A087

DESIGN: Dragonfly ~ Rotor
Go to Prototyping note at bottom of this page.
Overview:

Tri-teetering rotor hub; with offset and undersling,

Outside Helicopters

xx.

Dragonfly

Characteristics:

OTHER: Helicopter Specification - Dragonfly

Scaling between 2-blade SynchroLite Rotors and 3-blade Dragonfly Rotors:

The scale multiplier for 2 to 3 blades while maintaining the same volume (mass) is: 1 * (0.87358)³ = 2/3

	SynchroLite:	Scaled to Dragonfly:	^{(1) Proposed Dragonfly:}
Rotor Radius:	8.667' ~ 8' - 8"	7.571' ~ 7' - 6.85"	8.0' ~ 8' - 0"
Chord: ⁽²⁾	0.4375' ~ 5.25"	0.3822' ~ 4.586"	0.50' ~ 6.0"
Thickness:
Aspect Ratio:	19.8		19.2
Effective Disk Area:	338.5		288.4
Disk Loading:	1.62		1.91
Blade Loading:	36.3		27.5
Rotor Solidity Ratio:	0.0448		0.0693

The values are increased to the next higher round figures. This will result in a slight weight increase but it will reduce the horsepower increase.

The SynchroLite has no taper whereas the Dragonfly does. With taper the blades will more easily handle the additional centrifugal forces that result from using smaller, faster turning rotors.

Reverse Velocity:

At the maximum regulated forward speed [55 knots], the location of zero airflow is at 1'-4" radius at azimuth 270º.

Quick Disconnect for Blades:

Rick A. says that the Hughes hub has a type of quick disconnect pins at the blade root and the drag link connection. Perhaps BCIT has a manual on this craft. Quick disconnect blades and tail would make the helicopter truly transportable.

Hub Spring:

There are two tie-bar assemblies. One is for the lead, the other is for lag and both work together for teetering. Location a Hub Spring at the central intersection of the three bars on each assembly will act as a lead-lag spring and also as a teetering spring.

The elastomerics should be located outside the tie-bar centers, between them and the mast. Because, they cannot be located inside and press against the control stick. Note the the elastomeric must leave room for the three collective rods to pass through.

I do not think that there is any reason for a hub spring.

Negative thrust, Droop Stops & Tie-rods:

The current design of the tie-bar assembly will not allow it to handle compression. Therefore, it cannot serve in any way to resist droop or negative thrust. Consider the idea of elastic droop stops, witch are permently in position. They could start resisting negative flap at a some to-be-determined angle.

Drag Link: ???

Consider drag links with some elasticity. This is to accept the lead/lag caused by the variation in drag at forward speeds. It might also reduce any 'shock loading' on the gears. Better yet, put elasticity in the arms of the rotor head [composite head?] that go out to the grips.

Alternatively: Consider locating the provision for lead/lag in the vertical blade attachment bolt assembly, if the amount of lead/lag is quite small. Basically the conection will have a 'sloppy spline and the gap between the two spline segments will be 'filled' with an elastomeric. Bushings will be required to maintain blade alignment. May be a dumb idea.

Folding Blades:

For view of SynchroLite folded blades, see: Design: SynchroLite ~ Rotor Disk - Folded Blades - View 2 The dragonfly has the blade pivot points spaced further apart and there are three blades per rotor. The dragonfly's blades will probably have to be folded both forward and aft; or removed.

Additional information OTHER: Flight Dynamics - Rotor Hub - Offset Teetering Hinge Concept

Interesting & Relevant Post by Vfrpilotpb on PPRuNe:

Mast bumping can occur on any "Teetering Head" system the R22, R44 B206 the Huey and many other well known Helicopters have that sort of head, Lu is quiet right in his explanation of the way to cause mast bumping, it is something that is normally drummed into most Helo pilots to avoid at all costs(the cost being your life and or that of your pax) with the rotor going one way and you and the heli going the other, it is apparently a problem that is easier for converted fixed wing pilots to get into than pilots trained on Helo from scratch, the main reason being a fixed wing pilots reactions are to push the stick forward at the sign of any problems this action will cause the Teetering head Heli to get into the condition of Negative G, this is a most dangerous position to get into and requires quick but accurate action to recover, it has been demonstrated to me by a very experienced Heli pilot, with the cyclic loosing all feeling of being attached to anything, very dangerous don't go there!

Some Musings on Clearance between Rotors: February 17, 2003

If the V-angle increases, the blade to hub clearance will improve but the tip to blade will stay about the same.
If the Stagger increases, the blade to hub clearance will improve but the tip to blade will stay about the same.

No coning angle, flap of offset was considered in the above.

If the offset is increase, the mast bearing will be subjected to greater moments, the control linkage must be a little more elaborate, a lesser angle of flap can probably achieve the same rate of angular velocity, but the V-angle and the Stagger might be reduced.
Restricting the lateral cyclic stick movement will reduce the flapping at azimuth 90º and might assure sufficient blade-to-hub clearance.

Prototyping and Testing: September 4, 2003

There is concern wether this craft can be manufactured for less than 254 lbs. and also if the reasons for building to 254 are compelling enough. Therefor consider prototyping the Offset teetering rotor at 254, because it is less exxpensive and partially designed.

Then. if there is a desire to go to the UniCopter I GW of 750 lbs. or to the inverse of the above SynchroLite blade reduction (shown above at 1 * (0.87358)³ = 2/3) then the increase on the tested rotor is only 11% for 750 lbs. or 14.4% for (550 * 1.5) = 825 lbs.

There will be no desire to increase the gap, therefor the hub and bearings MIGHT require some beefing up. If there is a desire for a slower rotor and a pusher prop, the centrifugal loads and thrust loads might even be lighter.

IE. DEVELOP THE ULTRALIGHT ROTOR FOR TESTING. Perhaps with simple spur gear or 90-degree bevel and use vebelts for the primary testing reduction.

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Prototyping the Blades:

Assume that six fiberglass covered spruce blades were produced and used to test the complete primary package. Then three of the blades could have their tips easily modified for taper or anhedral or whatever. The three modified and re-modified blades could be tested against the three unmodified blades; on a single rotor.

Same Page ~ Different Craft: ~ DESIGN: SynchroLite ~ Rotor ~ DESIGN: UniCopter ~ Rotor

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Last Revised: November 12, 2003