407 tail rotor
Thread Starter
Joined: Jan 2001
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From: Everett, WA, USA
407 tail rotor
The Delta hinge on a Bell 407 is backwards. In other words the pitch change induced by tail rotor flapping is in the direction where it increases flapping rather than decreasing it like on most other helicopters.
Does anyone know the real reason why it is like that? To me it seems like an unwise design but maybe there is a good reason for it. No one I have asked has ever had a good answer. Makes me wonder about those 407 accidents where the tail rotor flapped too far.
Does anyone know the real reason why it is like that? To me it seems like an unwise design but maybe there is a good reason for it. No one I have asked has ever had a good answer. Makes me wonder about those 407 accidents where the tail rotor flapped too far.
Joined: Jan 2008
Posts: 35
Likes: 7
From: USA
407 T/R Issues
During it's infancy, there were a number of accidents where the tail rotor made contact with the boom.
The solution was:
Lower VNE until real fix can be found. Lower IAS = Less Flapping.
Move tail rotor further from the boom.
And finally, add the pedal stop system, which is airspeed sensitive. Below certain airspeed, full pedal travel allowed. Above that airspeed, travel would be restricted to prevent gross inputs.
Story I always heard was that Bell assumed the "real problem" was pilots inadvertently slamming full pedal in at cruise. Use your imagination on the scenarios leading to that.
I flew the aircraft for 7 years in temperature ranges from +35 to -40 C and I never had any issues with the tail rotor or the pedal stop system.
The solution was:
Lower VNE until real fix can be found. Lower IAS = Less Flapping.
Move tail rotor further from the boom.
And finally, add the pedal stop system, which is airspeed sensitive. Below certain airspeed, full pedal travel allowed. Above that airspeed, travel would be restricted to prevent gross inputs.
Story I always heard was that Bell assumed the "real problem" was pilots inadvertently slamming full pedal in at cruise. Use your imagination on the scenarios leading to that.
I flew the aircraft for 7 years in temperature ranges from +35 to -40 C and I never had any issues with the tail rotor or the pedal stop system.
Joined: Dec 2001
Posts: 1,835
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From: Philadelphia PA
The use of full left pedal in flight was never proven - it was strongly suggested, but there never was any proof - and personally, I've never ever heard of anyone using full pedal in forward flight.
Joined: Apr 2007
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From: Center of the Universe
I've never ever heard of anyone using full pedal in forward flight.
Joined: Mar 2002
Posts: 814
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From: Iceland
The story making the rounds a few years ago
So at the very least the instructors at Bell are in the belief that was the reason.
There are no issues with the tailrotor any more so problem solved.
Joined: Mar 2011
Posts: 226
Likes: 25
From: Arizona
The OSN's that Bell issued mentioned that on one occurrence the pilot stated that he did not have both feet on the pedals during cruise flight at 120+ KIAS when the aircraft started the rapid yaw. That's probably where the presumption of full pedal inflight comes from. Sounds likely. It appears that the pedal stop has solved the problem.
The original thread is a question about the negative delta 3 on the 407 tail rotor. "Helisphere", if you're still out there, the delta hinge minimizes flapping by taking the tail rotor natural flapping frequency away from resonance with the aerodynamic frequency. The natural frequency is 1 per rev, aerodynamic loads are 1 per rev. A negative delta reduces the natural frequency to less than 1 per rev, positive delta increases it to more than 1 per rev.
The original thread is a question about the negative delta 3 on the 407 tail rotor. "Helisphere", if you're still out there, the delta hinge minimizes flapping by taking the tail rotor natural flapping frequency away from resonance with the aerodynamic frequency. The natural frequency is 1 per rev, aerodynamic loads are 1 per rev. A negative delta reduces the natural frequency to less than 1 per rev, positive delta increases it to more than 1 per rev.
Joined: Apr 2010
Posts: 98
Likes: 1
From: Kansas
The tail rotor "hub" consists of the Main yoke and the flapping bearings. the two flapping bearings are mounted at a 30 degree angle(Delta Hinge). This hinge offset causes the pitch of the blades to change when the blades flap. my best way to describe this is... we need to compensate for the dissymmetry of lift in forward flight. on the main rotor we could use cyclic to trim it out, but the tail rotor only has "collective" control and therefore needs to be able to do it on its own. so... the flapping action of the TR blades equalizes lift on the advancing and retreating blades without pilot action... The OH-58D also states that full left pedal in cruise may cause flight loads that may cause boom contact... and do not do it. Seems to be working for us here in the Army.
