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HelicopterMain Discussion › New Approach to Collective Pitch Setup
07-11-2008 05:49 PM  9 years agoPost 21
rcarlson123

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Colchester, VT - USA

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In the world of Fluid Mechanics, the airflow after the rotor disk wouldn't matter. The airflow into the disk is important though.
If that's true, how do you explain ground effect?

- Ross

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07-11-2008 05:53 PM  9 years agoPost 22
SSN Pru

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Taxachusetts

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this has a limit though as I could imagine a solid plate disk mounted to the skids that would be large enough to serious affect the lifting capacity of the rotor disk.
You missed this, right above your quote.

You could very well manufacture a solid disk and attach it to the skids which would provide a reactive force counteracting the lift produced by the blades.

I am fairly certain that Ground effect increases the pressure differential from the upper side of the airfoil to the lower side. That's what my gut is telling me anyways.

I am not an aerodynamicist though.

And airflow into the disk upright and inverted are not the same. Inverted, there is a much 'dirtier' path for the air before interacting with the rotor blades which would decrease the efficiency of the rotor disk.

Again, I am not insinuating that I am absolutely correct here, just thinking outloud

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07-11-2008 06:45 PM  9 years agoPost 23
Heli 770

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USA.

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umdpru
I was just asking Kyle about the speed because I too have read like Riq has the same thing he said. Well I started thinking, If what you say is true, Then why are all the props on outboards, out drives, and trolling on the rear?
I don't know the answer but I sure would like to.

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07-11-2008 07:32 PM  9 years agoPost 24
rcarlson123

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Colchester, VT - USA

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It's a common misconception that airfoils cause lift due to a pressure differential above and below the airfoil. It's even taught that way in textbooks and by flight instructors. The reality is a bit more complex ... the main source of lift comes from the airfoil redirecting airflow downward behind the wing (or rotor blade) ... the equal and opposite reaction is the lift.

Another common misconception is that ground effect is the result of air compressing underneath the airfoil as the aircraft nears the ground ... the reality is that ground effect is the result of less updraft near the leading edge of the wing. Here's a good explanation of how lift really works, as well as ground effect:

http://www.allstar.fiu.edu/AERO/airflylvl3.htm

Of course none of the above explains why a heli might need more or less pitch to hover inverted as opposed to upright ... I can only assume (as others have) that it has something to do with the heli fuselage interfering with airflow.

- Ross

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07-11-2008 08:53 PM  9 years agoPost 25
Heli 770

rrProfessor

USA.

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Riq
Have you read the book by Paul Tradelius? That's were I got my info. on flying inverted.

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07-21-2008 07:36 AM  9 years agoPost 26
mchammer

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California,USA

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Put the bird down and turn clockwise or counter clockwise both long links from the swash to the grips or bell mixers. Do this until the "Feel" of positve climbouts and negative is the same. You'll probably end up with something uneven like positive 11.5 and negative 12.5. You want to make sure the swash is not binding on full positive or negative.
the only negative side to this is if you are doing 3d maneuvers that require 0 deg of pitch to look good ie:death spiral etc. zero deg of pitch is no longer at center stick. you will have to get used to wherever it now is either below or above true center stick on the gimbal.

Peace Through Superior Firepower!!!

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07-21-2008 11:28 AM  9 years agoPost 27
buburub

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Bayside, NY

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I currently have a revised approach. Set it up mechanically (increase or decrease links) to get true center - the smoke is not going up or down during idle up mode on the ground. Increase pitch on the swash menu until the side that feels weakest (upright,inverted) finally feels strong. Then you would decrease the pitch curve on the stronger side to balance things out. So if the negative side finally feels strong enough and you need to kick things down on the positive to even it up, it might look like this (0,25,50,70,90), This way you can acheive true center with both positive and negative side feeling the same.
Bub

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07-21-2008 01:36 PM  9 years agoPost 28
SSN Pru

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Taxachusetts

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If what you say is true, Then why are all the props on outboards, out drives, and trolling on the rear?
Good question.

Azipods are mounted in a tractor configuration on ships.

It is a clear fact that a "pusher" configuration has a much dirtier upstream than a tractor configuration. That's why "pusher" configured propellers of any type are preceeded with very streamlined body's such as cowlings or outdrives.

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07-21-2008 02:12 PM  9 years agoPost 29
SkateFreak

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Cambs UK

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I dont think the fuse has that much of an effect to be honest,
when you think about it the amount of lift is not equally spread throughout the disk, if anything its minimul at the center of the disk and gets stronger as you move out...
The amount of lift in the center of the blades is proberbly minimul and therefore the majority of the power translates throughout the mid/outter blade, and if your canopys out there...
you gotta have one ugly bird

-Jvr

Non-3D heli pilots are planker spys trying to bring down the heli community from the inside - Topher

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07-21-2008 03:57 PM  9 years agoPost 30
SSN Pru

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Taxachusetts

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I dont think the fuse has that much of an effect to be honest,
when you think about it the amount of lift is not equally spread throughout the disk, if anything its minimul at the center of the disk and gets stronger as you move out...
An obstruction does not have to be directly in line with the strongest lift to cause lift loss.

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07-21-2008 04:03 PM  9 years agoPost 31
Pistol Pete

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Seffner, FL

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Something tells me airflow into the rotor disk is disrupted by the fuse or pod when inverted...
most lift is created by tips of blades.

EDITED

bolded the key word.

~~Enjoying the hobby one flight at a time~~

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07-21-2008 05:55 PM  9 years agoPost 32
rcarlson123

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Colchester, VT - USA

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I wouldn't say it's created by the tips ... but, the amount of lift generated by any given section of a rotor blade does increase with distance from the hub. That's only because the airspeed increases towards the tips. This is assuming the angle of attack and chord of the blade doesn't change along its length, which is generally true for RC rotor blades, as far as I know ... I don't think that's true for all real-world helis though.

- Ross

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07-21-2008 07:11 PM  9 years agoPost 33
Topher

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Rochester, Michigan

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Real world helis normally have a constant chord but they do have blade twist for the obvious reasons. I'm thinking its a combination of squishing the dampers and turbulence created by the airframe is the root cause of this issue.

will wash your heli for a quarter

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07-21-2008 10:08 PM  9 years agoPost 34
TulOkFly

rrApprentice

Tulsa. OK - USA

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Think about it. The heli is being subjected to the forces of lift, drag and gravity.
Gravity is a constant, obviously.
When the heli is right-side-up the blade disk is creating lift and blowing air downward which hits the fuse and due to drag this high velocity airflow is pushing the fuse downward. This cancels out some of the lift.
When the heli is inverted the high velocity airflow is not hitting anything.
If the pitch and headspeed is identical for both positive and negative there is more net lift when inverted.

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