About Airborne Magazine

Pushy Cat

by Ron Passlow

Plan No.684

Features Construction Articles

Plan Detail:
Plan No. 684
Price: AU$54.00 (2xSheets) 
plus P&H (AU$5.00 within Australia).

Order Details:
Order On-Line

By Phone:
(03) 9333 5100
ByFax:
(03) 9333 5099
By Email:
By Mail:
Airborne Plans Service: 
PO Box 30 Tullamarine,
VIC, Australia, 3043

A Construction article, but why?

Don't most people these days buy ARF models? And that is fine if you only want the standard type models, but if you would like something a little different then the only way to get that is either to build it yourself or pay to have it built. Even if you only fly ARF models, you will at some time have repairs or upgrades to carry out on these models and if you do not have any experience with building models you will have to learn on the run or get assistance from someone who knows how to fix your problem. But if you have at least built one model "Off the Plan" then you will have some idea of what is needed.

Off the plan building is also very satisfying because you start with a pile of wood and you sculpt it into your latest plane that you can be proud of and want to show your skills to the other members of your club. Anyway back to this construction job of the Pushy Cat. It should go without mention that when you receive a plan you should not start building until you have studied the plan thoroughly and worked out your own plan of attack. After studying the plan carefully I decided to start by building the hardest part first, the wing. Before starting to build I usually kit the plan. This means that I cut out all the parts that I can before I start assembly because this ensures that I have enough balsa and ply to complete the main construction and also makes the actual building process proceed more smoothly and quickly.

The Wing
This wing is built in three sections and is also built upside down over the plan. The first job is to cut out the ribs accurately. You may have a favourite way of transferring the rib outline onto your balsa, but I used the old and tried method of tracing each rib onto tracing paper and then transferred this to the balsa. This method has the benefit of not damaging the plans with pins being pushed through to mark the outline and leaving the plan a perforated piece of paper that will rip more easily along these lines. As this is a tapered wing, each rib has a different profile so make sure that the tabs (to assist with building) at the rear of each rib are cut accurately so the wing will end up being built straight.

I chose to build the centre section first because this was the more complicated section. With the fact that the wing is built upside down it is not possible to put the wing joiners (J1 and J2), for the outer panels, in place until this section is removed from the building board. I started with pinning down the main spars for the two halves and then trial fitting the ribs in place before gluing to ensure that all is aligned correctly and that the wing joiner (J3) slides through the slots in the ribs. IMPORTANT - I use plastic wrap over my plans to protect them during all building because this stops you gluing parts of your model to the plan. I found that ribs W1 to W3 for both sides of the centre section needed to be slid over the joiner before fitting the ribs in place on the top spar (which is the spar pinned down on the board). Then the ribs were pinned in place, ensuring they were at 90o to the building board and then the 6.5 mm balsa "trailing edge" spar is pinned in place at the rear of the ribs and then all the joints were glued and left to set. Remember that ribs W3 and W6 are made from ply to support the undercarriage and also to give added strength to the wing joiner and as such should be glued with a better adhesive. I used epoxy for these joints. The leading edge is then pinned in place and glued. Next, the top (bottom - remember the wing is upside down) spars are fitted (I joined them with an opposite mitre joint to spar pinned down) and undercarriage blocks are added and epoxied in place.

You will note that there are some balsa blocks in this centre section that need to be fitted before any sheeting is applied. These are for the dowels, the wing bolt holes and where the exhaust exits through the wing. This last one I chose to use a balsa tube rather than a block to help reduce any extra weight and also so I could place this in at a later time to suit the location of my exhaust. The centre section then has the sheer webbing glued in place between the top and bottom spars and then it is sheeted with 1.5mm balsa before removing from the building board.

Once you have the wing off the building board it is possible to insert and glue the joiners for the outer panels and sheet the entire section with 1.5mm balsa and trim off the building tabs on the ribs.

