Rotors

Locked
XieXie
Member
Member
Posts: 8
Joined: January 15th, 2011, 12:55 pm
Division: C
State: NJ
Has thanked: 0
Been thanked: 0

Re: Rotors

Post by XieXie » January 18th, 2011, 6:35 pm

Less weight is better, but by no means should 0.6 grams keep your helicopter from flying. I've never gotten my helicopter to less than 6 grams (something I should probably start soon :lol: )

gabuilder
Member
Member
Posts: 16
Joined: January 16th, 2011, 9:47 am
Division: C
State: GA
Has thanked: 0
Been thanked: 0

Re: Rotors

Post by gabuilder » January 19th, 2011, 9:25 pm

I've had a problem similar to IronChef.

Basically, my rotors spin quite slowly, too slowly to move. I originally thought it was the friction of my bead "bearing" under the tension of the rubber band, but I realized later that it was spinning VERY quickly when the rotor is not attached (20-30 times per second at least).

I then tested it with old airfoil curved propellers from Wright Stuff. With those, it spun much faster, leading me to think that it is caused by:
a) the shape of my rotors (plain rectangles, without airfoil curves)
b) the size of my rotors (additional air resistance)

However, the small rotors did not generate the necessary lift. I'm not sure if there's something else I'm not considering, but from all that I've tested it seems difficult to balance resistance and lift with my rotors. Any help? My rubber band is strong and wound many, many times (600-700 at least), and my bearing, again, is also fine.

Any input is greatly appreciated.

User avatar
eta150
Member
Member
Posts: 269
Joined: March 11th, 2009, 3:48 pm
Division: Grad
State: PA
Location: Kville
Has thanked: 0
Been thanked: 0

Re: Rotors

Post by eta150 » January 20th, 2011, 4:32 am

gabuilder wrote:I've had a problem similar to IronChef.

Basically, my rotors spin quite slowly, too slowly to move. I originally thought it was the friction of my bead "bearing" under the tension of the rubber band, but I realized later that it was spinning VERY quickly when the rotor is not attached (20-30 times per second at least).

I then tested it with old airfoil curved propellers from Wright Stuff. With those, it spun much faster, leading me to think that it is caused by:
a) the shape of my rotors (plain rectangles, without airfoil curves)
b) the size of my rotors (additional air resistance)

However, the small rotors did not generate the necessary lift. I'm not sure if there's something else I'm not considering, but from all that I've tested it seems difficult to balance resistance and lift with my rotors. Any help? My rubber band is strong and wound many, many times (600-700 at least), and my bearing, again, is also fine.

Any input is greatly appreciated.
What size was the motor? Old Wright Stuff rubber (.08"-.1") probably isn't thick enough, and if you are using it, you should safely be able to get at least 900 turns out of it. Try .125" if you want it to spin faster.
#ACESWILD

User avatar
smartkid222
Exalted Member
Exalted Member
Posts: 774
Joined: June 22nd, 2008, 8:12 am
Division: C
State: NY
Location: Western Long Island
Has thanked: 0
Been thanked: 0

Re: Rotors

Post by smartkid222 » January 20th, 2011, 2:39 pm

Think about the pitch of the rotors.
Also, pictures are always good.
Image 2008 NY BLG Champ
2010 NY Helicopter Champ

gabuilder
Member
Member
Posts: 16
Joined: January 16th, 2011, 9:47 am
Division: C
State: GA
Has thanked: 0
Been thanked: 0

Re: Rotors

Post by gabuilder » January 20th, 2011, 2:51 pm

I have been studying new wing designs, but won't lowering the pitch lower the lift allowed?

Also, I realize that turning the rubber band more will provide a faster spin. However, isn't this a temporary fix, as the extra 300 turns or so (or however many) will soon be used and then no more lift will be provided? For example, if it is not spinning fast enough at 600 winds, but does at 900, then it will only fly from the 900 to 600 turns, and then fall at a slowed pace.

A third question: why does a thicker rubber band provide more lift? Shouldn't all bands, tightened enough, provide similar lifts? The differing "rubber band constants" (not sure what to call it) can be compensated by extra length and more winds, right? Essentially, my logic is that 2 grams of the same rubber should provide the same force, ultimately, regardless of the thickness.

