Frank's Dumas Bearcat RC conversion- 30" span- Covid build #6

[Frank v B]

Started cutting out my next build.  The Dumas 30" span rubber power to RC electric conversion.  Bought the kit about 8 years ago from Michael Rogo at a TEMAC mini swap meet.  Thank you Michael.
After cutting out the parts, studying the construction and checking some internet projects,  I have made the following decisions:

1) It needs significant strengthening everywhere.  Have come to the conclusion that Dumas rubber models were designed to be very good rubber models (read as "very light").  The construction is light and the wood is almost contest grade...for rubber models.  The three Guillow's models I have built (Arrow, B-25, DC3) prove that they are very strong and were probably marginal rubber flyers.  The overbuilt nature made them excellent RC conversions built as supplied (much harder balsa).  Very little strengthening needed.

2) It must have ailerons. The rudder gives a lot of adverse yaw when flown as a 3 channel. See this web report https://mrm1018.tripod.com/mikesparkslowindoorrcflyers/Bearcat.html

3) It will have retracts.  This is actually a very big model with lots of chord in the wing and depth to the airfoil.  See the GWS retracts in the photo (left side).

4) The inside box frame will be 1/16 balsa and not built-up per plans.

5)  This model will be heavy for a rubber plane but very light for RC.  It will not be overpowered.... or it will be over-powdered on the first landing. 

6) Should I be tempted to add flaps?

Each part has been labelled and each plastic bag contains a sheet's worth of parts.

Frank

[GuyOReilly]

Started cutting out my next build.  The Dumas 30" span rubber power to RC electric conversion.

6) Should I be tempted to add flaps?

Frank

Of course add flaps!! As if you needed to ask. 

[Frank v B]

After careful consideration* of Guy's recommendation this model will have flaps.

F.

* one nano-second!

[Andy Hoffer]

Started cutting out my next build.  The Dumas 30" span rubber power to RC electric conversion.  Bought the kit about 8 years ago from Michael Rogo at a TEMAC mini swap meet.  Thank you Michael.
After cutting out the parts, studying the construction and checking some internet projects,  I have made the following decisions:

1) It needs significant strengthening everywhere.  Have come to the conclusion that Dumas rubber models were designed to be very good rubber models (read as "very light").  The construction is light and the wood is almost contest grade...for rubber models.  The three Guillow's models I have built (Arrow, B-25, DC3) prove that they are very strong and were probably marginal rubber flyers.  The overbuilt nature made them excellent RC conversions built as supplied (much harder balsa).  Very little strengthening needed.

2) It must have ailerons. The rudder gives a lot of adverse yaw when flown as a 3 channel. See this web report https://mrm1018.tripod.com/mikesparkslowindoorrcflyers/Bearcat.html

3) It will have retracts.  This is actually a very big model with lots of chord in the wing and depth to the airfoil.  See the GWS retracts in the photo (left side).

4) The inside box frame will be 1/16 balsa and not built-up per plans.

5)  This model will be heavy for a rubber plane but very light for RC.  It will not be overpowered.... or it will be over-powdered on the first landing. 

6) Should I be tempted to add flaps?

Each part has been labelled and each plastic bag contains a sheet's worth of parts.

Frank

This is a very interesting post with very thoughtful analysis.  I had always thought of adverse yaw as a phenomenon resulting from using aileron without coordinating rudder (especially on high aspect ratio wings), not from use of rudder without aileron.  The geometry of the rudder versus the roll axis is most intriguing.  A great troubleshooting example.  Thanks for this @Frank v B .

Andy

[Frank v B]

Getting going....

- did some internet searches on flaps (size as a % of span).  Interesting result-  On each side, the flap measures 60% of span and the aileron only 40%, measured as the projection from the fuselage side.  Have drawn them on the plan.
- first change-  The kit expects you to build a rectangular inner frame onto which you glue the formers (4 pieces per former) to make the fuse round.  To heck with that.  Made the box out of 1/16" sheet.  I did the count- the current box uses 5 pieces.  As designed it needs 104 pieces.  I am trying to ride out Covid; not serve a life sentence! 

Transferred it from the plans onto the balsa with the high tech dot/matrix X/Y axis method (plans over the wood and use pin pricks through the paper).    Connected the dots.

Frank

[Frank v B]

The inner fuse frame is finished.

F.

[Andy Hoffer]

Hey @Frank v B

Could you please redo photo #3456 with better focus? I am having trouble reading the print on the package just to the left of your epoxy bottles?

I'm sure it will offer great pedagogical insights into this project.

Thanks so much.

Andy

[Frank v B]

Andy,

No thanks.  Keep trying.


F.

[Frank v B]

Stabilizer/elevator done.  Changed the design significantly to make it stronger.  The wood size was roughly doubled but the spacing was increased.
Once everything is dry, the rounded Bearcat stabilizer shape will be sanded into it.

Yes, the photo shows an oops in the leading edge (center).  When dry, I will cut it across and glue a piece onto the front.

F.

[Frank v B]

Photo 62- Fin/rudder- the wood has again been at least doubled in size.  The original plans use 1/8" square balsa.

Photo 61 shows the de-oopsed stab.  Once dry, it will be cut back to fit properly.

F.

[GuyOReilly]

Good Morning @Frank v B ;
Before @Andy Hoffer mentions anything about pins (one of is specialties) kindly note that in picture #61 the left red pin is quite correct; however, the right pin should be green: the white one should be at the back.

I love following your builds; such great work!
Guy

[Andy Hoffer]

Good Morning @Frank v B ;
Before @Andy Hoffer mentions anything about pins (one of is specialties) kindly note that in picture #61 the left red pin is quite correct; however, the right pin should be green: the white one should be at the back.

I love following your builds; such great work!
Guy

Excellent work @GuyOReilly !  @Frank v B should consider himself blessed.  He cannot help but get this one totally right with his devoted staff of real-time super-diligent auditors on duty.

I will have more to say after I finish transcribing his pin codes to nautical, then to ICAO, then to Dutch, and finally to (sanitized) English.

Andy

[Frank v B]

Guy,

Two white head lamps, red and green pins properly oriented into an arrow and a single white tail light.

Happy now?

Frank

ps: 15 minutes of my life I will never get back.

[Michael]

Nice project.

My opinion: no flaps. It's a small and light plane. The unnecessary weight, complication and installation won't contribute to a better flying experience.

Also, make the landing gear removable (plug-in). You'll probably be belly-landing the plane in the grass.

My rule: KISS.

[Frank v B]

Michael,

Yours is the view of a rational person.

Here is my irrational thought process:
- add flaps?  ... adds 2 Covid-busy days to the project even if I never deploy them.  That is good.
- add retracts- adds 4 days of Covid-busy tinkering.  Take off on wheels and belly land.  This is what Jim Spence taught me when he got tired of re-gluing ripped retracts every landing.  I use this approach with my Superportster all the time now.

The additional weight is easily overpowered with a brushless motor.

Back to building:
- sorted out all the formers.  To get the round shape, each former has 4 parts (top, 2 sides, bottom).  40 pieces in total.  Argh!!
- the formers on the top have been glued onto frame box.  Note that I have glued the lightning hole of the stab mount back inside.  Prefer the strength over the weight saving.


Frank