Now you can build the two outer panels, again upside down over the plan. This is fairly standard construction except ensuring that rib 6a is set at the angle shown on the dihedral guide to allow the outer panels to have the correct dihedral. Again I pinned the top spar to the plan and built the wing over the plan. The plan shows wing tips are made from balsa block but I chose to make them by laminating pieces of 6.5mm balsa. This is usually cheaper to make and also most of us who have scrap pieces of balsa lying around will often find enough to make these wing tips without having to buy extra block balsa that can be expensive. Also, it is necessary to build the ailerons and the built up trailing edges for the wing panels and attach the trailing edge sections to the wing panels.

Once these panels are completed it is possible to join the outer wing panels to the centre section, ensuring that the correct amount of dihedral is achieved for each outer panel. The dihedral is shown on the plan and I chose to use a 32mm spacer under the rib W12. Do not forget that there are two servos in each of the outer wing panel, one for the aileron and one for the inverted V tail and these servo bays need to set up and you need to ensure that you can get the servo leads through the wing to connect them.

The wing is now basically finished except for the final shaping of the leading edge, the fitting of the dowels, the drilling of the holes for the wing hold down bolts and the cutting the hole for the exhaust. Now I put the wing aside so I could build the fuselage.

The Fuselage
This is fairly straight forward except that I believe a building jig is a great aid to building the fuselage. If you donÕt have one, they are not hard to make and you can use it for years to come. I started by gluing the fuselage doublers to the fuselage sides, ensuring that you make one left and one right side. It is easy to make two of the one side. I would suggest that when you glue these doublers in place, it would be a good idea to slightly curve the sides towards the ply doubler by placing spacers under them and clamping them down. You will end up with two curved fuselage sides bending toward the ply. I did not do this and found it difficult to bend the fuselage sides down to F1 and F2 even with a jig.

Next I glued F3, F3a, F4 and F5 in place along with the spruce longerons. All this was done in a home made jig as seen in the accompanying photo. Once these were clamped in place I inserted F1 and F2 (with the nose wheel bracket attached) and clamped the jig in tight to let the glue set. I used aliphatic resin for F1 to F4 and epoxy resin on F5 because this is the firewall and needs extra strength to cope with engine vibration. I set this aside to dry for 24 hours and then I was able to fit and shape the top and top sides of the fuselage with 9.5mm balsa. Once removed from the jig I was able to glue in place and shape the bottom sheeting of 9.5mm balsa. The nose block was them carved and sanded to the desired shape and glued in place.

Now comes the cockpit area which is basically the canopy with a floor and back and front wall installed. This may sound simple but let me tell you it is not as easy as it seems. The floor of the cockpit is 3mm balsa and I found that it was necessary to glue strips along each side to give you something to glue the canopy onto. Before gluing the canopy in place you need to think about whether you want to have a pilot in there or not and if you want to paint the inside of the cockpit. To get the cockpit area to mate to the fuselage properly I used plastic food wrap over the fuselage and when I was ready to glue the canopy to the cockpit floor and ends, I fitted it in place on the fuselage and clamped it there with rubber bands and allowed it to dry.

Probably the most difficult part of building the fuselage was the installation of the engine mount and making the cowl fit around the engine. This plan has the engine laying over at 45o from inverted to allow the muffler to go through the former F5 and have the exhaust exit down through the wing. Some engines will not allow you to do this with the standard muffler. I used my OS 46 FX but I had to modify the muffler so that it would clear the wing. Modifying the face of the exhaust flange to give me the correct angle did this. You may choose to use a Pitts style muffler or what ever, this is your choice.

Be aware that installation of the fuel tank is another area where you need to be careful because this is a pusher set up and the front of the tank needs to be facing the front of the fuselage and not to the engine.

With this being a pusher I knew that I had to keep as much weight up front as possible and so mounted the servos just behind the nose block with the battery pack between them and still had to add nose weight (more on this later).

The Booms and Tail Feathers
I started with the tail feathers because these are a little different to the norm. I built the two halves separately and then joined them using the ply joiners (TJ). This all sounds quite simple but it is important to have this all done accurately.