Thanks for the help, though!

User avatar
eta150
Member
Member
Posts: 269
Joined: March 11th, 2009, 3:48 pm
Division: Grad
State: PA
Location: Kville
Has thanked: 0
Been thanked: 0

Re: Rotors

Post by eta150 » January 20th, 2011, 3:01 pm

gabuilder wrote:I have been studying new wing designs, but won't lowering the pitch lower the lift allowed?

Also, I realize that turning the rubber band more will provide a faster spin. However, isn't this a temporary fix, as the extra 300 turns or so (or however many) will soon be used and then no more lift will be provided? For example, if it is not spinning fast enough at 600 winds, but does at 900, then it will only fly from the 900 to 600 turns, and then fall at a slowed pace.

A third question: why does a thicker rubber band provide more lift? Shouldn't all bands, tightened enough, provide similar lifts? The differing "rubber band constants" (not sure what to call it) can be compensated by extra length and more winds, right? Essentially, my logic is that 2 grams of the same rubber should provide the same force, ultimately, regardless of the thickness.

Thanks for the help, though!
First, he meant that you should increase the pitch.

Second, torque out of a rubber band is not constant. There is something called a torque curve. In the curve, the first period of winding provides very little torque, and that is why the helicopter descends. The middle period is flat. In Wright Stuff, and in very good helicopters, this is when they cruise at the ceiling. The last period climbs very sharply, creating enough torque for a climb. With only 600-700 turns, you may not be getting far enough into the last part of the torque curve to get enough power to reach the ceiling. And yes, that's the point, to have it reach the ceiling and then fall slowly.

Third, the thicker the rubber, the higher the torque. While there may be the same amount of energy storage in any 2g rubber band, there will be more torque in a thicker one, meaning there will be a higher output over a shorter period of time.
#ACESWILD

gabuilder
Member
Member
Posts: 16
Joined: January 16th, 2011, 9:47 am
Division: C
State: GA
Has thanked: 0
Been thanked: 0

Re: Rotors

Post by gabuilder » January 20th, 2011, 8:15 pm

The advice on rubber motor torque was exactly what I needed. I had my first flight--a great feeling after so much work! Thanks!

L2
Member
Member
Posts: 1
Joined: November 5th, 2010, 1:02 pm
Division: C
State: TX
Has thanked: 0
Been thanked: 0

Re: Rotors

Post by L2 » January 21st, 2011, 2:08 pm

How do you make and angle the rotor to exact angle, if you are using balsa wood stick and tissue paper? Post picture if possible. What dimesions works best for helicopter? :D

User avatar
aubrey048
Exalted Member
Exalted Member
Posts: 270
Joined: January 8th, 2010, 9:43 am
Division: C
State: AL
Location: Rocket City
Has thanked: 0
Been thanked: 0

Re: Rotors

Post by aubrey048 » January 26th, 2011, 11:19 am

Uhh, what exactly is a prop hanger? I've been seeing this term all over the place and would really like a definition . . .

Also--do I NEED one for a helicopter??
Plotting the function of the universe for efficiency without your permission.

Projected 2011-2012 Events: Anatomy, Microbe Mission, Disease Detectives, Tower, Optics, Helicopter.
Past Events: Anatomy (7th), Helicopter (6th), Mission Possible (1st), Write It Do It (4th, 8th), Ornithology (5th).

User avatar
illusionist
Member
Member
Posts: 942
Joined: March 20th, 2010, 4:13 pm
Division: C
State: MI
Has thanked: 0
Been thanked: 0

Re: Rotors

Post by illusionist » January 26th, 2011, 11:25 am

It's much simpler than you think...
A prop hanger is simply a device/mount that holds the wire shaft of the propeller. It allows the propeller to spin inside.
Here are some examples:
http://gallery.scioly.org/details.php?image_id=3252
http://gallery.scioly.org/details.php?image_id=2957 (the red thing by the right side of the picture is a prop hanger)
2012-2013 Building Event Captain
Rule 7d. "Event Supervisors are allowed to break any competitors' devices" -bearasauras

Locked

Return to “Helicopters C”

Who is online

Users browsing this forum: No registered users and 1 guest