Helicopter tails, thick vs slim ones

[helmutkohl]

I don't know how to explain it

but basically on some helicopters you see a thick tail like this, its basically a smooth slope from the main body to the tail rotor

on other helicopters, you have thinner tails like this

are there any pros and cons to these designs?

I assume one pro to the thinner tial is the ability to have a rear ramp/door

 

[yasotay]

Mission, weight, survivability. Modern combat helicopters tend to have 'thick' tails as they are designed to take hits from heavy caliber weapons. Many of the thin tail aircraft are older models (like the Mi-2 above) that were concerned with weight much more as the engines did not produce the power of modern turbines, or they are civil platforms that are concerned with minimizing weight for cost considerations.
From a combat helicopter perspective I would much prefer a thick tail boom as there is more open space to miss critical components and more internal strengthening. There is a well known survivability lesson that fighters are more susceptible to damage because all of the critical systems are stuffed into a smaller space. This holds true for helicopters as well. While modern missiles make this somewhat less valid, there are still a lot more bullets being launched at helicopters than missiles.
Modern engineering technology might make this less valid, but the number of "thin tail combat helicopters being proposed in minimal.

 

[riggerrob]

From the title, we got the initial impression that you were asking about the difference between stock tail fins on Bell UH-1 Huey versus the slim chord tail fins update kit offered by Boundary Layer Research out of Washington State.

BLR offers a half-chord tail fin to reduce fin area while increasing airflow through the tail rotor in the hover. Since many civilian Hueys spend most of their working lives in the flying crane mode, it is cost-effective to optimize them for control while hovering.

BLR also offers single strake and double strake kits that can be added to the circular cross-section tail booms of Jet Rangers, Hueys, A-Stars, etc. BLR strakes "kill" adverse yaw (lateral lift) created by main rotor down-wash while hovering. This reduces the load on the tail rotor and reduces the incidence of reversed flow. Reversed flow can be especially problematic when hovering with a cross-wind. Strakes "stall" any un-wanted lift. The Canadian Armed Forces fitted similar strakes to their Sikorsky Ch-124B Sea KIng helicopters during the mid-1980s.
Deep tail booms can provide more side-ways lift ... er ... help the tail rotor ... because their oval airfoil section produces more lift.

 

[riggerrob]

Small diameter tail booms retain the correct spacing (between the main and tail rotors) at the lightest weight (Robinson, Jet Ranger and the PZL pictured above).
OTOH deeper, slab-sided tail booms can provide structural or aerodynamic advantages. Consider how Sikorsky Sea King S-61 has a deep, slender tail cone that ends in a rectangular bulkhead that holds tail pylon hinges. The deeper tail cone allows for wider spacing on hinges and more room for fold hydraulic cylinders, control cables, etc.
Note how later, larger Sikorskys (CH-53 Stallion and Skycrane) just use rectangular bulkheads in their tail booms.

For aerodynamic advantages, see my post above about how Boundary Layer Research offers STC kits that help the tail boom to help the tail rotor in providing anti-torque side-ways thrust.

 

[publiusr]

The thickest were fat ducts for NOTARs as I recall.

 

[Dynoman]

I don't think the tail boom of most helicopters, including attack helicopters, provides significant ballistic protection. I would think that the stresses (loads) associated with maneuvering, which would be transferred across a larger tail boom would protect the tail rotor transmission from bending moments. Locating the transmission shaft on top of the boom would help to reduce impacts from below (e.g. terrain, small arms gunfire, etc.) Here are a few tail sections from attack and military utility helicopters. Note: the attack helicopters longerons, stringers, and formers are positioned closer together than civilian helicopters. The additional structure is able to absorb more damage than the lighter designs.

 

[Mark Nankivil]

The thickest were fat ducts for NOTARs as I recall.

Which is a large air duct for the NOTAR system. Might have the control runs for the vents but nothing else.

Mark

 

[TomcatViP]

A large tail section is favorable for torque structural resistance (see inertia matrix properties in your old mechanical science book).

That's the infamous something / I in the classical beam theory, where I stands as the beam section matrix of inertia.

In other words, it might be more interesting to increase the section size of the tail boom to counteract the tail rotor torque while still keeping the mass low.

Aerodynamics variables also play in, with the smooth transition of fuselage to tip tail having a better Cx in forward flight (cruise regime) if that has to be favored.

Mechanical considerations also impact the overall choice of configuration, with a shaft kept as straight as possible as it is the most efficient and reliable.

 

[helmutkohl]

i do agree that in general most military applications of helicopters tend to have thicker booms

although there are some exceptions as far as modern helicopters go

the Dhruv

Ansat

not sure where to place the H160 as its somewhere in the middle, but its interesting they opted for that tail over the thicker one its predecessor had. the Dauphin

 

[Silencer1]

IMHO, the comparison of similar size/purpose helicopter could be more useful. For example, compare modern assault helicopters or transport ones.
I see, that most single-rotor attach helicopters have "slim" booms, and sole exclusion is coaxial-rotor Ka-50.
Transport helicopters are much more different: however, presence of large clamshell doors didn't made similar Mi-8 and Mi-26.

 

[publiusr]

With a hollow fat tail...with shafts enclosed-might you have it be a coandish duct-jet...serve some other purpose?

 

[yasotay]

i do agree that in general most military applications of helicopters tend to have thicker booms

although there are some exceptions as far as modern helicopters go

the Dhruv


Ansat


not sure where to place the H160 as its somewhere in the middle, but its interesting they opted for that tail over the thicker one its predecessor had. the Dauphin

I might argue (incorrectly perhaps) that these three helicopters started out as civil projects that became military utility helicopters.

 

[riggerrob]

With a hollow fat tail...with shafts enclosed-might you have it be a coandish duct-jet...serve some other purpose?

To truly utilize Coanda Effect, the hollow tail boom needs an internal fan and narrow slots down one side to blow air parallel to the outer skin similar to a McDonnell-Douglas NOTAR.
The external strakes (ala. BLR) just stall airflow when cross-winds come from the wrong side.

 

[publiusr]

I wasn't think NOTAR...maybe a gas turbine exhaust or something...seems like chopper lay-outs had less tinkering done than fixed wing designs. I like trying to imagine different lay-outs.