Pondering Peanut Power and Propellers

I’ve been building Rubber-Powered Free Flight Peanuts for just about 40 years.  I imagine that I have built 50 or more Peanuts.  And yet, I still am looking for better performance.  Where can we find those last little bits of performance?

There are a couple of obvious considerations:  subject matter, stability designed in, straight building, and weight are some of the most obvious.  For several years, I’ve been predicting basic performance or a potential subject by measuring the wing area and guessing at what weight I can build the model.  By dividing the weight by the wing area, we can come up with Wing Loading.  Having kept track of all of my models for an extended period, I can compare a potential subject to past models and, knowing how past models flew, I can guess that a new model with similar numbers might fly roughly as well as the old model.

Checking my records, I see that my Peanuts have had wing loadings as low as 0.22 g/sq in (Fike, BD-4) and as high as 0.46 g/ sq in (Stuka).  The average of 13 designs that have been built and flown (and have detailed records) is 0.33 g/sq in – that’s a good target for those of you who are looking to adopt this sort of tool.  But I have come to believe that this is just the start – there are other considerations – such as your power pack; the prop and rubber combo.

Everyone knows that the prop and rubber combo is important.  In fact, you can also add that to your “predictor” tool – just keep track of what all of your old models have used and when you get a new model and calculate the Wing Loading, you can start with a similar prop and rubber combo.  Everyone should do this – and they probably do, even if is inst as formal as keeping a spreadsheet.  But…there’s more…

When I first started building, I used plastic props, as everyone does.  In fact, one of my first Peanut designs that I did in the late 80s shows that I used the Guillows prop (now hated by me).  But my go-to props were the Peck props – 5″ and 6″ grays that were – and still are –  the standard for so many.

As I started to look for better performance and build lighter, I started using wood/carved props.  These were designed from formulae found in such places as Don Ross’ book and elsewhere.

Both the Maule and the Stallion were built very light (for me) and flew away.  But I still used mostly plastic props until maybe 8 years ago.  I started using Superior Props (6″ is the smallest we make) and stacked props.  But the problem was – I would almost always get beat at a larger contest.  I might be able to get 60 seconds or so.  And I had some Peanuts that I just could not get to fly, even given relatively low Wing Loading and lots of rubber.

Last year, as I got beat again in Peanut by Pat Murray, I asked him about his Peanut Fairchild.  It would zoom up and get great long flights while I was still stuck around 45-60 seconds.  He told me he uses a 4″ plastic prop.  I was amazed – that is a tiny prop.  But it got the wheels turning.

In the meantime, I had been working with Archie Adamisin, helping him sort out the 3D printing of propellers, mostly for larger and sport models (Archie sells some great P-30 props, possibly game-changers).  With his help, I printed a 4-blade prop for my Peanut Corsair – and I started getting some really nice flights with it.  But look at the prop … its not very wide.

I had been trying wood props with a 5″ diameter and up to 1″ blade width.  I got decent (sub-60 second) times, but this “skinny” 4-blader really allowed the model to fly better.  By the way, the model weighs about 10 grams (0.27 g/sq in WL) and uses a loop of 3/32″ rubber.

The next adventure was my Peanut Stallion.  Again, I tried to build as light as I could and it came out to be 7 grams (0.32 WL – its got a really narrow wing).  I had imagined and hoped that I could fly it on a loop of 1/16: rubber. I decided I wanted a scale-looking prop and changed the parameters to print out a narrow-bladed, 4″ diameter, 5″ pitch, 3-bladed prop (and spinner).  While I figured it would work, it did much better than I imagined and I get around 70 seconds indoors with regularity.

Things started to click – or gears started to turn – or something.  In my most recent Peanut build, I projected that I could get similar results with the BD-4 if I could keep it light.  My model again weighed 7 grams, but has a fatter wing so it has a 0.22 WL.  I took the Stallion prop, changed it to 2-blade, increased the diameter to 5″ and the pitch to 6.25″ and kept the blades skinny.  Compare this picture to a Peck 5″ prop.

The result – on a loop of 1/16″ rubber – was 84 seconds indoors.  Astounding for me.

Here is a question – have I (and to a greater extent – we) been over-propping our Peanut models?  We all have been using 5″ diameter plastic props with a fat blade (Peck) because that is what has been available. We also know that a higher pitch than is available on the plastic props is better for duration.  These props that I have printed have narrower blades and a higher pitch than the Peck props, especially for the 1/16″ motors.  The 3/32″ motored models have a wider blade.  I am a convert – I’ll be printing props for my Peanuts from now on.  I feel I have two good sets of data – for 1/16″ powered light-weight Peanuts and for 3/32″- powered middle-weight Peanuts.  But of course, I’ll keep experimenting and adding data points.