Item 0909

DESIGN: UniCopter ~ Rotor - Disk - Dihedral [R ?] at Blade Tip (Bladelet )

Reasons for:

1. It is said that the additional lift of the winglet is the same as that of a wing where the span was increased by the span of the winglet. I suspect that tip dihedral (winglet/bladelet) will be an advantage on an ARR since it should reduce the moment arms of thrust and drag slightly.
2. It may also reduce the tip vortex.
3. For speed stability, if the bladelet was at 45º it might want to raise the nose of the craft (when the blades are at 180º azimuth) and lower the tail of the craft (when the blades are at 0º azimuth). This may help reduce longitudinal static instability. It may thereby reduced the size of the horizontal stabilizer, plus the drag and downward force that it creates. This is in addition to the pre-cone angle.
4. Probably going with some anhedral over the span of the blade, if it is necessary, to compensate for any lack of absolute rigidity in an Absolutely Rigid Rotor.
5. The 'footprint' of the helicopter will be slightly smaller.

Notes:

• If the lift at azimuth 180º is 5 lbs and at azimuth 0º is -5 lbs then the downward force at the horizontal stabilizer can be reduced by 10 lbs. This results in a 10 lb reduction in required rotor thrust.
• The amounts of the forces are relational to the craft's forward velocity.
• Concern:- If the blade tip has sweep and dihedral then these forces will want to cause negative pitch when the blades is at azimuth 180º and positive pitch when the blades is at azimuth 0º. Perhaps have the front of the tip extend forward of the blades leading edge so that the upward and the downward force form the dihedral is equally located for and aft of the feathering axis.
• Make:- Put the upward bend at the tip into the upper web of the spar only. It will be difficult to put it into the lower spar.
• A smaller tail ≡ a smaller diameter boom ≡ less vibration from downwash and a higher elevation pusher prop or larger pusher prop.
• This dihedral tip will not require an increase in blade strength since the tip's lift will be at azimuth 180º, whereas the blades greatest out-of-plane stress will be experienced at azimuth 90º.

The current plan is to include a pre-cone. For additional information see; Rotor - Disk - Pre-cone

These notes are probably totally irrelevant and incorrect

Advancing Blade Concept and Dihedral: The following was done with a single rotor in the Access database. The advancing blades have a greater angle of attack and thrust then the retreating blades. If the advancing blade is not extremely rigid it will flap up (virtual flapping), forming a cone and therefore represent dihedral, such as found on a helicopter with a single rotor.

A comparable change in the angle of attack in an advancing and retreating blade will have an approximate 3:1 difference on the thrust between the advancing and the retreating blades.

 Collective Pitch: Thrust: Change in thrust for 1 change in angle of attack Retreating Blade: 0 0 1 79 Difference = 79 Advancing Blade: 6 1015 7 1235 Difference = 220

Initially displayed: September 11, 2002 ~ Last Revised: September 30, 2004

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