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HelicopterMain Discussion › headspeed and 3d
10-03-2005 11:52 PM  12 years agoPost 1
brian12

rrApprentice

Chicago

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Why are headspeeds for 90 size machines lower then 50 size machines. For instance for a YS91 engine most people recomend 1750 headspeed for 3d. With a hyper 50 most people recomend 1900-1950 for 3d (some even go higher).

Now i understand that where your engines maximum rpm is that has a lot to do with where you set your head speed, but the more head speed you have wouldnt you have more crisp flying with more power?

Another question is what headspeed do the pros with 90 size ships use for 3d? Do any really go as low as 1750 for hard 3d?

1750 headspeed seems low for hard 3d, but judging from using the search button it seems thats what everyone runs if there using a YS91.

Thanks for any thoughts.

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10-04-2005 12:34 AM  12 years agoPost 2
Richie Sabin

rrApprentice

Niagara Falls, NY

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Rotorhead speed is lower in 90 ships because of the engine RPM Engines have a curtain RPM range which produces the most power. the max practical engine RPM is 16,000 which the Hyper 50 max practical RPM is 20,000 which gives you the ability to run a higher headspeed with the 50

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10-04-2005 12:42 AM  12 years agoPost 3
dkshema

rrMaster

Cedar Rapids, IA

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You can also get away with a lower head speed because you are swinging a larger diameter rotor disk.

Dave

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10-04-2005 01:14 AM  12 years agoPost 4
MJWS

rrKey Veteran

Airdrie, AB - Canada

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Motor RPM is part of it, but we get around that with gearing. You can easily get the head of a 90 over 2000... a couple Japanese pros were doing this. Slam on 7:1ish and crank it up.

Part of it is the forces on the rotor head with large heavy blades when you spin them at very high rpm. The idea is to maintain some margin of safety. One of the math wizards can do the math for you... but suffice it to say 720's spinning at 2100rpm generate a lot of force on our little rotor heads.

Bottom line is we need to generate a specific amount of thrust for a given move. Larger blades swung slowly are much more efficient then small blades spinning very fast. (Think plank prop vs. Rotor blades.) The drag increases far more quickly than our thrust when we crank up the blade speed.

But we aren't really looking for pure lifting power for 3D. We want to be nimble and light in the air, so there are trade offs necessary. The other thing to consider is the energy stored in the disc at high RPM with the smaller heli's. We can use this inertia to our advantage.

Lots of things to consider beyond motor RPM.

Mike

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10-04-2005 03:40 AM  12 years agoPost 5
AirWolfRC

rrProfessor

42½ N, 83½ W

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Rotor tip speed is where it's at.

For "nimble" flight characteristics, you want a relatively high rotor tip speed. 0.5M is a good hi-ish number. For 720 blades and a 6.5" rotor hub, the rotor disk diameter is 5.135 ft. The head speed comes out to 1130 * .5 * 60 / 3.1416 / 5.135 = 2101 RPM.

You just need to verify that everything will hold together at that speed.

Before you start crying that's too high, keep in mind that the Robinson R22 has a tip speed of 0.6M, 20% higher yet. Of course, The R22 disk loading is higher too.

Wolfgang

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10-09-2005 08:28 PM  12 years agoPost 6
AirWolfRC

rrProfessor

42½ N, 83½ W

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Oh, I forgot, for 720 SAB blades at 2100 RPM,
that's 900 lbs trying to pull the blades off the rotor hub.

At 1800 RPM it's only 666 lbs.

Wolfgang

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