Grumman F7F Tigercat Scratch Build

[DamoRC]

Been mulling this one over for a while and tinkering with the model in Sketchup. Time to pull the trigger on building this.

The model in Sketchup is pretty much done. The wings are just placeholders and need more work. The curved spine of the original is pretty hard to match but I think it will look okay.

Rough specs:
Wingspan: 46 inches
Length: 40 inches
Power: 2 x E-flite Motor 10 (1300kV) wearing 9 x 6.5 three bladed props. Will Probably use 2 x 2200mAh 3S in parallel
Target AUW: Think I can keep it to 4lbs or less

Plans - I will post plans here once the build is finalized.

Build - I will post links to the posts where I describe the build steps here.

This is the final product....

Wing Build

The scale was driven by the available motors and subsequent estimates of what diameter 3-bladed prop I could use and buy. I happen to have two E-flite Motor 10 units which I got from some guys at the field (one was surrounded by a beat up Timber). On the Timber, whose AUW is just over 3 pounds, this motor spins a 12 x 4 prop. One approach for dealing with blade number conversions was described as calculating the prop load as D^3 x P x Sqrt (N-1) where D is the diameter, P is the pitch, and N is the number of blades (apologies for the math). So for the 12 x 4 2-bladed prop this works out as approx. 6900 (not sure what the units are supposed to be). I had a rough idea that a 9 inch 3-bladed prop would produce much the same load. Then I found 9 x 6.5 B-25 3-bladed replacement props at Motion RC. If you run the numbers on this prop, the load is 6700. Pretty close. To check the numbers I measured the static thrust, Amps, and RPM for the two props at full throttle.

12 x 4, 2-blade prop generated 4.4 lbs of thrust at 10400 RPM pulling 31 amps
9 x 6.5, 3-blade prop generated 3.4 lbs of thrust at 9700 RPM pulling 38 amps

So the three blade props are pushing the motor a little on the hard side, but it should be fine. Also should have no issues with power if I can hit the target weight of 4 lbs.

DamoRC

 

[Michael9865]

Subscribing

 

[Jaxx]

That looks great! I need to learn SketchUp.

 

[Namactual]

It looks like you are off to a great start on another great build Damo.

 

[foamtest]

That model would look great in an airshow.... just saying...

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[DamoRC]

Thanks Guys!

That model would look great in an airshow.... just saying...

↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓

Yep - I need to respond to your poll - I seriously thought about this but trying to ship this (I will not be at FF Ohio), even in pieces, would be crazy expensive. Let me mull on this some more before committing to anything.

DamoRC

 

[Crow929]

That fuselage looks mighty skinny, are you going to have enough room to fit a battery?
Looking forward to seeing this one, that's a cool looking plane!

 

[foamtest]

Thanks Guys!



Yep - I need to respond to your poll - I seriously thought about this but trying to ship this (I will not be at FF Ohio), even in pieces, would be crazy expensive. Let me mull on this some more before committing to anything.

DamoRC

It's completely okay if you can't, you don't have to participate at all if it is a problem, I just saw a new fitting design and was just advertising it a little in case you were not aware of it. This also gave me an idea for flite fest though if I can make some F-22 kits up it would help get more people involved at the event itself. Then all that the people need to provide is a little assembly time and some servos! Or even better I might be able to get FT to supply a few since they are all getting auctioned off, idk though just thinking out loud.

 

[DamoRC]

That fuselage looks mighty skinny, are you going to have enough room to fit a battery?
Looking forward to seeing this one, that's a cool looking plane!

Good Point - I think I will have just enough room depending on how far forward I have to place the batteries. I am planning on using 2 x 2200s which may need to be stacked on top of each other.

She is skinny though....

DamoRC

 

[foamtest]

That is true I never realized how skinny of a bird that was, well the fuse just needed some guns and a pilot so...

 

[DamoRC]

This also gave me an idea for flite fest though if I can make some F-22 kits up it would help get more people involved at the event itself. Then all that the people need to provide is a little assembly time and some servos! Or even better I might be able to get FT to supply a few since they are all getting auctioned off, idk though just thinking out loud.

