Logo
Builders' Plan Gallery  |  Hip Pocket Web Site  |  Contact Forum Admin  |  Contact Global Moderator
October 22, 2019, 06:47:29 AM *
Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with email, password and session length
 
Home Help Search Login Register
Pages: [1]   Go Down
Print
Author Topic: Carved balsa props  (Read 387 times)
0 Members and 1 Guest are viewing this topic.
ironchefmpls
Copper Member
**

Kudos: 0
Offline Offline

United States United States

Posts: 5

Topic starter


Ignore
« on: October 02, 2019, 10:23:07 AM »

Hi all,

I am getting close to finishing my first carved balsa prop. It is an 8.5" prop for a FAC Moth from Volare. I have a couple questions for the community:
  • Have I sanded the blades thin enough?
  • What is a good way to construct the hub bearing? It looks like I could glue a piece of brass tubing in the hole and drill or ream the ID to fit the 0.047" shaft
Any advice would be appreciated. Thanks!

-Jeremy
Attached files Thumbnail(s):
Carved balsa props
Carved balsa props
Carved balsa props
Logged
Viperkite
Bronze Member
***

Kudos: 0
Online Online

United Kingdom United Kingdom

Posts: 62



Ignore
« Reply #1 on: October 02, 2019, 10:57:18 AM »

As another novice carver (I have only done 3), I would not take any more off until it has been tried. Then if it works ok, leave it. I just use a bit of brass tube on mine, but I also fit a freewheel. Mine are only for sport models so performance isn't critical.
Logged
ironmike
Platinum Member
******

Kudos: 87
Offline Offline

United States United States

Posts: 1,131



Ignore
« Reply #2 on: October 02, 2019, 01:30:27 PM »

I would use an oversize (5/32)brass tube piece
with a filed ramp cam in the front end.
Bush up the inside to fit your prop shaft.
Logged
strat-o
Gold Member
*****

Kudos: 7
Offline Offline

United States United States

Posts: 385



Ignore
« Reply #3 on: October 02, 2019, 02:27:28 PM »

This prop doesn't look to me like it has the right proportions to be an effective prop for rubber power.  Typically props for rubber have a lower aspect ratio with maybe 2-3x the blade area (for the given diameter) of what yours is.  The key to getting the shape right is to start with a balsa wood blank that is often shown on the plans, especially with older models.  The initial blank, when carved properly will dictate the pitch. When you carve to the edges, it will give you the correct area and pitch.

Marlin
Logged
billdennis747
Titanium Member
*******

Kudos: 51
Offline Offline

United Kingdom United Kingdom

Posts: 3,680



Ignore
« Reply #4 on: October 02, 2019, 02:41:09 PM »

The last photo looks very thick. I would thin it to 3/32 at max camber
Logged
Red Buzzard
Silver Member
****

Kudos: 4
Offline Offline

United States United States

Posts: 124



Ignore
« Reply #5 on: October 03, 2019, 12:30:22 PM »

What Bill says, then down to about .040" at the tips. The edges will get nicked-up so, once you're done with carving/sanding run a bead of CA around the perimeter, then smooth that off with 320.  As many will say you can make a freewheeler almost bomb proof by rounding the top edge of the nose block so it falls out with a blade strike.

Besides, now that you have done one, another is fairly easy if you decide to change the shape.

RB
Logged
ZK-AUD
Platinum Member
******

Kudos: 50
Offline Offline

New Zealand New Zealand

Posts: 1,143



Ignore
« Reply #6 on: October 03, 2019, 09:51:11 PM »

Hi there ironchef  

I've carved a few wooden props now and I'll share a couple of things that have worked for me.  

First of all acknowledging the other great advice you have been given.

Thinner is better but of course that is a balance against the strength of the wood.  The harder it is the thinner you can go without being too concerned about breakages.  I use rock hard block that you can't depress a thumbnail into - that's  my gauge!!   https://www.hippocketaeronautics.com/hpa_forum/index.php?topic=23478.0  There are some pictures on my FROG Jupiter thread in reply #13.  You can tell just looking at that wood that it's hard and stringy.  That prop is 13" but thinned to around 1/16 - 3/32,  and even less at the tips.  It's really powerful and efficient.

The other thing that occurred to me is that the propshaft hole should be drilled at the beginning before you start carving the blank otherwise it can be hard to get her square and that just ruins your life

There'a another thread you may be interested in - the guy (not me) is carving a prop for a Ted Evans Jaguar and goes into a full tutorial on the prop from reply#72  https://www.hippocketaeronautics.com/hpa_forum/index.php?topic=4228.100

I agree with Strat-O's comment that the design of that prop is not what we normally expect, for rubber  though it could just be the way the photo was taken.

If you dig around on the forum there is bound to be a table showing the usual dimensions and proportions for  fine, medium and coarse pitched props in varying diameters - Also plenty of great advice in Don Ross' book on building and flying rubber powered model aeroplanes

Cheers, Mike
Logged
ironchefmpls
Copper Member
**

Kudos: 0
Offline Offline

United States United States

Posts: 5

Topic starter


Ignore
« Reply #7 on: October 04, 2019, 10:20:55 AM »

Thanks everyone. I sanded it down considerably from when I took the pictures. For the shape, I attempted to copy the shape of the Czech P-30 prop, but the block didn't suit the shape. So it wound up kind of weird and narrow. I'm sure it will make the plane go, but I have another block on order.
Attached files Thumbnail(s):
Re: Carved balsa props
Logged
Red Buzzard
Silver Member
****

Kudos: 4
Offline Offline

United States United States

Posts: 124



Ignore
« Reply #8 on: October 04, 2019, 01:21:59 PM »

Hi Ironchef,

Mike's advice as well as his Jupiter prop are great. When looking at sample blocks from other designs of similar wingspan, beware that freewheelers generally have a coarser pitch to lower the drag when in glide mode. I suggest you then only consider freewheelers. There are a bunch of them out there as you might imagine.

Also standard blocks are always carved to the edges of the rear surface first before shaping the final shape. If you shape them first it will upset the helix (pitch distribution) of the original design.

Later designs for folders don't always follow this rule.

Bill

Logged
Pages: [1]   Go Up
Print
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.21 | SMF © 2015, Simple Machines Valid XHTML 1.0! Valid CSS!