Sideslips?

[FlyAF]

Found this excerpt online from Frank Robinson on R22s and don't quite understand what this man means when talking about adverse roll characteristic. What is a good roll characteristic when it comes to helicopters and why is rolling into the turn better during a sideslip? Negative Dihedral???

SIDESLIP WITH THE R22
Concerning the caution against excessive sideslips in the R22 flight manual, this was in part due to a misunderstanding by the FAA. In the Army training film on mast bumping, it showed excessive side slipping as one cause of mast bumping. This was true for the Army Bell Cobras and Hueys, because both aircraft have high centers-of-gravity and relatively low side silhouettes due to their high-mounted powerplants and low-mounted tailcones. During a severe sideslip, the resultant fuselage drag could be below the center-of-gravity and cause the helicopter to roll out of a turn, instead of into the turn, i.e. a negative dihedral or adverse roll characteristic. Airplanes prevent this by having wings with positive dihedral.

I am just a wee student pilot, any references/knowledge and fun facts anyone has on Sideslips would be greatly appreciated, Thank you

[Disguise Delimit]

You might be wanting to slip to the right during descent, so your nose is pointed left of the flight path. You will have some right cyclic input to start the flight path in that direction, but then take some of it out to maintain the slip. The drag on the fuselage will want to swing the body further out to the left, as it hangs under the mast. This brings the mast to lean further to the right, and reduce the clearance between the rotor head and the mast. 

You don't want to roll quite as much as this drag is making you do, so you counter with a bit more left cyclic. This further reduces the clearance, and it can be possible to then get a mast bump if anything upsets the balance. You will also be doing a tap-dance on the pedals to keep the nose pointing where you want it, and the secondary effect of pedal is roll - which might be the tipping point for a mast bump.

[iChris]

On 2/6/2024 at 8:26 PM, FlyAF said:

Found this excerpt online from Frank Robinson on R22s and don't quite understand what this man means when talking about adverse roll characteristic. What is a good roll characteristic when it comes to helicopters and why is rolling into the turn better during a sideslip? Negative Dihedral???

I am just a wee student pilot, any references/knowledge and fun facts anyone has on Sideslips would be greatly appreciated, Thank you

Blade flapping results from conditions referenced to the flight path (relative wind) rather than the orientation of the fuselage. In forward flight, with no sideslip and the rotor trimmed perpendicular to the shaft, the helicopter is in equilibrium. Let's simulate a theoretical side slip condition. Suppose the flight direction is suddenly changed so that the helicopter is flying directly to the right without changing fuselage heading or control settings. In that case, the blade over the tail becomes the advancing blade and the one over the nose the retreating blade. Since the cyclic pitch no longer corresponds to trim conditions, the rotor will flap down on the left side and up on the right side. The asymmetrical velocity distribution over the rotor tilts the thrust vector to the left, creating a moment arm about the CG and producing a rolling moment to the left, countering (dampening) the right roll. 

 Notwithstanding this theoretical side slip condition, sideslip angles are far less than 90°. Still, the trend is the same: helicopters roll away from the approaching wind (sometimes called blowback or flap-back). The same characteristic found on airplanes with dihedral (both wings slanted up) is referred to as positive-dihedral on rotors, even though the source is different. Positive dihedral is only possible for the R22 and other teetering rotor system helicopters if the rotor is positively G-loaded, allowing it to produce thrust. Without a viable thrust vector, rotor flapping alone cannot change the fuselage attitude. 

 Positive dihedral is a desirable characteristic that helps the pilot. With a negative dihedral, a sideslip would tend to roll the aircraft into an ever-tightening right roll. Too much positive dihedral, on the other hand, can also be undesirable, often causing an over and undershooting around an equilibrium position, increasing the pilot's workload.

Don't forget all helicopters have a small degree of inherent sideslip, and any excessive or unnecessary sideslip increases rotor flapping. Moreover, low-G and negative-G flight environments eliminate rotor dampening on teetering rotor systems. A good roll characteristic is one in which the helicopter can roll into a coordinated turn with equally balanced lateral acceleration. Like in level flight, the pilot will feel no left or right acceleration (slip or skid), remaining straight in the seat. The radius of the turn held constant throughout.

Dihedral Effect: the tendency of the helicopter to roll away from the approaching air during sideslip. 

