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$50.00 Scratch Build Contest Entry - F7F Tigercat

Old 09-06-2013, 03:43 AM
  #1  
pmullen503
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Default $50.00 Scratch Build Contest Entry - F7F Tigercat

My entry will be the F7F Tigercat. I wanted to do one for a long time, ever since I saw one fly at the EAA fly in several years ago. This thread also got me juiced up. Originally designed as a heavy fighter for the US Navy, it failed Navy tests but was adopted by the Marines for a variety of roles.

As a model it has a lot going for it: Nice broad wing, ample stabilizer and an almost comically large rudder. It has huge propellers so no problem powering it with efficient outrunners. I had begun to draw up plans a few years ago but was stymied by the long spindly nose gear and small front wheel. Flying off of grass looked questionable for the size I was contemplating. The narrow fuselage and nose gear that retracted through about 110 degrees only made things more difficult. But after the success of my He-219, I began to reconsider the Tigercat. The He-219 had a similar problem with the nose gear so I left it off and made a belly lander instead. That plane is a blast to fly and so I'll do the same with the 'Cat. I dug out the uncompleted plans and finished them up.

Specs:
1/12 scale
WS 51.5 in.
Length 45 in.
Area 440 sq in.
Target weight 40-48 oz.
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Old 09-06-2013, 05:54 AM
  #2  
kyleservicetech
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Originally Posted by pmullen503 View Post
My entry will be the F7F Tigercat. I wanted to do one for a long time, ever since I saw one fly at the EAA fly in several years ago. Originally designed as a heavy fighter for the US Navy, it failed Navy tests but was adopted by the Marines for a variety of roles.

As a model it has a lot going for it: Nice broad wing, ample stabilizer and an almost comically large rudder. It has huge propellers so no problem powering it with efficient out runners. I had begun to draw up plans a few years ago but was stymied by the long spindly nose gear and small front wheel. Flying off of grass looked questionable for the size I was contemplating. The narrow fuselage and nose gear that retracted through about 110 degrees only made things more difficult. But after the success of my He-219, I began to reconsider the Tigercat. The He-219 had a similar problem with the nose gear so I left it off and made a belly lander instead. That plane is a blast to fly and so I'll do the same with the 'Cat. I dug out the uncompleted plans and finished them up.

Specs:
1/12 scale
WS 51.5 in.
Length 45 in.
Area 420 sq in.
Target weight 40-45 oz.
A fellow club member had a giant scale version of this model, with 950 square inches of wing. The club member wanted to electrify this model. Using a pair of Hacker A60 motors, two 10S LiPo packs, along with all the servos and so on, this lead brick with wings was going to weigh in at about 35 pounds.

The wing loading came out to some 85 ounces per square foot. A gasser version wasn't much better. I convinced him to sell the model for what he could get out of it.


Bottom line, be very careful of weight with this model!
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Old 09-06-2013, 04:16 PM
  #3  
pmullen503
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Originally Posted by kyleservicetech View Post
A fellow club member had a giant scale version of this model, with of wing. The club member wanted to electrify this model. Using a pair of Hacker A60 motors, two 10S LiPo packs, along with all the servos and so on, this lead brick with wings was going to weigh in at about .

The wing loading came out to some per square foot. A gasser version wasn't much better. I convinced him to sell the model for what he could get out of it.


Bottom line, be very careful of weight with this model!
Good advice for any model.

85 oz/sq ft! Must have been fiberglass. I don't think you could get 35 lbs of balsa (much less foam) into a model of that size!

Mine should come in around 16 oz/sq ft or less with over 100% thrust to weight ratio so it will fly well and still handle some wind.

Anyone else notice numbers being stripped out of replies?
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Old 09-06-2013, 05:48 PM
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kyleservicetech
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Originally Posted by pmullen503 View Post
Good advice for any model.

85 oz/sq ft! Must have been fiberglass. I don't think you could get 35 lbs of balsa (much less foam) into a model of that size!

Mine should come in around 16 oz/sq ft or less with over 100% thrust to weight ratio so it will fly well and still handle some wind.

