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Old Aug 06, 2008, 01:06 AM
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Popular Science Hydrofoil

I just wondered if anyone has ever tried to build the old Popular Science Hydrofoil model. I found the reprint on the web again the other day. I remember wanting to build it when I was a kid (I am dating myself here a bit).

Popular Science Hydroplane (Hydrofoil)

I am really an airplane guy, but I plan to give it a shot - go electric - and make it an RC model. Maybe something like a Eflite Park 370 motor, 3s 900mah lipo, 10amp ESC. All just a theory yet, but I have started cutting some balsa.

Just looking for any hints and tips - like how I am going to make all that balsa semi waterproof.

Thanks in advance...
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Old Aug 06, 2008, 01:52 AM
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Auckland. New Zealand
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Quote:
Originally Posted by cyberhoops
All just a theory yet, but I have started cutting some balsa.
That is how most of my builds happened.
An idea............a sketch.......bit of Q&A on the forums......Build !!
Run.....observe.......alter.......run......SMILE !!!
I currently have a home design scratch build Cat, Hydro, Rigger and a Canard.
All are 05 electric on 6 cells and honk on the water.

I am very keen to add a foil to the fleet however it will be prop drive in the water and needs to be quick.

This forum seems to have some people who know a LOT about foils so I will begin a foil design discussion soon.
Quote:
Originally Posted by cyberhoops
Just looking for any hints and tips - like how I am going to make all that balsa semi waterproof.

Thanks in advance...
For the balsa I use liberal amounts of varnish. Allan over at Astec (U.K) agrees that it is a good low cost build method.
I love it !
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Old Aug 06, 2008, 01:52 AM
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PDX, OR
Joined Dec 2002
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A several coats of sanding sealer, or satin finish laquer will work.
Post your progress here as you build.

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Old Aug 06, 2008, 11:56 AM
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785boats's Avatar
Brisbane- Australia
Joined Jul 2007
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That appeals to my love of experimental vintage. It's on my list of things to do.
Regards.
Paul.
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Old Aug 06, 2008, 01:26 PM
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Old Aug 07, 2008, 10:54 AM
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Cutting some part

Well, I have started cutting parts for the hydrofoil, and I have already started making minor changes to the design.

For the "floats" on the end of the main spar I had some 1/2 x 3/4 leading edge balsa, so two pieces of that glued together and rounded make up the floats. That makes them 1/2 x 1 1/2 inch floats instead of 3/4" diameter. Minor change. For the main spar I used a 1 inch wide strip of balsa instead of the 1 1/4 inch balsa from the plans. I am going to reinforce it slightly with a carbon fiber tube which really was not an option in 1968.

I also cut the main cabin to have a flat stern intead of rounded. It will be slightly easier to mount pushrod exits on the flat surface. The other big change to the cabin is that I will need a hatch to access the radio equipment.

The main spar is also supposed to be at a 3 degree angle of attack. If my math is correct then that is {1/8 inch offset over 1 inch} - 1/16 inch over 1 inch to get between 3 and 4 degrees. I made a small 1 inch wide shim from 1/8" inch balsa that tapers down to get the right angle of incidence. (lots of sanding) That will be glued to the floor of the cabin under the main spar. Then I removed the 1/8" shim and replaced it with a 1/16" shim because the math was wrong as pointed out by Andy in a later post.

For steerage, the current plan is to add a water rudder on the back of the stern foil spar. Since the water line looks to be 3" below the spar I found a suitable rudder at Tower Hobbies - the "AquaCraft Rudder Assembly" for the "Nitro Hammer". I will just need to modify the back of the spar to allow for a mounting bracket.

My current motor theory is the Eflite Park 250 motor spinning a 5x4.5 prop. That will run on 2 Lipo cells with about a 7 amp max current draw and 50 watts of power. Overall weight of the motor, speed controller and battery will be about 2.3 ounces. Since the original design could be powered by an .020 to .049 motor - that should be plenty of power without much extra weight over the original .049 motor and fuel.

I like it so far.
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Old Aug 07, 2008, 04:24 PM
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More little tweaks

It seems that the original plans have the stern foil mounted to the side the strut, rather than centered. Another little tweak here. Since I want a water rudder tucked in behind the stern foils I am going to change things to center mount the rear foil assembly. I am going to shorten the rear spar 3 inches, the length of the rear foil, and sandwich the rear foil between two additional pieces for support.

It looks pretty beefy, but I need some added width in back to mount the rudder bracket anyway, so it seems like a decent plan.
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Old Aug 08, 2008, 12:09 AM
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More of the rear foil assembly

Here is the rough sanded rear foil assembly. I added two small carbon fiber braces in recessed slots on either side of the vertical fin so that they are flush to the surface. I thought the fin had too much wiggle and I think the carbon fiber is an improvement. (But then I tend to overdesign everything.)

The spar going back is light plywood.

There is also a 3/4 inch wide expoy and thin cloth 'V" brace on the bottom side of the V foils for support. I just don't like those flat strip joints.

