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5' Rutan Pond Racer

bpw823

Junior Mastermind
#1
Howdy everyone!

It's been a while since I've been able to post any new projects, It's been a hectic time transitioning from a senior in high school to a college freshman, but I've settled into a groove and have found my outlet back into the hobby! I'm now an Aerospace Engineering major at Texas A&M and I joined the Society of Flight Test Engineers, a group dedicated to the design, building, and flying of model airplanes, helicopters, and drones. As such a casual place to unwind after a week of classes, I jumped straight into a project I've wanted to tackle for a while now.

Pond Racer.jpg

I've always held admiration for Burt Rutan and his crazy ideas, and to reflect his ambitious goals I've decided to design and construct a 5' foam-board model of the Pond Racer and see how fast I can make it go. I'll be running two Cobra c2814 motors off of their own dedicated 4s batteries and swinging two 8x4 props in the hopes of achieving the most velocity out of my setup. I'll set it up for differential thrust in order to simplify my streamlined servo layout. I already have a good amount of the plans laid out and I'm hoping to have it built and ready to fly for FliteFest Texas (which I am planning to attend!)

I'll keep posting as progress comes along. My priority IS class so if exams are coming around or anything else, I "probably" won't be focusing so much on the airplane for the immediate time.
 

willsonman

Builder Extraordinare
Mentor
#5
This one was on my list at one point but after doing some math, I decided against it. The wing loading is the biggest challenge but at your larger scale, it seems much more practical.

One thing that stands out to me is the use of 8x4 props. While you would be on the line here in terms of power output, your prop pitch would really be hampering your top speed. Even swapping out to a 7x6 would up your top speed by ~30MPH. BUT, the 8x4 may be a good place to start. All this data comes from eCalc's propCalc online application.

Very much looking forward to whatever you come up with as I love this subject and I always love seeing how people would approach a model that I too want to build.
 

Mid7night

Jetman
Mentor
#7
Guys, we've DONE the math, you don't need silly things like "WING AREA" as long as your props are beating the air into submission around you! XD

Good luck with the Pond Racer! Looks very slick!

For "maximum velocity" however, you will want to look into higher-pitch props, maybe even something over-square. I had very good results on my SK3 using an 8x10.
 

bpw823

Junior Mastermind
#9
Thanks for the feedback! The compatibility of props and motors is something I definitely need to be enlightened on. I'll start with the 8x4 and work my way up to higher pitch props and "expand the envelope" so to speak. Wing area is definitely a concern so I'll see what it takes to beef it up a little.
 

willsonman

Builder Extraordinare
Mentor
#11
Guys, we've DONE the math, you don't need silly things like "WING AREA" as long as your props are beating the air into submission around you! XD

Good luck with the Pond Racer! Looks very slick!

For "maximum velocity" however, you will want to look into higher-pitch props, maybe even something over-square. I had very good results on my SK3 using an 8x10.
Not all of us are quite capable of flying literal "Bricks" for aircraft. :p
 
#14
With so much of the propeller being covered up by the spinner couldn’t he go with a larger diameter propeller? Some sort of mysterious “mathematical formula?” 😃 I doubt it would be 2” spinner allows 2” larger propeller. Would it be like 2” spinner allows 1/2” larger propeller without stressing the motor? 🤔
 

willsonman

Builder Extraordinare
Mentor
#15
Yeah, don't fret too much about having too much prop exposed. A great example in my Gee Bee Z. It has a frontcowl that is 8" in diameter but I swinging an 11" prop. Only 1.5" exposed on each prop and she still goes like mad. The majority of the work of a propeller is done at the tips so generating enough thrust should not be a problem.

A larger diameter would still require a higher pitch. If the kV of the motor is constant then the only way to increase speed is to increase the pitch of the prop. The diameter will do nothing for speed. So if you are maxed out in current draw but you need more speed... increase the pitch and decrease the diameter. When I ran the numbers I was surprised that the 7x6 props would actually draw less current than the 8x4. Math can really screw with your brain sometimes.
 

Tench745

Active member
#16
There's an equation for the load on a prop, which is useful if you're comparing two different props and don't want to break out a whole calculator.
The equation is as follows:
Load= (D^3)*P*sqrt(N-1)
Where: D= Diameter, P= Pitch, and N= # of blades

So for a 7x6 you'd have
Load=(7^3)*6*sqrt(2-1)
Load=(343)*6*sqrt(1)
Load=343*6*1
Load=2058

That 8x4 would be
Load=(8^3)*4*sqrt(2-1)
Load=2048

Looking at the two numbers you can see that they will be very similar in the power required to spin them. According to this equation the 8x4 would require slightly less current, contrary to what Wilsonman's calc says, so it's not perfect. I believe the calc uses real data collected from different brands of prop, whereas the equation above is just a general formula. I find it useful when figuring out what prop to use when upsizing, downsizing, or going to a multi-blade.
 

rockyboy

Skill Collector
Mentor
#17
Very neat! Is the load value in watts?

I've always done the thumb sized estimation of 1 inch diameter can be traded for 1 inch of pitch or adding 1 more blade (i.e. 2 to 3 blade) and while I haven't burned anything up because of this, I'm sure I haven't been getting all the performance opportunity out of my gear either.

Thanks!
 

Tench745

Active member
#18
I actually have no idea what units the value is in. As far as I know it's an arbitrary value, since motor speed can change the actual amp draw. If I recall correctly the formula was created for combustion engines, so might not correlate directly to anything in electrics.
 

willsonman

Builder Extraordinare
Mentor
#19
eCalc is based on real-world data, which is why you pay for the full-access version. Someone has to get that data. I like general calculations like this but I've also been known to be a bit of a perfectionist bordering on clinical OCD when it comes to modeling. :p
 

bpw823

Junior Mastermind
#20
eCalc is based on real-world data, which is why you pay for the full-access version. Someone has to get that data. I like general calculations like this but I've also been known to be a bit of a perfectionist bordering on clinical OCD when it comes to modeling. :p
Guess I know who to shout out to as I get closer to the final assembly ;)