Anyone else notice numbers being stripped out of replies?
Yeah
I had the Engine cowling in my workshop, taking measurements, and trying to figure out where to put that many batteries. That cowling was nearly a foot in diameter.
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Old 09-06-2013, 06:28 PM
  #5  
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Originally Posted by kyleservicetech View Post
Yeah
I had the Engine cowling in my workshop, taking measurements, and trying to figure out where to put that many batteries. That cowling was nearly a foot in diameter.
Lots of room in those huge cowls. Mine are about 5" diameter and I may have to tweek that so I can use $tree 3 liter bottles to make the cowls.
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Old 09-06-2013, 10:45 PM
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I love the looks of that plane one I want to build someday. Looking forward to seeing your progress with this build.
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Old 09-07-2013, 02:14 AM
  #7  
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Originally Posted by pmullen503 View Post
Lots of room in those huge cowls. Mine are about 5" diameter and I may have to tweek that so I can use $tree 3 liter bottles to make the cowls.
Yup
The club member wanted to go with those A123's. There was room inside that cowl for 24 of those A123's, the ESC, and the Hacker A60 motor. And, all of that stuff got heavy.
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Old 09-07-2013, 11:02 PM
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pmullen503
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I got started on the fuselage. I'm making it from heat formed FFF. I like this method because it makes a light, rigid and cheap fuselage. The other big advantage for me it that you can scratch build from a 3-view without CAD. (If you are interested in the details of the method go here and look at my last seven builds.)

I started with a decent 3-view and blew it up to 1/12 scale. Then subtracted a 1/4" from the side view and cross sections to account for the FFF.

To make the molds, I cut out the side view in 1/2" plywood for the backer boards. After that, I used the cross sections printed out and glued to cardboard to rough cut XPS foam blocks and glued them to the backer boards. I leave gaps between the blocks so I can slip in cardboard to trace the cross sections so I can compare the right and left halves.
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Old 09-07-2013, 11:09 PM
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Thats neet, Keep up the nice work Chellie
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Old 09-07-2013, 11:16 PM
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Once the blocks are glued on, I shaped them to the final dimensions. I start by block sanding to a nice smooth curve. Then I start checking the cross sections by sliding the cardboard templates into the slots. The 3 view I used was pretty good, all but one of the cross sections looked OK. I sanded down to about 1/8" larger than the final dimensions. After that, I don't rely on the templates. I only worry about getting both sides the same and keeping the curves fair. I don't know if the sections are accurate or were accurately traced, so I go by what looks and feels right. I don't mind shaping the molds so I got the two halves very close, better than 1/32". You can easily get away with a 1/8" variation between the halves at this size, you usually can't see or feel the difference.

This is the first time I've tried to mold the rudder with the fuselage. We'll see if it works......
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Old 09-07-2013, 11:56 PM
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Very impressive.
Do you 'skin' the foam with something?
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Old 09-08-2013, 12:51 AM
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pmullen503
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Originally Posted by quorneng View Post
Very impressive.
Do you 'skin' the foam with something?
Stay tuned. I'll wrap the molds in fan fold foam and bake them to form the fuselage halves.

The photo below is from my He-219 project.
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Old 09-08-2013, 05:17 AM
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The molds are wrapped in FFF and baked to form the fuselage shells. A 1x3 wooden spine was screwed to the back of the mold to keep it flat.

First, the FFF is covered in PVC packing tape overlapped 1/4 to 1/2". The tape is applied to the film side. The film comes off when the tape is removed and actually makes the tape removal easier. The packing tape functions as the female part of the mold.

The taped FFF is then rolled over a pipe to pre-form it into a curved shape to make it easier to tape to the mold. The FFF is then taped to the mold. Finally, it's baked in my oven (foil faced foam box with a modified space heater inside) until it reaches 100C (usually 15 to 20 minutes.)

When it's cooled, The spine is removed and the formed shell is trimmed flush with the top of the backer board. I'll leave the tape on until the halves are joined to protect them from dents and stray blobs of glue. The weight of the first half was 62g. Without the tape and film it should weight about 38g. or about 12g/sqft.
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Old 09-09-2013, 01:58 AM
  #14  
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Getting ready to join the fuselage halves.

