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HelicopterLow Head Speed Helicopters › Low head speed you may be heading for problems
08-06-2007 04:33 PM  10 years agoPost 1
eyeinsky

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Fall River, Nova​Scotia, Canada

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There are two big concerns that I have found two be possible catastrophic fault when lowering the head speed under heavy loads experienced by most AP helicopter.

Blade stall
This is may occur under certain condition. You will recognize this for most part the aircraft will react by pitching or flaring opposite to the direction the aircraft is traveling.

It is not possible however to predict at what point the rotor will stall each time due to forward speed because many factors come in to play; rotor load, temperature, altitude and maneuvers undertaken. Basically the down wind blade air flow in forward flight, but not only, backward and sideway can also experience this condition. The air speed become to slow combined with the high angle of attack of this blade causing the blade to stall starting with the tip and will continue to move inward until control is lost. Because of gyroscopic precession the aircraft will react 90 degrees to the stalled blade area.

When a stall is experienced the corrective action is to reduce forward speed, reduce pitch and increase rotor speed.

Rotor disc coning
This is condition is cause by lower rotor speeds. Basically as the rotor speed is decreased the blades will cone more. The end result is that the blades carry more upward load and less centrifugal load. If the point of structural integrity is reach in maneuvers (G force) the system will fail.

Another bit of info I came across. This pertains to Auto's, most full scale helicopter will not successfully auto rotate between 12 and 300 feet with no fwd speed. Pilots are trained to operate in the safe zone when ever possible. Does any one find this to be true with the small helicopters?

Ground resonance is a self-excited vibration that can happen when the helicopter is on the ground, which may manifest slowly and gradually build up or appear very rapidly. It is caused by one blade leading and one blade lagging at the point of contact with the ground. The feed back from the landing gear especially if on uneven ground can start this condition. This can damage the helicopter if not corrected. The best thing to do if this condition occurs is lift off the as fast as possible it will correct its self immediately. Same goes for landing. Has any one found them selves in this condition?

I have seen this At the time I reduced power she sure rattled on the ground. Next time I will take off.

Just little useless info

Hard job competing with gravity.

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08-06-2007 07:39 PM  10 years agoPost 2
eyeinsky

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Fall River, Nova​Scotia, Canada

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Unsymmetrical air foil verse symmetrical airfoil blades.
Unsymmetrical air foils center of pressure move in greater span FWD and AFT when the angle to relative wind is changed. For this reason it not the preferred airfoil for rotor wing aircraft. The blade pitch changes constantly, leading to instability characteristics in the rotor head. That why it not common design used in the full scale rotor aircraft. Some manufactures are experimenting with an airfoil design that changes through the span of the blade and have had good success.

Just little useless info

Hard job competing with gravity.

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08-06-2007 10:25 PM  10 years agoPost 3
Load Factor

rrApprentice

Colleyville, Texas

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I think your evaluation sounds good for Full Scale....but as far as blade stall....I think with 1300 head speed being of the norm, it would be pretty hard to stall even downwind with a pretty good tail wind at that....JMO.

I've auto'd my Bergen from a 30' hover more times than I care to mention, and she landed perfect (Main gear eating machine!, actually it was the guy setting the mesh)....it is fast but very doable. There is soo much energy stored in our little rotors....I wouldn't try to do one from 50'.

My very first instructor could pull off the most spot on auto's from 15' hover in the R22. He demo'd those when I had a around 5 hours, it was truly amazing! He is just one BA copter pilot!

Awesome info.....!!

Tanner

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08-07-2007 12:23 AM  10 years agoPost 4
eyeinsky

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[Quote]I think your evaluation sounds good for Full Scale....but as far as blade stall....I think with 1300 head speed being of the norm, it would be pretty hard to stall even downwind with a pretty good tail wind at that....JMO.

This is based on physics the relation ship remain constant that is a fact. All air foil are subjected to the same laws regardless of scale. The numbers may change in some cases but the laws will always apply.On the other end.

If the RPM is to high tip speed on the up wind blade combine FWD air speed will reach the speed of sound lift will be diminished and will continue inward till control is lost. This force will be felt 90 degrees from the area that lift is lost causing nose to pitch down. The opposite goes for blade stall. It is a fact. Operating with slower RPM you should reduce your speed or you could experience blade stall.

Like I said useless information

Hard job competing with gravity.

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08-07-2007 12:46 AM  10 years agoPost 5
46Taylorcraft

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AZ

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All air foil are subjected to the same laws regardless of scale.
yes but with rules. look up "Reynolds Number"

I've never experienced a blade stall before. I think to get that to happen in an RC heli you'd need to have disk loading closer to that of a full scale.

Our "model" heli's are lightly loaded when compared to a real chopper.

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08-07-2007 12:56 AM  10 years agoPost 6
eyeinsky

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Fall River, Nova​Scotia, Canada

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Yes that is correct, that is why I have brought it up because the wing loads are getting closer to this range. I my self have taken the head speed way down and flew the machine around. All I am saying is I started to think about it.

Hard job competing with gravity.

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08-07-2007 01:04 AM  10 years agoPost 7
46Taylorcraft

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AZ

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this is where multiblade heads may start to become a desireable option. You can reduce the headspeed without having to increase AOA.

Parasite and induced drag (possibly?) would be higher requiring more power but for really heavy loads it could be something of interest.

I think a 3 blade setup would be perfect. The increase drag by adding the 3rd blade would be significantly offset by the removal of the flybar/paddles.

Also some believe a 3 blade head does not have the "thump thump" effect that a two blade rotor does which could mean smoother video.

