanter

[McG 6969]

Still following here, Lev.

Even if I don't understand why - after all those wing modifications - you don't want to have nice lightweight build-up ailerons.

Missus Isabella should be sóó proud of you.

Very neat build as always...

Cheers

Chris

[Levanter]

Hi Chris

Hi Chris
A quick excuse dressed up as an explanation

Because I moved the ailerons outboard I did not needs to have such a large chord. In fact the size I wanted happened to coincide exactly with some short lengths of trailing edge stock I had lying around. Amazing that

I did think about built up ailerons but due to the wonderful coincidence and an equal dose of laziness, the stock material won through.

I do now have some other problems with the wings that I will come onto shortly

Levanter

[Levanter]

I am trying to make an effort to incorporate something that I have not done before on every new build. This time around I decided to try riblets. I thought it would enhance the look following my other modifications as well as perhaps make the wing a bit more efficient. I didn't bother to plot out the riblets as the difference between each one is virtually nothing. The tiny difference in the tapered wingtip area was easily sanded to the correct profile using the adjacent ribs as a guide.

The wing tips ended looking like this. The sheer webs get smaller and smaller out towards the tip as they have less and less work to do. The outboard sheer web is just long enough to provide a landing for the last riblet.

The block on the right hand side of the photo is a block to make a hard point for the strut attachment.



Here is a view of the inboard section of the port wing before joining. I almost always use the false leading edge method that I first saw on a Peter Miller design.



Always fun to have a pre-covering jigsaw. This one had a definite purpose though to get a rough idea of the balance point. Most bits and pieces are just taped in place but the motor (OS LA 10) is bolted in place, the pilot is in her position and the undercarriage fixed. Making an allowance for the covering which always seems to add tail-weight I worked out that the rudder servo could go at the rear. The elevator could go in the cabin where there is plenty of room and a normal sized Rx battery would give me the fine adjustment somewhere around the middle of the cabin.

All back into pieces for the finishing which seems to take even longer than getting to this point.

Levanter

[Marco Penk]

Hi Levanter,

I am the Isabella designer. It is nice to see someone does build the model.

To overcome potential nose heaviness, you could have extended the fuselage length a bit. But I think that a standard 4 cell RC pack far back in the cockpit will be sufficient to get the CoG right.

I deliberately designed the wing spar further back and well below the airfoil outer shape. I am always dissatisfied when the main spar and semi spars protrude into the airfoil. Air does not like to flow around sharp corners. I believe that some US designers favor wings with visible spars and semi spars at the very critical first 30% of the airfoil. By having the big recess the iron on film cannot attach to any spar, thus making the airfoil perfect according to its design specifications. Of course, the airfoil at the middle section between two wing ribs is slightly different from the airfoil exactly on the ribs, but again, I have rather a slightly different airfoil than a sharp edge. The Isabella wing provides a very smooth shape, which air really loves.

Designers in the 30s actually measured and specified the airfoils that were created by the wing covering hanging down between the ribs.

I used the same design methodology also on my glider Carina, which is a very nice flier with a MH30 airfoil.

The Isabella wing gets very strong as soon as iron on film is applied. On the other hand, beefing it up a bit is not a bad idea, if modelers decide to put in some extra power. My Isabella was first powered by a 2S power application and later with an Enya Quicky 09. The plane was not used for aerobatics.

I get lots of requests for bringing Isabella as a CNC kit to the market. In fact my Kadett is a smaller version of Isabella when you compare the wings.

Keep up the good work!

Cheers,

Marco