One of the characteristics of this kind of construction is that you are "off the plan" almost immediately and are building on the shells themselves. To get an accurate reference line to lay out the wing and stabilizer locations I drew a line on a board (bench was occupied with a different project) parallel to the edge. I used the reference line off the backer board and marked it's location on the ends of the shell then taped the shell to the board. I taped a marker to my square and set it to match the line on the bench. As long as I kept the square parallel to the board, I could accurately mark the fuselage reference line as it rose and fell. Trying to use a long ruler, even a flexible one, just doesn't work. I poke through the foam with a pin to transfer the reference line to the inside of the shell when I need to.

The stabilizer is almost 24" wide but the fuselage where it mounts is only about an inch wide. That's a lot of leverage so I added 1/32" balsa sheet to reinforce where the stabilizer mounts. That area is a compound curve so I picked the best line I could find for the balsa (least compound curve.) I steam bent the balsa and clamped in place to dry with cloth strips. When it was dry I glued it into place.

Foam alone is not strong enough for the wing mount so it too got some reinforcement. I used some very hard 1/16" balsa and ran a piece from the wing saddle all the way to the tail. The fuse is so narrow and the tail so wide that I figured it would be a good idea. I pre-bent this too to roughly match the fuselage and clamped it while the glue dried by replacing the shell back onto the mold and wrapping it with pallet wrap. (I bought a roll of it to ship a piece of equipment 17 years ago and am still using the same roll!)

I slipped fresh cardboard into the gaps in the mold and traced new cross sections. I'll use these to make the formers. I laid out the paths for the elevator and rudder pushrods and marked them on the former templates. I'll make the formers from foam.
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Old 09-09-2013, 06:20 PM
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pm...some very nice building techniques. I read through those links on your laser wing building. Thought, where do you find two perfectly flat building boards, but it quickly hit me that they don't have to be flat. Your technique guarantees a straight wing every time.....thanks for the tips....
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Old 09-09-2013, 06:36 PM
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Originally Posted by Brner View Post
pm...some very nice building techniques. I read through those links on your laser wing building. Thought, where do you find two perfectly flat building boards, but it quickly hit me that they don't have to be flat. Your technique guarantees a straight wing every time.....thanks for the tips....
Well done! Most people don't pick that up. That's the key thing about the laser method; accuracy. I'll start on the wing soon.

Got the fuselage internal structure done and the halves joined. Pictures when my Internet connection starts working again.
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Old 09-10-2013, 04:34 AM
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The first photo shows the internal structure. I cut out the formers using the newly traced templates. Holes were cut for the carbon fiber rudder pushrod and the flexible cable for the elevator. The elevator pushrod sheath is installed. I'll add the end support for the elevator pushrod sheath after I cut out for the stabilizer. I'll have to make a small door to access the elevator horn to connect the pushrod. The formers are glued into one side first and checked to make sure the seam will close up when the other shell is glued on. Only a couple formers needed some sanding.

The balsa sides and bottom for the battery box were added by first cutting slots in the formers, gluing the formers into place and then sliding the balsa into the slots and gluing. Only after the balsa is glued in place are the centers of the formers removed. That way the smaller formers aren't distorted or fall apart when gluing. I plan to use a 2200mAh 3s battery but made the battery box just big enough to fit a 4s battery or slightly larger 3s.

I marked the edges of the shells with a green highlighter marker. This will make the vertical center line easy to see. Often the glue joint is good enough to vanish after sanding. I use white Gorilla glue to join the shells. It foams up to fill any gaps between the shell and formers. It is a pain to sand so I'm careful to not let much foam up on the outside. Leaving the tape on also keeps the glue from getting on the outside foam. Any glue on the outside will be carefully excised with an Exacto knife. You really can't sand dried Gorilla glue flush with the bare foam without sanding the foam around it away too.
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Old 09-10-2013, 10:16 AM
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quorneng
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pmullen503
I appreciate the tail end of the fuselage on a Tigercat has a particularly small cross section but how much of the stiffness do you reckon comes from the balsa inserts?