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08-07-2007 01:59 AM  10 years agoPost 8
eyeinsky

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Fall River, Nova​Scotia, Canada

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Reynolds Number
If an airplane wing needs testing, one can make a scaled down model of the wing and test it in a wind tunnel using the same Reynolds number that the actual airplane is subjected to. If for example the scale model has linear dimensions one quarter of full size, the flow velocity would have to be increased four times to obtain similar flow behaviour.

So let’s think about how this is applicable to the stall speed of airfoil. 4 times the speed produces 4 times the drag produces 4 times the lift requiring 4 times the thrust. The wind speed is slowed down the angle of attack is increased for given airfoil it will stall at the same point regardless of scale or would stall at an airspeed it 4 time higher. That does not make sense to me. This means the stall speed would be 4 time higher, this would make it worse for the small scale aircraft.

Some one help me out this dose not make sense to me

Hard job competing with gravity.

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08-07-2007 02:10 AM  10 years agoPost 9
eyeinsky

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Fall River, Nova​Scotia, Canada

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The 3 blader would be sweet. Blade load would be 1/3 the angle of attack would less to produce the same lift. The rotor diameter could be smaller there for tip speed would reduce not getting close to the speed of sound. I also think it would have less vibration.

Hard job competing with gravity.

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08-07-2007 05:13 PM  10 years agoPost 10
AceBird

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Utica, NY USA

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this is where multiblade heads may start to become a desireable option. You can reduce the headspeed without having to increase AOA.
---

I think a 3 blade setup would be perfect. The increase drag by adding the 3rd blade would be significantly offset by the removal of the flybar/paddles.
For AP I think I would opt for a four bladed rotor and still keep the flybar. The drag of the flybar/paddles would not equal the drag of just one additional blade but all that means is more power. So a twin cylinder (or two engine) power plant seems right for heavy lifting applications. My only reservation is I think the Predator mechanics are a bit light for this type of project.

Ace
What could be more fun?

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08-07-2007 06:02 PM  10 years agoPost 11
eyeinsky

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Four blades and a fly bar wonder how that would look like, keeping in mind gyroscopic precession. The force will be felt 90 degrees from input. That is beauty of Bell Hiller fly bar system it takes that in to account. You can remove the fly bar directly in put the blades. You just must compensate for gyroscopic precession.

I would go with odd number of blade 3 or 5 because I have herd from the full scale guys. They say the odd number systems have less vibration.

You could use a smaller disc higher RPM not having to up grade the power plant.

Hard job competing with gravity.

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08-07-2007 09:25 PM  10 years agoPost 12
AceBird

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Utica, NY USA

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Four blades and a fly bar wonder how that would look like, keeping in mind gyroscopic precession.
Two of the blades do not have cyclic. These blades would line up directly above or below the flybar. They only add lift which is what you need for a camera ship. These blades do not necessary have to be the same length as the cyclic blades. You will need enough cyclic control on the other two blades to be able to fly or stabilize the craft. This should not be a problem for an AP ship because they are not noted for fast forward flight.

We were talking about a specialized craft for hauling a camera right?
They say the odd number systems have less vibration.
I was under the impression the more the merrier. Why would three be better than four? I can’t see why.
You could use a smaller disc higher RPM not having to up grade the power plant.
If you make the disc smaller and increase the RPM what do you gain; a smoother ride? My intent would be to increase reserve lift so you are not so close to loosing it with a heavy load. As an added bonus a twin or duplicate engine with a four bladed rotor would also smooth things out. I actually like the idea of a second engine more so than a twin because of the redundancy. The other advantage of two single engines is they are cheaper than 1 twin.

Ace
What could be more fun?

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08-07-2007 11:23 PM  10 years agoPost 13
eyeinsky

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Fall River, Nova​Scotia, Canada

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Do one better get rid of the fly bar, with 45 Degree angle on the blade grip pitch link arms off set the swash plate input by another 45 degree to complete the 90 degree required for Gyroscopic precession. Then go with what ever number blades you want all with collective and cyclic control.
If you keep RPM higher the centrifugal force will keep the cone flatter there for reducing the blade up ward load. Smaller disc can turn faster more blade share the load. This is big advantage .

Hard job competing with gravity.

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08-08-2007 03:39 PM  10 years agoPost 14
eyeinsky

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Fall River, Nova​Scotia, Canada

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Interesting read Low Reynalds number heli.

http://www.aviationtoday.com/rw/pro...gines/1788.html

Hard job competing with gravity.

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08-08-2007 05:51 PM  10 years agoPost 15
AceBird

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Utica, NY USA

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If you keep RPM higher the centrifugal force will keep the cone flatter there for reducing the blade up ward load.
But if the cone angle is greater it will be more stable. Still thinking AP here. As you add blades the load on each blade decreases (shared load).

Ace
What could be more fun?

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08-08-2007 09:01 PM  10 years agoPost 16
eyeinsky

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Fall River, Nova​Scotia, Canada

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Yup

Give and take that is what the operator has to decide. Low head speed you should slow your maneuvering reduce G load. Slow fwd speed to prevent blade stall. The full scale guys train and brief on emergency procedures. It something a good operator should do to protect the public, your self and your equipment. If you approach it the same as a civil aircraft, crashing is not an option. 100% of all crashes are preventable there is all ways a reason. If you eliminate the cause there will be no crashes.
Pretty simple concept.

Hard job competing with gravity.

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HelicopterLow Head Speed Helicopters › Low head speed you may be heading for problems
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