horizontal stabilizer design question

[chris pochari]

Curious as to why some helicopters, bell 407, 429, as365, ec135, ec145 have fins at the end of the horizontal stabilizers and so many others like the as350, ec120, ec130, uh 1, have regular horizontal stabilizers. If the advantages to the fins at the end of stabilizer is so great why don't all helicopters have them? And it seems like most attack helicopters, ah 1, ah 64, mi 24 hind have conventional stabilizers.

Sorry if this question seems stupid I'm just trying to learn as much as I can. A lot of this I can't get from my already massive helicopter book collection

[Eric Hunt]

The vertical fin/endplate does a couple of things - it makes the horizontal part more efficient by reducing the vortex around the tip (see the number of airplanes that have winglets for this purpose)

Secondly, they add to the vertical fin area to give some directional stability. But on the LongRangers, the tailboom is longer than the JetRanger, and the normal vertical fin, being at a longer moment, has more effect anyway - so surprisingly, the endplates are turned in the opposite direction to the fin to take away some of the (excessive) stability.

 

The 135/145/ BK 117 family only have a shortish tailboom and need to generate a bit more force, hence the endplates.

 

But wait for some designer to come along and give more info, or maybe iChris who seems to have access to a lot of electronic information to give you the real story in words of one syllable or less.

[CharyouTree]

The AH-64, much like the UH-60 doesn't actually have a stabilizer, but in fact a stabilator. If the apache is anything like the Blackhawk, it's automatic variable angle of incidence based mostly on airspeed, but also lateral accelerometers (out of trim conditions) and pitch rates (gusts) can make it adjust in small up or down increments. The hawk is also connected to the collective position transducers to reduce "collective up, nose up" variations due to increased/decreased downwash on the tail boom.

 

As far as the vertical bits, I assumed they were like winglets as well, so I can't add anything Eric didn't already cover.

[Azhigher]

The 206/407's need all the TR help they can get, hence the endplates. The AS350 just has a massive rotor with TR authority abounding, so no need. The 130 has such a giant vertical stabilizer (Sail) that it doesn't need any more hanging out on the vertical stabs.

 

/sarcasm

 

I... have nothing useful to add.

[Guest pokey]

This is a bit of a different situation than the winglets on fixed wings. Back prior to successful results from the Sperry autopilots, Piasecki found that their HUP Retriever tandem wanted to fly 'sideways' (because each rotor was searching for 'clean air'). They put on these huge barndoor style endplates in an effort to make it fly rite.

 

[iChris]

If the advantages to the fins at the end of stabilizer is so great why don't all helicopters have them? And it seems like most attack helicopters, ah 1, ah 64, mi 24 hind have conventional stabilizers.

 

horizontal stabilizer design differences1.jpg

 

Eric Hunts post above stated the main reasons for these endplates.

 

Rather than being directly behind the fuselage and rotor hub, some engineers found that placing vertical endplates on the end of the horizontal stabilizer made for a more effective horizontal stabilizer and vertical endplate, since the endplate is out in relatively cleaner air. The vertical endplates being displaced from the tail rotor, also minimized their effect on tail rotor flow patterns.

 

These vertical endplates aid in unloading the tail rotor thereby further reducing tail rotor flapping in forward flight. Furthermore, endplates are often incorporated to address directional stability issues. Engineers have difficulty predicting analytically whether a design will be stable or unstable. The aerodynamic environment at the tail rotor and empennage are influenced by the disturbances produced by the main rotor and fuselage.

 

As an example, this helicopter with SAS off, Dutch roll was unstable without endplates on the horizontal stabilizer but stable when small canted endplates were installed. The dramatic change in stability was much more than could be predicted by any known analytical method and must be attributed to some peculiar flow conditions in the main-rotor wake. In the final design, they went from the T-Tail to a conventional horizontal stabilizer and found endplates were not necessary. Overall design considerations, trade-offs, advantages, and disadvantages along with flight test, weigh-in to determine whether endplates are necessary.

 

 

Edited April 9, 2017 by iChris

[chris pochari]

 

Eric Hunts post above stated the main reasons for these endplates.

 

Rather than being directly behind the fuselage and rotor hub, some engineers found that placing vertical endplates on the end of the horizontal stabilizer made for a more effective horizontal stabilizer and vertical endplate, since the endplate is out in relatively cleaner air. The vertical endplates being displaced from the tail rotor, also minimized their effect on tail rotor flow patterns.

 

These vertical endplates aid in unloading the tail rotor thereby further reducing tail rotor flapping in forward flight. Furthermore, endplates are often incorporated to address directional stability issues. Engineers have difficulty predicting analytically whether a design will be stable or unstable. The aerodynamic environment at the tail rotor and empennage are influenced by the disturbances produced by the main rotor and fuselage.

 

As an example, this helicopter with SAS off, Dutch roll was unstable without endplates on the horizontal stabilizer but stable when small canted endplates were installed. The dramatic change in stability was much more than could be predicted by any known analytical method and must be attributed to some peculiar flow conditions in the main-rotor wake. In the final design, they went from the T-Tail to a conventional horizontal stabilizer and found endplates were not necessary. Overall design considerations, trade-offs, advantages, and disadvantages along with flight test, weigh-in to determine whether endplates are necessary.

 

 

Wow, thank you very much