The SB>1 Defiant questions answered!

Vertical Magazine asked Sikorsky and their senior test pilot about the SB>1 Defiant!


Vertical: Why coaxial main rotors?

Sikorsky test pilot Bill Fell: Coaxial main rotors provide the best of everything a helicopter has to offer. The lift on the advancing halves of legacy rotorcraft is limited by the requirement to make the same amount of lift on the retreating half of the rotor. In X2 the rotors are balanced as a system of two rotors with a dissymmetry of lift allowed on each rotor but balanced between the two.

Photo by Eric Adams

Vertical: Why a rear propeller?

Sikorsky test pilot Bill Fell: A rear propeller makes sense from an efficiency perspective. The advancing blade concept requires less power in high speed flight in large part due to the focus on the good lift on the advancing half of the rotor. Power is then available for use in the prop. A prop is the most efficient propulsive device in this speed category. The prop is in the rear for a combination of operational necessity and efficiency.

Photo by Eric Adams

Vertical: Isn’t the coaxial rotor design the same one that has been used for decades on Kamov helicopters?

Sikorsky test pilot Bill Fell: No. The Kamov design is an articulated rotor system and as such is required to have a balance of forces on each rotor. Retreating blade stall will limit forward speed. So while the rotors may delay blade stall due to lower loading on each rotor they still have physical properties that are fundamentally different than X2 rigid rotors.

Photo by Eric Adams

Vertical: How difficult will it be to track and balance the main rotor system?

Sikorsky test pilot Bill Fell: This is accomplished the same way as rotorcraft today. The rigid rotors tend to have a fairly tight track to begin. Part of our test program is to figure out all the sensitivities to weight and pitch rod adjustments. We have a great deal of history and experience in this regard.

Photo by Eric Adams

Vertical: Can the main rotors contact each other in flight?

Sikorsky test pilot Bill Fell: We engineer aircraft rates allowed through the flight controls to limit rotor tip convergence. These rates are actually higher than where a pilot would feel comfortable maneuvering the aircraft, even in combat under fire. As a rule we allow 2/3 of the static separation for maneuvering and retain 1/3 as a safety factor.

Photo by Eric Adams

Vertical: How does the rear propeller work?

Sikorsky test pilot Bill Fell: The rear prop is geared very much like a tail rotor. There is a driveshaft out of the main gearbox back to the prop. The engine(s) turn the gears in the box and the box turns the rotors and prop. The prop has a clutch such that it can be disengaged from the system and either freewheel or brake. Brake mode is great for ground operations especially in tight places like a ship or forward arming site.

The prop is controlled by a switch on the collective. Move forward for more thrust and aft for less thrust.

Photo by Eric Adams

Vertical: Was the rear propeller engaged during first flight [of the SB>1 Defiant]?

Sikorsky test pilot Bill Fell: No. That [was on subsequent] flights. Another beauty of the prop is it can sustain damage and the aircraft is still capable of helicopter-like speeds of 160 knots.

Photo by Eric Adams

Vertical: Will the main rotor system experience retreating blade stall at high speeds?

Sikorsky test pilot Bill Fell: No.

Photo by Eric Adams

Vertical: Is it possible to autorotate the aircraft?

Sikorsky test pilot Bill Fell: Yes. In fact, in high-speed flight the collective is already very low.

If one engine [in the SB>1 Defiant] fails, the flight control system reduces prop thrust automatically and we can continue at a lower speed. In most cases we have single engine hover power if we have burned off some fuel. If both engines fail the FCS reduces prop thrust. This deceleration back drives the rotors and sustains RPM. The pilot has the time from say 250 knots until 120 knots to find somewhere to land. Then the pilot will most likely disengage the prop and autorotate like any other helicopter.

Photo by Eric Adams

Vertical: The design seems really complex. Does it outperform a standard helicopter well enough to merit the complexity?

Sikorsky test pilot Bill Fell: There are less parts on our two rigid rotors than a single articulated rotor. Less parts equals less maintenance. The design is not complex…but a well thought out integration of existing proven technologies.

Independent of the higher speed this aircraft has incredible deceleration capability via significant negative pitch on the prop. Deceleration that will happen in a level attitude and much faster than a helicopter.

The ability to declutch the prop and alter rotor RPM will set a new bar in acoustic signature. There are also improved maneuverability and survivability aspects of this technology.

Photo by Eric Adams


Every time I see this thing, it looks awesome! The future is now people!


I absolutely love this thing. It’s amazing! Great Q&A.

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I love that Sikorsky did this!

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