10% Bonus for two Blade Design

[Tom-TTom2015]

Dear Coaches-

We started to compete in-person for the season now. We noticed that lots of teams are shooting for the 10% bonus, whereas we have not started yet and felt left behind.

As a group, we had a brainstorming. Someone good at math calculated that 4 blades FFM design ( 6 total blades for top and bottom) has almost same blade surface area as the 2 blades J&H 24-25 Hurricane style design. However, 2 Blades seems requiring “high” blade rotation speed to maintain its stability, which in turn consumes lots of rubbers, considering Hurricane already has a much shorter rubber.

Only case is that:

From the NFSS YouTube, that 24-25 Hurricane flew very well in super high ceiling of almost 100 ft. But it’s that descending height gave it almost 2 minutes descending time.

So, our question is that what’s the advantage of the 2 Blades design against 4 Blades?

Many thanks,

[coachchuckaahs]

The main advantage is that it has a 10% bonus!

The bonus was put in place presumably to reduce the complexity as some teams had difficulty in building the FFM 4-blade rotor. However, the FFM kit still features a 4-blade rotor.

Most high endurance AMA helis (super lightweight) have two blades, and fly fine. IMHO, there is not an inherent stability issue with 2-blade helis. But, Dave at FFM has found a stable solution with four top blades, and stuck with it this year.

Both FFM and J&H kits have changed considerably from 2024-25 season, so comparisons to the 2024 kit are not current.

The two bladed rotor is simpler to build, and easier to hit the minimum mass goal. The 4-blade rotor this year is a bit larger, and therefore heavier, and many teams are struggling to hit min mass. Since heli's are highly dependent on mass, this is an important aspect. Yes, the FFM kit can be built to mass, but it requires careful application of glue to well-fitted accurate joints. This raises teh difficulty, but if you meet min mass, the FFM kit should be very competitive.

The J&H offering is changed considerably this year, and initial reports are very favorable.

I have not built either kit this year. My team built FFM last year, but chose a custom build this year. While our initial flights had some stability issues, teh climb was very straight. During letdown I believe the rotors were stalling at lower rubber power. We improved with wider rubber, and with reduced pitch angle on our lower rotor. We have NOT optimized yet, but the results have been promising. It still will fall off late in descent when the blades stall. We have not yet played with top rotor pitch angle either. Our 2-blade rotors are currently turning slower than last year with the 4-blade system. I do not believe high RPM is needed for stability.

Your motor stick length should NOT impact your rubber sizing much if at all. You should be able to have a rubber loop twice as long as the hook to hook length without much issue if wound carefully. This should likely be more than optimal rubber mass even on the shorter MS.

In my opinion, the bonus is a win win. It pushes people to try a less complicated layout, and if the pitch/diameter/rubber space is properly explored it appears to be a flyable design. Unlike some bonuses in the past, which encouraged a more risky design, I believe there is a 2-blade solution that will not give up performance but will gain you the bonus.

I would say do not be afraid to try the two-bladed designs, whether from a kit or from scratch, but be ready to explore a range of tip pitch angle adjustments as well as rubber widths to avoid stalling on letdown.

Coach Chuck