View Full Version : Why the forward sweep?
Flingwing207
26th Mar 2006, 20:35
http://www.helipost.com/Art/tailrotor.jpg
Near as I can figure it is to balance the aerodynamic force across the feathering axis, reducing left pedal effort. (This on a S300 series.)
slgrossman
26th Mar 2006, 21:08
Looks like that arrangement puts the leading edge perpendicular to the flapping axis.
-Stan-
Heli-Ice
26th Mar 2006, 22:22
This one looks a lot cleaner than on my 300...
Flingwing207
26th Mar 2006, 23:16
Looks like that arrangement puts the leading edge perpendicular to the flapping axis.
-Stan-That's because of my quick & dirty illustration - I had the Delta-3 hinge backward, it is now corrected.
Jack Carson
26th Mar 2006, 23:22
The Delta angle between the pitch change axis and the flapping axis is called a delta three angle. Doing this creats a blade pitch change with blade flapping. Most teetering tail rotors utilize this type of design. Typically blades are oriented normal to the feathering axis while the pitch change axis is offset. Look at a Bell 206 tail rotor.:8
Flingwing207
26th Mar 2006, 23:29
The Delta-3 is known, it is the forward sweep vs feathering axis I am ciphering on. It might also serve to enhance the Delta-3 effect?
Jack Carson
27th Mar 2006, 00:28
It looks to me as if this is a backwards way of achieving Delta - 3. Do you know which if any helicopter tail rotors do it this way? Even though it appears as of the blades are swept forward they are normal to the axis of rotation.
autosync
27th Mar 2006, 02:05
Hmmm, never really thought of this before, so i am going to hazzard a guess here...
The foward sweep can reduce the diameter but gives the same surface area.
May also give extra strength to the cantilever.
This relates to the Delta -3 in that it just provides the right amount of AoA as it passes through the feathering axis.
Graviman
13th Apr 2006, 18:21
Just come across this thread...
Due to the profile drag (surface friction and normal pressure) any high speed rotating blade will also generate a centrifugal component of flow. Sweep forward causes a pressure trying to drive the flow inward. Since tail rotors operate at a fixed RPM it is relatively easy to choose the foward sweep so that the two effects cancel, so you get lower input torque for the same rotor thrust. It also reduces tip noise.
Fans used for cooling in particular are tending towards "sickle" shape fans. Again the forward sweep is to minimise tip outflow.
Mart
Edit: Thanks Vaqueroaero...
vaqueroaero
13th Apr 2006, 19:48
Due to the parasitic drag (surface friction and normal pressure)
Is it not profile drag rather than parasite?
Since tail rotors operate at a fixed RPM it is relatively easy to choose the foward sweep so that the two effects cancel, so you get lower input torque for the same rotor thrust. It also reduces tip noise.
So is there any reason as to why this doesn't work on the main rotor?
Here's my guess. The part of the tail rotor inboard of the airfoil part is a tube. It's too big to fit very far back on the skinny airfoil. So, they put it where it would fit, then used the forward sweep to get area ahead of the feathering axis to get the control forces low. They were able to get away with this due to the short, stiff characteristics of the assembly. Why nobody does it on a main rotor is because the aeroelastics of the long, wiggly blade would cause an instability with the tip jacked so far ahead of the feathering axis.
-- IFMU
The forward sweep allows a component of the centrifugal force to counter the drag, reducing the load on the hinge. I believe the R22 main rotor has and element of this in its design and I know that the Lynx MRH has the 4 blades shifted forward slightly for this reason.
The forward sweep allows a component of the centrifugal force to counter the drag, reducing the load on the hinge. I believe the R22 main rotor has and element of this in its design and I know that the Lynx MRH has the 4 blades shifted forward slightly for this reason.
If the MR blades are shifted forward, wouldn't that just be pre-lag? Shifting the whole blade, feathering axis and all forward of the hub centerline is a common tactic to reduce stresses with respect to bending in the lag direction. The S300 main rotor has this feature too.
-- IFMU
IFMU - yes you are right but I reckon the forward sweep will achieve much the same and is easier to engineer.
NickLappos
15th Apr 2006, 03:07
Flingwing is right, I think. The arrangement reduces the "propellor moment" which is the resistance to feathering that the blades present to the pilot when he pushes the left pedal.
The forward sweep increases the stress on the blade roots, if the blades were lagged, ("pre-lag") this would reduce the stress, as it would align the blades with the line of force they experience. This leading sweep increases the bending at the blade root.
Graviman
15th Apr 2006, 12:02
The arrangement reduces the "propellor moment" which is the resistance to feathering that the blades present to the pilot when he pushes the left pedal.
I was wondering why not just set the whole aerofoil forwards, or even just use an aerofoil with a rearward shifted centre of pressure. Of course this only works for one AOA, so forward sweep neatly covers the problem at all pedal positions. I imagine there must be a few forward offset rearward sweep designs too - this also balances the drag/centrifugal bending moment. Carefull tip design is probably the way to go for noise...
Mart
NickLappos
15th Apr 2006, 12:48
Graviman,
Just a guess, but I believe the design shown us is flight test fix for a basic problem found so late in the development cycle that this expediant was the fastest and cheapest way to get the pedal forces down without dedesigning the whole blade (a 2 to 3 year process.)
Graviman
16th Apr 2006, 11:04
Hehehe. Nick, is that an admission that even aeronautical engineers/TPs sometimes have to find a quick bodge? ;)
Mart
Flingwing207
17th Apr 2006, 11:03
I was wondering why not just set the whole aerofoil forwards, or even just use an aerofoil with a rearward shifted centre of pressure.
MartProbably because as IFMU said earlier, the blade root is just a tube progressively flattened out and bent forward. It forms the leading edge of the blade (with a bit of fiberglass and a stainless-steel skin wrapped around it). If you look at something like the new A109 blades, it seems they've used your ideas.