Dihedral Effect
Slip vs Skid

Edited February 16 by iChris

[FlyAF]

On 2/15/2024 at 11:43 PM, iChris said:

Blade flapping results from conditions referenced to the flight path (relative wind) rather than the orientation of the fuselage. In forward flight, with no sideslip and the rotor trimmed perpendicular to the shaft, the helicopter is in equilibrium. Let's simulate a theoretical side slip condition. Suppose the flight direction is suddenly changed so that the helicopter is flying directly to the right without changing fuselage heading or control settings. In that case, the blade over the tail becomes the advancing blade and the one over the nose the retreating blade. Since the cyclic pitch no longer corresponds to trim conditions, the rotor will flap down on the left side and up on the right side. The asymmetrical velocity distribution over the rotor tilts the thrust vector to the left, creating a moment arm about the CG and producing a rolling moment to the left, countering (dampening) the right roll. 

 Notwithstanding this theoretical side slip condition, sideslip angles are far less than 90°. Still, the trend is the same: helicopters roll away from the approaching wind (sometimes called blowback or flap-back). The same characteristic found on airplanes with dihedral (both wings slanted up) is referred to as positive-dihedral on rotors, even though the source is different. Positive dihedral is only possible for the R22 and other teetering rotor system helicopters if the rotor is positively G-loaded, allowing it to produce thrust. Without a viable thrust vector, rotor flapping alone cannot change the fuselage attitude. 

 Positive dihedral is a desirable characteristic that helps the pilot. With a negative dihedral, a sideslip would tend to roll the aircraft into an ever-tightening right roll. Too much positive dihedral, on the other hand, can also be undesirable, often causing an over and undershooting around an equilibrium position, increasing the pilot's workload.

Don't forget all helicopters have a small degree of inherent sideslip, and any excessive or unnecessary sideslip increases rotor flapping. Moreover, low-G and negative-G flight environments eliminate rotor dampening on teetering rotor systems. A good roll characteristic is one in which the helicopter can roll into a coordinated turn with equally balanced lateral acceleration. Like in level flight, the pilot will feel no left or right acceleration (slip or skid), remaining straight in the seat. The radius of the turn held constant throughout.

Dihedral Effect: the tendency of the helicopter to roll away from the approaching air during sideslip. 

Dihedral Effect
Slip vs Skid

Man thank you so much for taking the time to explain that it helped tons, still digesting and working through some of the technical points of what you're saying. Haha took me too long and a re-read to realize "TV" meant Thrust Vector in that illustration, by the way a beautiful drawing of an R22 lol. So with a hypothetical EXTREME negative dihedral, side slipping would almost immediately tighten that right roll, and bring the rotor mast intoooo mast bumping fast af? Just asking to check my understanding on that part. Anyway thank you again!!

[iChris]

On 2/6/2024 at 8:26 PM, FlyAF said:

During a severe sideslip, the resultant fuselage drag could be below the center-of-gravity and cause the helicopter to roll out of a turn, instead of into the turn, i.e. a negative dihedral or adverse roll characteristic. Airplanes prevent this by having wings with positive dihedral.

 

On 2/18/2024 at 10:02 PM, FlyAF said:

So with a hypothetical EXTREME negative dihedral, side slipping would almost immediately tighten that right roll, and bring the rotor mast intoooo mast bumping fast af? Just asking to check my understanding on that part. Anyway thank you again!!

Don't get too hung up on the negative dihedral end of the characteristic; the positive dihedral respective to rotors is a characteristic similar to an airplane with wings slanted up; however, the source of the effect is different. On an airplane with wings slanted up, that positive dihedral characteristic is part of the design; it will always produce a positive effect. Being true to its characteristics, the rotor will always flap away from the relative wind regardless of the fuselage's orientation. In this case, the negative effect is due to the sideslip and the relative wind striking the right side of the fuselage, creating the right roll. However, you are correct; hypothetically, a negative dihedral on the part of the rotor would exaggerate the right roll.

 It's not the right roll that's the problem; it's the pilot's reaction to the right roll with left cyclic. If there's a positive G load on the rotor, the positive dihedral (dampening) automatically assists in correcting the roll rate due to the sideslip, and the pilot reacts with a moderate amount of cyclic force to level the helicopter. However, if there's no positive G load on the rotor to produce thrust and dampening, the result is an increasing roll rate. If the pilot reacts abruptly with the left cyclic, there's a problem. The pilot's initial response with cyclic yields no response as the rotor is unloaded and cannot produce thrust to change the fuselage attitude. The fuselage and the rotor shaft are rolling to the right, while the abrupt application of cyclic has the rotor flapping abruptly to the left, prelude to mast bumping.

In Flight Mast Bumping

Edited February 28 by iChris
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