Duration

[mrsteven]

Traditional built-up props use elliptical blades since they are much more efficient as far as drag goes. The only reason (that I can think of) why all SO helicopters flare at the tips is because it is substantially easier to build them that way (with just two straight spars instead of forming the whole thing on a carved prop block).

yeah thats true. I see that youre the event superviser for helicopters at nationals. Any advice on how to improve our helis?

[chalker7]

We are talking about two different drag forces here. You are thinking of the drag force vector in the vertical direction, while I'm talking about the drag force vectors perpendicular to the rotor's motion.
While drag is good for parachutes to prevent things from falling more rapidly, it is never good on propellers/rotors. It is a waste of energy preventing the rotors from spinning freely, thereby reducing the speed of the rotor, thereby reducing the lift off of them. Less lift is always a bad thing. The elliptical shape reduces this type of drag (which I suppose you could call rotational drag).
The vertical drag comes primarily from the surface area of the rotors, which would not be too difficult to match (given a few calculations) between an elliptical and wedge shaped rotor.

[chalker7]

Any advice on how to improve our helis?

There is no hidden secret (that I'm aware of). Just build a lot of them until you consistently get strong and cleanly built helicopters at 4.0 grams. Then keep flying them a lot, test out different rubber motor sizes, always use a torque meter and keep lots of notes. This event is more about the testing and understanding of how your helicopter flies than the construction, just as Wright Stuff was.

[new horizon]

Then would this L/D ratio just be your pitch? The higher pitch, the more drag, and therefore the more torque needed to fly. So perhaps this drag is unnecessary. I dunno, the kits have been rather successful in our state, I think the top 5 or so (except your team, sj) used that kit in competition. Probably because it's easy to build so a lot of experimentation could be done. (though I would think that your flat design is easier to build than the helical design that they give)

Oh and chalker, I was merely making an analogy toward parachutes and their intended purpose. I understand how less lift would be bad, but I was just thinking the intended design was to conserve winds as it would take more time for one wind to complete if there was more rotational drag, so would there be an advantage to this design if you take a longer time to run out of winds?

[bwy]

We are talking about two different drag forces here. You are thinking of the drag force vector in the vertical direction, while I'm talking about the drag force vectors perpendicular to the rotor's motion.
While drag is good for parachutes to prevent things from falling more rapidly, it is never good on propellers/rotors. It is a waste of energy preventing the rotors from spinning freely, thereby reducing the speed of the rotor, thereby reducing the lift off of them. Less lift is always a bad thing. The elliptical shape reduces this type of drag (which I suppose you could call rotational drag).
The vertical drag comes primarily from the surface area of the rotors, which would not be too difficult to match (given a few calculations) between an elliptical and wedge shaped rotor.

I'm just wondering, if you reduced the friction, wouldn't the rotors spin faster? I thought slower rotors were better (as long as they can still keep the helicopter flying, of course) since they unwind the rubber band slower and thus the helicopter flies longer. That was what I saw at states, at least.

[illusionist]

Hey sj, if you don't mind, could you share your vertical spacing and tip-to-tip distances (for the pitch)?
If not, I completely understand. You've done the work and clearly achieved results. I've tried many pitches to get it to work on 1/8, and I have one last pitch left that I want to try.

[mrsteven]

Hey sj, if you don't mind, could you share your vertical spacing and tip-to-tip distances (for the pitch)?
If not, I completely understand. You've done the work and clearly achieved results. I've tried many pitches to get it to work on 1/8, and I have one last pitch left that I want to try.

^^ Not all that aware of alot of the helicopter lingo, but what is 'vertical spacing'

[new horizon]

Most likely the distance from the most top point to the most bottom point on a rotor.

[chalker7]

I'm going to address several issues at once, so I apologize if this jumps around.

Conserving winds is good, but wasting energy is bad.

Think of it like this, (this might be a strange analogy) the rubber band is your gas tank. You only have so much gas (or rubber mass) to provide so much energy. If you are driving down the highway, you'll go further on a single tank if you aren't dragging a giant parachute behind your car. The parachute is like the drag on the rotors.

If you have more efficient rotors that turn more easily, you will be able to fly on thinner motors. Thinner motors can get more turns on them and will provide less torque (thus spin the rotors more slowly). But, since the rotors require less torque to turn, the helicopter will still be able to fly and since you will have more turns, it will be aloft longer. That's the ideal scenario anyways, it takes a lot of experimentation to get that far.

The second part of that is the whole pitch/rubber matching thing we talk about sometimes but I don't know if it's ever been discussed in detail.

Higher pitch does have more drag, but that is a separate issue from the shape question. All of these characteristics (shape, speed, pitch, lift, drag, torque) are individual components that affect one another in a complicated relationship. Adjusting one will likely affect many of the others, but there is no one solution to everything. This is the part of testing and flying that is so critical and is what will separate the very best teams at many states and nationals from the ones who just bought one of the competitive kits.

[chalker7]

Also, the L/D ratio (lift over drag) is not simply the pitch. The pitch is how far the rotor would travel in one rotation if cutting through a solid material (like a screw). It is a distance measurement based on geometry, I imagine most Science Olympiad helicopters have a pitch somewhere between 20 and 40 cm, probably mostly on the low end of that.

The L/D ratio is a measurement of how much lift the helicopter is providing vs. how much drag is holding is back from flying. I don't know much about professional helicopter design, so I don't know if every drag component is generally added into that or if they even calculate such a ratio for whole helicopters. That ratio is most commonly used (as far as I'm aware) for individual airfoils.