1126

Item 1126

DESIGN: UniCopter ~ Fuselage - Tail - Twin Boom

Note: It is appearing that a single boom, coming from the center of the propeller, might be the best. This is because there appears to be no reason for a horizontal stabilizer. See the bottom of web page 1105.html

Consider this for the vertical stabilizer; OTHER: Flight Dynamics - Control - Twin Vertical Stabilizers

Drawing:

Notes:

Strength:

The two booms will be laterally braces at the rotors, the propeller saddle and the tail. This will allow their width to be less than their depth and a vertical NACA 0050 profile (2:1 ratio) might be ideal. The thick profile is of very little disadvantage because the induced velocity from the rotor will be much less than that of the propeller. This is particularly true at high forward speeds when the rotors are partially off-loaded and also the shallow angular airflow will make the blade appear to have a lower thickness to chord ratio,
The use of twin booms offers additional advantages, such as; they do not interact with the high velocity air from the propeller and they should better distribute any vibration resulting from the streamtubes of the rotors.

Form and Shape

The profile will be NACA 0050 from the front (or back) of the vertical stabilizer to the supporting location over the propeller. From here forward the NACA 0050 shape will change, to a circle, and then to a horizontal oval. The width of the oval will be that which fits the final reduction and air intake etc. The depth of the oval will be in accordance with the following.

In the side view, the centerline will be a straight and horizontal line running from front to back. The top and bottom lines will both be straight lines tapering towards each other, from front to back. They will be equidistant from the centerline at all locations along the boom's span.

In the plan view, the centerlines of the two booms will be equidistant from each other, at least from the VS to the brace over the propeller.

Aesthetically and aerodynamically it might be advantageous to give the booms a twist. The airfoil profile could be pointing directly upward at the aft end so that they align with the VS's and also minimizes the swirled downwash from the two rotors. At the forward end, where they attach to the fuselage, the trailing edges could be canted inward so that they align with the edge of the fuselage that is curling upward and outward. Note that the idea in the last sentence will want to reduce the size of the propeller. Some other solution is required.

Tail Boom Incursion:

The blades of the rotor on the same side as the boom are the only ones of concern. This is because the blades of the other rotor are at a higher elevation when they pass over the boom. This is good since the other rotor's blades are leading with the tips.
Having the 25% chord line of the NACA 0050 profile boom will result in the top of the boom being lower at the tail end, and thereby more clearance. Taper will result in the inner portion of the blade to make any contact first. Twist will negate this advantage.
A small ramp could be located on the outer side of the boom at about 75% R to assure that if a blade was ever to strike the boom it would 'climb' the ramp first.
A high ramp with a 'crushable' top will allow the determination of how close the blades get.

Airflow:

In hover, just about all of the induced downwash will be aft of the propeller and therefore passing down the NACA 0050 profile.
At fast forward speed, most of the airflow will be over the NACA 0050 airfoil and since inflow angle will be quite shallow, the airfoil will appear to have a smaller thickness to chord ratio.
Could the contraction of the propeller's wake and the downwash from the rear of the rotors cause a horizontal contraction of the wake in the plane of the twin tail booms? If so, consider angling the booms inward in the horizontal plane, after passing the propeller. This should reduce drag slightly and position the base of the vertical stabilizers slightly further forward and slightly higher.

General:

The twin boom is a means to get the top of the prop disk up close to the bottom of the rotor disks.
If twin vertical stabilizers are envisioned, they will require a larger vertical gap for the blade tips because the blades from each rotor will pass through this gap at two different elevations.
I think that this might be better achieved by using the 'Y' configuration.
If the booms are made strong enough than the bracing at the rotor hobs, propeller saddle and HS should provide enough rigidity for the tail. For high rigidity the booms should have foam cores.
The plane twin boom arraignment causes the booms to be unsupported for quite a length. Therefore, locate a saddle over the propeller and brace the saddle to the fuselage at the location where the drive shaft to the rotors is located.
The centerline to centerline of the booms is currently 33". The center to center of the rotorhubs is 28.3." It would be better for airflow, from the rotors, if the distance between the booms was closer to that of the hubs, since this will allow more air to reach the props from the unobstructed sides. Opposing this is the fact that any further reduction will put the trailing edge of the NACA 0050 booms (the lower portion) directly into the streamtube of the propeller tips.
If the blades are given a 3º pre-cone then at the pusher prop the booms could be 1.88" higher and the diameter of the propeller could be approximately 4" greater.

Increased Clearance between Boom and Blade:

Wishbone boom to put the rear portion of the boom in the center where the blades ar slightly higher. This does nothing for the area where the coard is the greatest.
Reduce the taper of the blade.
Move the maximum chord out furthere on the blade.
Give the masts a forward tilt.
Lower the rear end of the booms.
Give the booms a banana dip downward and outward near the propeller.

The best might be 5.

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Last Revised: January 17, 2007