The two halves were built over the plan individually in a similar manner to that used building the wing. I pinned the bottom spar to the plan and glued in place the ribs and attached the 6.5mm balsa leading and trailing edges. Then the top spar was glued in place and the tail centre (TC) was fitted and shaped. The top 1.5mm balsa sheeting was then glued in place with aliphatic glue and this was left to dry over night. Once dry it was removed from the building board and the bottom sheeting was glued on. The two sections were then test fitted together until they were at the correct angle and alignment and then they were glued together. The trailing edge of the centre section was built up from 12.5mm balsa and sanded to the desired shape. Next the ruddervators were built over the plan, making sure they were kept warp free.

The booms were built individually over the plan. This was really a simple matter of building as if they are small fuselages and as such needed to be kept straight and true. I was tempted to build them on their side, but decided that this would not be the best way to have them both come out true down the centre line of the boom. Whilst building the booms you need to look at how you will install your snake controls for the ruddervators. Once the booms were finished I measured off the plans their centrelines and marked these on the building board and attached the booms to the building board in preparation of fitting the inverted V tail. This process needs to be done with patience and care and not rushed so that the correct alignment is achieved. It entailed quite a bit of trial fitting and sanding and then the final gluing in place and then the fillets are glued in place and the whole set up sanded to shape. This configuration of booms and inverted V tail are then removed from the board and test fitted and checked for alignment before gluing to the wing. I needed to enlarge the holes in ribs 6 and 6a to allow a better angle for the snake to pass through and down the booms.

The inverted V tail presented its own problems for set up. First, I was going to set this up as a V tail with ruddervators and assign rudder to an extra channel for the steerable nose wheel. But my radio had problems because I could get the elevators to work in the right direction but the rudder control would be wrong and if I reversed the settings the rudder would be right but the elevator would be wrong, so I opted to have just elevator on the V tail and use the rudder channel for the nose wheel.

Finishing
I chose to use the method I am comfortable with and that is with doping, painting with spray enamel and heat shrink covering for the wings. The entire airframe was filled and sanded before doping. I gave it two coats of dope, sanding between coats, and then I painted the entire area to be painted white and then masked off the parts needed before painting the colour.

When all assembled with the 6 servos (2 for ailerons, 2 for ruddervators, 1 for throttle and 1 for steerable nose wheel) are installed and the engine is fitted it is time to balance the model. The balance point needs to be 11.5cm from the leading edge and to achieve this I had to add 12oz of lead to the nose.

To the Field
The model attracted a lot of attention from all corners because of the unusual design of a pusher with twin tail booms and inverted V tail.

The first trip to the field was an eventful one. After all the setting up and tests completed it was time to fly. The motor fired up and I taxied out to the strip and lined up and gassed her up and she was in the air within about 10 meters and after I had turned to the down wind leg the motor cut and I called out Òdead stickÓ, but one of our helicopter pilots, who was not standing in the pilots area, did not hear me and as I turned to land across the strip he flew in front of me so I had to turn away from him causing the plane to loose too much air speed and it hit the deck fairly hard. Breaking the tail off and one wing outer panel. The reason for the dead stick I believe was two fold in that not really thinking I put the tank in with the front of the tank to the motor, but I should have had the front of the tank to the front of the plane. The second reason was sufficient cooling and I cannot emphasise too much that the motor in a pusher configuration especially needs good cooling particularly for running up the engine in the pits as you do not have any prop wash over the motor to cool it. Two weeks later with it repaired and with the tank in the correct way around I was back at the strip and this trip I was able to fly a couple of flights to test Pushy Cat out. It is quite a docile flyer because of the large wing area, but it does handle well with no real vices that I could detect. It again was airborne very quickly and it was fun to fly doing loops, rolls etc. With the ailerons set on low rates (5mm either way) the roll rate was quite slow and even on high rate (10mm either way) the roll rate was not too fast. When coming in to land this plane just wants to keep on flying because of the semi-symmetrical aerofoil and large wing area.

wing frame

wing frame

fuz frame

framework from underneath

framework from above

ready to finish

finished

taxiing out

spacer
Copyright © 2004-2005 Airborne Magazine. All rights reserved.
Designed and managed by: SCAD
This page was last modified on the August 24, 2006