The airfoil shapes have been rough sanded, but I won't try to complete that until I get the trim tabs installed. I guess they will be some kind of thin aluminum strips, and not tin can material as stated in the plans.
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Old Aug 08, 2008, 07:40 AM
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Illinois
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Quote:
Originally Posted by cyberhoops
The main spar is also supposed to be at a 3 degree angle of attack. If my math is correct then that is 1/8 inch offset over 1 inch to get between 3 and 4 degrees.
1 degree is 1/16" over 3 9/16" (approx). 3 degress would be 3/16" over 3 9/16", or 1/16" over about 1.2". I think you have a little too much angle there.

The way to check this is like this:

The circumference of a circle is Pi * D. Start with a circle 360 1/16" steps (each being a degree of arc). That is, the circumference is 22.5" (360 / 16).

Divide that by Pi: 22.5" / 3.14 = 7.17" diameter. Divide that by 2 to get the radius = 3.58" (3 9/16 = 3.5625 - close enough!).

Please correct me if I screwed up!

Quote:
... 50 watts of power. ... that should be plenty of power without much extra weight over the original .049 motor and fuel.
An .049 in that era was more than 50W but less than 100W power. You might want to upgrade the motor slightly.

I ran a 27" hydrofoil (different design - basically a rigger with stainless skid fins on both sponsons) on 250W (6 cells NiMH in a racing configuration). This boat was noticeably faster than when I ran it as a pure rigger - and as a rigger it gave me a season District championship.

Andy
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Old Aug 08, 2008, 11:28 AM
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Angles and power

Let me think about this:

Quote:
1 degree is 1/16" over 3 9/16" (approx). 3 degress would be 3/16" over 3 9/16", or 1/16" over about 1.2". I think you have a little too much angle there
I think this method breaks down. If we test the approach above using a 45 degree angle would be 45/16 over 3 9/16. That would be 2.8 inches over 3.56 inches. However, 45 degress should have equal lengths and be 3.56 inches over 3.56 inches.

EDIT - EDIT - EDIT

Andy - All that being said you are still correct. The offset is wrong and the angle is too big.

EDIT - EDIT - EDIT

I used the theory where the opposite side of an angle is the radius * sin(x). For a shallow angle like that my 1 inch wedge has a radius that is still really close to 1" - so

sin(3 degrees) = 0.052
sin(4 degrees) = 0.069

Since 1/8" = 0.0625 - it is actually much closer to 4 degrees but in a decent range.

WRONG - 1/8 = 0.125. 1/16 = 0.0625. Using 1/8 gives an angle between 7 and 8 degrees - not 4.
---

Meanwhile - I didn't realize the 049 had that much kick. I think I will adjust my motor up slightly. I have no idea what kind of speed to expect from this hydrofoil. I think I will be surprised if it hits 10 mph, but you never know.
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Old Aug 08, 2008, 03:43 PM
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Auckland. New Zealand
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Quote:
Originally Posted by AndyKunz
I ran a 27" hydrofoil (different design - basically a rigger with stainless skid fins on both sponsons) on 250W (6 cells NiMH in a racing configuration). This boat was noticeably faster than when I ran it as a pure rigger - and as a rigger it gave me a season District championship.

Andy
Andy that is excactly the sort of information I am after.
Do you have any pics of this boat of any futher info.
If so can you put it on my Foil discussion thread, http://www.rcgroups.com/forums/showthread.php?t=904901 or PM me.
I don't want to hijack this thread because I am following it keenly.

Cheers
Simon.
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Old Aug 11, 2008, 07:22 AM
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Illinois
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I still have the boat (or I will, when the moving van arrives in another 2 weeks), but it's a pure rigger again and all the holes, etc. are filled.

Andy
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Old Aug 11, 2008, 07:25 AM
AndyKunz's Avatar
Illinois
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Quote:
Originally Posted by cyberhoops
I think this method breaks down. If we test the approach above using a 45 degree angle would be 45/16 over 3 9/16
That's why I used the word "approx" - it's only good for small angles, and it's only an approximation even there. Something you can use to get you in the ballpark.

I only ever use it to get a quick sanity check on a setup.

Andy
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Old Aug 11, 2008, 10:58 AM
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Well, now that Andy had checked my sanity and adjusted my angle...

My next step was getting a start on the cabin and the motor mount. I cut way back on the size of the motor mount from the plans. A small electric motor just does not need the massive mounting structure as described in the plans. The plans called for a plywood motor mount that was some 2 1/2 inches deep. I ended up with a 1 inch wide plywood mount with two supports left and right that is wrapped in a bit of balsa.

Here are a couple pictures of the work in progress. The center section between the motor mount and the front section will eventually become the hatch area.

I think this current design is heading toward being a bit nose heavy since I will not have the same weight of an 049 engine to add behind the balance point. It looks like the radio gear will have to be located as far aft as possible.
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Old Aug 11, 2008, 12:38 PM
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Seattle
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Framed up

Here is an image with the main body framed up and ready for the hatch section. The body has a very "vintage" look, which was the intent here. If I make another version I think I will redesign the body shape to be more "modern". { Which translates to more complex curves and harder to build. }

I also picked up some 0.016 aluminum sheet stock at the local hobby shop. I think that will work for the trim tabs on the stern foils.

Jim
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