These are great ideas - perhaps this could also be a way for those not attending to help the event, i.e. buy a kit from FT specifically for the auction.

DamoRC

 

[foamtest]

That is true, now i just need to learn how to recruit a bunch of people out of a large audience to help at the event itself.

 

[Grifflyer]

Looks like you're off to a great start Damo.

...now i just need to learn how to recruit a bunch of people out of a large audience to help at the event itself.

Hijack the speaker system....

 

[thenated0g]

Looks great. I have always been a fan of this kind of shaped plane like the DH103 hornet. That skinny fuse on the tigercat would make a great sloper as well. I also really need to start learning sketchup.

 

[foamtest]

Hijack the speaker system....

Was already planning on it lol

 

[DamoRC]

Looks great. I have always been a fan of this kind of shaped plane like the DH103 hornet. That skinny fuse on the tigercat would make a great sloper as well. I also really need to start learning sketchup.

Based on the work you are doing, particularly with the F-100, I don't think you need Sketchup!

Some more details and the wing build (next post). I wanted to model the motor mount for these E-flite motors so I took a picture and a couple of measurements, imported the picture into Sketchup and drew the part. Then I could scale it to the measurements taken so that it was represented accurately in the model.

DamoRC

 

[DamoRC]

Wing Build

Sometimes I have trouble modelling the wing accurately in sketchup in that the model I make does not always fold to the same foil shape. For this wing, I am doing it a little differently by making the wing first, then taking a picture of the airfoil and using that picture imported into Sketchup to model. I am only really interested in making sure that the wing cut-outs are pretty accurate.

The wing is pretty straightforward, a basic FT Style flat-bottomed wing with a little twist (literally).

The wing panels cut and assembled. Upper and lower panels cannot be cut from one sheet so they have to be cut separately and taped together.

A pocket is removed so that a wood spar can be added for strength. The wing will be thin (spar is 1/2 inch wide single layer of foam on the flat) so this additional reinforcement is needed. There will be another "dihedral spar" between the two halves to add strength to the join. A pic of the pocket for this is below.

Pretty standard pocket and install for the aileron servo. Because the wing is thin a pocket is also needed in the upper wing surface. By outlining the servo with red sharpie and test folding the wing, the impression of the servo is transferred to the upper surface so that the pocket can be positioned accurately.

An extra, short, pocket is added to the upper surface to align with the lower dihedral spar pocket. This allows for a reasonably broad spar to be installed for more strength.

The wing is glued and folded in typical FT fashion (bottom panel on the table, glue the LE, spar, and TE, fold over and hold down until glue sets) except the TE is not glued down at the wing tip where the aileron cut out is placed. Instead, once the rest of the wing is glued and folded the wing is turned upside down and then the TE at the tips is glued by pushing the lower surface down onto the upper surface. This adds a slight twist to the wing at the tips. I have no idea if this will work or not.

The dihedral spar is cut from a paint stir stick, test fit into the pockets, and ultimately glued in when the final assembly of the wing is performed. Per the model in Sketchup, if one wing half lies flat on the bench, the lower surface tip should be 4 inches above the bench. I was pretty close to this when the wing was finished.

Tomorrow I will model the wing back into Sketchup so that the wing cut out can be drawn in. Then I hope to build the tailfeathers before tackling the fuse and nacelles.

DamoRC

 

[Jaxx]

Thanks for the details about the design techniques. I'm taking it all in. Hope to apply what I'm learning here in the near future. The completed wing looks great!

 

[jpot1]

Looking great! I take the same approach and create the airfoil, take a picture, import into Inkscape and resize. To be safe I then test print a section of the plans and cut just the wing cutout and test fit. This helps eliminate problems if the picture you import is slightly angled.

 

[Namactual]

That wing looks great Damo, I really like how thin it is.

I hear ya on the actual wing folds not quite matching what you draw in the computer. I had a little better luck measuring the radius of some foam test folds and using that data to dictate the the material length between the top and bottom plates mathematically. It's still not perfect, but much closer. One of these days I am going to try sketch up to unwrap models. Right now I am unwrapping all of my models by hand.

The little bit of washout at the tip definitely can't hurt. Every little bit helps.

Great work as always.