Would using just the shell be possible on a more generous section?
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Old 09-10-2013, 02:11 PM
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quorneng,

As you would know, the issue is not really stiffness; just like your many planked depron models, the stiffness is there. I'm concerned about the foam failing under compression. Adding the balsa gives the foam compressive strength. The 1/32" stab reinforcements add little weight (they wouldn't register on my scale). The total weight for all the balsa added was only 20g or about 1.6% to the AUW. Also, a significant amount of the foam will be sanded away in that area shaping the fin.

This is the first time I've added a piece like this. Normally, the fuselage is wider and the stab span smaller and so has less leverage in a bad landing or a mishap during transport. What I've found important is the long balsa piece from wing to tail. The first model I did with this technique was a Ki-46 Dinah which had just terrible p-factor issues. I cartwheeled the first take off attempt and the fuselage didn't break but the foam compressed just in front of the stab on one side. That was harder to fix than a clean break.

It's possible to build very light with this technique. I have a 54" powered glider made from tissue covered $Tree foam with a wing loading of 7 oz./sqft but that's strictly a 5 mph winds or less airplane and has little dent resistance.
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Old 09-10-2013, 03:22 PM
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I added a 2.5mm bamboo reinforcement to the rudder TE and balsa to the fin LE.

The bamboo comes from a roll up blind I bought a few years ago from the bargain bin at the local builders supply. It has a lifetime supply. I planed a flat on the bamboo and then heat bent it to the shape of the TE. I glued it on using carpenters glue. The LE got a tapered 1/4" balsa piece.

I don't know if I'll be able to use the foam rudder once it's cut off or if it will make more sense to build a new one but the wooden TE and LE will help when I shape the fin. The green marker on the seam really helped at this step to keep things centered.
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Old 09-11-2013, 02:02 PM
  #21  
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Fascinating...
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Old 09-12-2013, 12:25 PM
  #22  
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Very cool indeed!
Crash
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Old 09-12-2013, 02:50 PM
  #23  
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Got to work on the rudder. After removing the tape and film from the back part of the fuselage (I'll left it on the rest for dent resistance), I shaped the fin and rudder. I realized that I should have included some ribs for the rudder to keep it's shape once it was cut out so I added them now. I cut slots and glued in more foam and sanded them flush. The foam is now about 1/8" thick in this area after shaping.

I cut out the rudder, added 1/16" balsa and more foam to the LE and sanded it to shape. The rudder is really thick so I added shrouds to cover the gap. I sanded in a recess (the tool was a piece of Formica with a square of sand paper glued to it) and glued in some maple veneer I had laying round. After that, I sanded the recess to fit the rudder. I'll use a pivot on the bottom and point hinges for the rest. I'll have to glue in mounting pads in the recess to mount the rudder to the fin.
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Old 09-12-2013, 04:01 PM
  #24  
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Very Nice I Like the recess Rudder assy, Thats going to look sweet, Take care, Chellie
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Old 09-13-2013, 01:46 PM
  #25  
pmullen503
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I made the stabilizer from hot wire cut foam core skinned with 1/32” balsa. I feel that a “real” airfoil (NACA 0009) on the stabilizer makes the plane fly better than a flat stabilizer. This also makes a very stiff, strong stabilizer and really doesn't take that much longer to make than a flat foam stabilizer with balsa reinforcement. It does weigh a little more but the benefits are worth it. The foam came from some packing material and is very light, core weighed only 4g (but very soft too). I have a thickness sander and made my own 1/32” by sanding down 1/16” balsa. I bought bulk seconds from Balsa USA for only $1.50/ sheet for 4”x48”x1/16” (at an air show so no shipping!)

I cut both halves from the same block of 2” foam. This guarantees that both sides end up the same size and any errors in the templates end up on the same side of the stab so no net rolling force due to inaccurate templates.

Tips are made from 1/32” balsa sandwiched with FFF. The balsa sticks out a bit so it can fit into a slot cut into the tip of the stab. This increases the glue area and makes sure it stays stuck during shaping. After shaping the tips, the halves were joined and the elevator cut free. A 1/8” balsa TE was added to the stab and a ” balsa LE added to the elevators.
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