Continuing of Discord Thread on Heli Design

[coachchuckaahs]

Adith:

On a two-bladed large rotor, we used all carbon and it came out plenty light. Coach Brian has used balsa for greater stiffness. My team is going for more flare to control early rotor speed. Initially we used 0.016x0.039" carbon for the TE (very stiff), and 0.020" rod for the LE (with elbow). We found this was way too flexible. We increased stiffness of the center portion of the LE by tacking on an additional 0.020 rod, and the flexibility is close to what we were looking for.

If you are building 2-bladed rotors, you can use various carbon to get stiffness desired, as weight should not be an issue. Ours built at 2.65g, though the strengthening brought it close to 3g. If you have a 4-bladed rotor, then weight is more of an issue. Balsa has better stiffness to weight but may not be as durable, and must be carefully selected. I like carbon due to uniformity. Coach Brian likes balsa for the stiffness. Some data shows very stiff rotors may be better, but we are experimenting with more flex just to see.

For ribs, they are not under a lot of load. We used 1/16" 6# balsa wet formed to shape, then sliced to 0.050 width. There is not a lot of wood involved, so selection is not critical.

Se Coach Brian's prior posts (maybe on Discord? I do not recall) for his notes on spar material. Seems like he was around 10# wood for carbon-free spars to match weight on FFM kit.

Coach Chuck

[dbailey]

Coach Chuck,

Two questions...
1) what do you mean by "flare"?
2) do you have a source for the CF pieces? I can find various diameters of rod easily (on Amazon), but what exactly is the .016x.039" (.4x1mm?) format you mention?

DB

[bjt4888]

@coachchuckaahs and @bjt4888 My Aeromartin jig for my long rotor design has finally finished printing. I plan on building it from purely balsa with no carbon. Do you reccomend that I use carbon on certain parts(like how FFM has bottom rotor carbon but top is all balsa)? I am running a fixed rotor design, so I don't know if this applies. What type balsa do you reccomend I use for the spars, ribs, etc?

Adith,

We determine balsa weight and density for spars and ribs by creating a weight budget for the entire heli. So, if your desired motor stick is 5.5 lb cu ft and is 10.5" long and is 3/16"x1/4", it will weigh about 0.7 grams. Add up weight of all parts and materials like this, including covering, glue, wire hooks, nose bearing, etc. and make the spars of the highest density 1/16"x1/16" square stick possible to achieve a weight of less than 4.0 grams. If you have time, look for a good sheet of 1/16" wood too. Strip spars from a few sheets to find the stiffest for particular density that you can. For ribs, we used again 1/16" square dimension and 6.0 lb cu ft. Covering weight is on Ray Harlan's website and for ultrafilm is about .142 grams for 100 sq in. Spray 3M77 glue, if applied correctly on a large heli adds about .05 -0.1 grams to each rotor (0.5 is good; 0.1 is not so good; use a quick spray).

Here's a balsa density calculator that I created in MS Excel that makes this part weight and wood density selection a little quicker: https://docs.google.com/spreadsheets/d/ ... ue&sd=true

I always intended to get this calculator on the NFFS website; just haven't done it yet. Please use the calculator by downloading it to your phone or computer. Do not use it as is since this link is on my personal Google drive. Only enter data in the green highlighted cells as the "Calculated Density" cell contains a formula that is not protected (you could overwrite it if you enter data here). Use a micrometer to get exact measurements (a lot of sheet wood labeled 1/16" is actually .065"-.067"). So, for example, if your total length of spars for 18" span rotors is approx 72" (calculate the exact measurement for your jig) and the wood density is 10 lb cu ft, the total spar weight is 0.8 grams. If 9.0 lb cu ft, the total spar weight is 0.72 grams, etc.

We did add about 2.5" of .020" carbon rod to the inner portion of the spars of the free spinning rotor to make it easier to handle. This was done after construction with a few dots of thick CA followed with a touch of thin CA.

Coach Brian

[bjt4888]

Coach Chuck,

Two questions...
1) what do you mean by "flare"?
2) do you have a source for the CF pieces? I can find various diameters of rod easily (on Amazon), but what exactly is the .016x.039" (.4x1mm?) format you mention?

DB

DB,

Flare refers to the rotor leading edge bending up a greater amount than the trailing edge during flight as airflow pressures affect the rotor. This effectively increases the rotor angle of attack relative to the airstream and slows the rotor spin rate down. The correct amount of flare and flare under high torque early portion of flight vs flare amount under lower torque later portion of flight is all determined by testing. Too much flare results in the rotor blade stalling leading to poor flight duration.

Good question.

Coach Brian

[coachchuckaahs]

As coach Brian noted, flare is the increase in pitch at high load by selective bending under flight loads. The leading edge is made more flexible than the trailing edge. The stiffness becomes a variable that impacts the amount of flare.

This adds several additional variables, and needs substantial testing to refine. Or initial guess was way off, the flare was too much, and the heli just flipped around instead of flying. We are still flying the same heli, with many patches to increase stiffness in select areas, many adjustments to the static pitch angles, etc. We are almost ready to build our next generation trying to incorporate our findings into the design.

A complete and accurate log is critical to understanding the flare, and to making the best guess at next adjustments, as well as feeding in to the next build.

As far as carbon, the 0.016x0.039" is a standard size carbon strip, used in many airplane designs for leading and trailing edge spars.

My source for all kinds of carbon is CST, at cstsales.com. shipping is not cheap, but they have a wide selection.

Coach Chuck

[Tom-TTom2015]

Dear Coaches-

Saw this note about flare and stiffness. We found it’s an interesting topic.

We learned Helicopter blade could have Coning effects, if the weight, lift and centrifugal force not balanced well.

What’s the difference between flare and Coning?

For Coning, we thought due to conservation of angular momentum, the blade may turns to rotate much faster, just like ice skater spinning. That’s probably not preferred.

Many Thanks

[coachchuckaahs]

Flaring is flexing of the leading edge more than the trailing edge, changing the pitch of the rotor. A poor man's variable pitch. If matched to rubber it can reduce power early flight and conserve winds. With multiple new variables involved, flaring as a positive tool should be attempted by a team willing to test a lot under very controlled conditions, and willing to redesign and build new to adjust stiffness. It is also important that stiffness be balanced blade to blade so that flaring is matched.

Coning both spars flex, making the rotor basket like. This does nothing to control the power. While the diameter changes some, that is small and probably a minor impact. I could see some dihedral-like impact which could improve stability, but also could hit ceiling with a blade. I don't see enough benefit to make this desirable.

Coach Chuck

[NEonGhOsT]

I used 1/16 square balsa abount 7 pounds per cubic feet, for the ribs I cut it out of 1/16 wood with a balsa stripper. I'll start using my laser cutter to cut these ribs soon.

[bjt4888]

I used 1/16 square balsa abount 7 pounds per cubic feet, for the ribs I cut it out of 1/16 wood with a balsa stripper. I'll start using my laser cutter to cut these ribs soon.

7 lb sounds a little light duty. We used 9-10 lb. But we also fixed one rotor directly to the fuselage to save 0.4g thus allowing more weight and strength for the spars.

Good luck with your build!

Coach Brian