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I'm a bit confused on the purpose of the horizontal stabilizer. From my understanding it's purpose is to keep the fuselage more or less straight and level during forward flight. If that is true then why does the fuselage follow the cyclic inputs so closely rather than moving just the rotor disc?

 

Also, I'm curious on how stable helicopters like the Bell 47 or Lama are in forward flight because they lack vertical stabilizers. Does this mean that the pilot must constantly use pedal input to keep the nose straight?

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In the basic choppers like the R22, the stabiliser is in a fixed position, meaning that it is set for the cruise position. Any airspeed other than that will have an attitude that is different.

 

More expensive machines have a moving "stabilator" which makes better adjustments for (usually) cyclic position, rather than airspeed. The modern electronic machines will automatically adjust the stabilator with airspeed to give the best results.

 

The 47 has tiny winglets on the end of the horizontal stab, but it is still pretty twitchy, despite being a huge blunt object flopping along at 60 kt. And any pilot worth the name is tap-dancing on the pedals to stay perfectly balanced, vertical stab or not. It should be automatic - raise the lever, add some left pedal and forward stick, and vice-versa.

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I don't recall the 47 (TH-13) being that twitchy, but perhaps that was because I went to it from the TH-55, which was really twitchy. The TH-13 was my instrument trainer, and it had one asset: If you could fly instruments in it, you could fly instruments in anything. I went from that to the UH-1, and thought I had died and gone to heaven.

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.

 

More expensive machines have a moving "stabilator" which makes better adjustments for (usually) cyclic position, rather than airspeed. The modern electronic machines will automatically adjust the stabilator with airspeed to give the best results.

 

.

which ones would these be?

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I'm a bit confused on the purpose of the horizontal stabilizer. From my understanding it's purpose is to keep the fuselage more or less straight and level during forward flight. If that is true then why does the fuselage follow the cyclic inputs so closely rather than moving just the rotor disc?

 

Also, I'm curious on how stable helicopters like the Bell 47 or Lama are in forward flight because they lack vertical stabilizers. Does this mean that the pilot must constantly use pedal input to keep the nose straight?

 

The horizontal stabilizer allows the aircraft to fly closer to a level pitch attitude at speed. Cyclic input still slants the disk allowing speed and direction changes. The fuselage will follow cyclic input, except that some nose down will be minimized.

A fixed horizontal stab's tail down/nose up effect will vary with speed, the faster you're traveling the more nose up it generates, but a fixed stab will not maximally level the fuselage in any other condition than that designed for, a particular weight, power, speed and atmosphere. All the other flight conditions will be compromises where the stab will have varying effects on level pitch.

If the horizontal stab is in the hovering rotor downwash column, like many Bells, it will keep the nose higher than a stab outside the downwash, as in many Hughes designs, which will hover more nose down. You need to think about the horizontal stab if you have any wind at the hover.

 

Some helicopters have cyclic linkages that change the pitch of the horizontal stab, like the Huey, where forward displacement pitches the stab more and more nose down until an engineer's calculation indicated that more forward should reduce the nose down to compensate fore increased speed. Some helos have 'elevators' on the stabs that do essentially the same thing. These are fixed mechanical linkages and compromises in other than assumed designed parameters. The pilot still needs to consider wind at a hover.

 

So far, I've been harping on relative wind directions because you can turn the tail and stab into/out of the wind and be surprised by helicopter attitude changes if you weren't anticipating the effect. The TH55/300 series had a related issue in that horizontal stab was slanted, taking the aircraft out of aerodynamic trim in one yaw axis is/was a very different experience than a yaw the other way. Once burned, twice shy...

 

I am told that some modern designs have adjustable horizontal stab incidence, some even automated for efficiency.

 

As to real old helos and vertical stabs, they doesn't seem to have been a consideration for a fixed airfoil to act in the yaw axis in a lot of designs. I seem to remember a triangular mini-fin on the aft end of the TH 13 tailboom where the TR drive shaft makes that little bend. I remember the 47 as being very stable compared to the TH 55.

Edited by Wally
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lets get to the "meat" of the TH-55 compared to the civilian version,:

 

1) the military took it upon themselves to chop about 2 inches off of the horiz stab, & install a 'stall strip',,,, AKA a small piece of angle "iron" aluminum strip on the top leading edge. (Now? let the TH-55/300 talk begin)----any one that has ever auto'd a 300 vs TH-55 knows there is a HUGE difference in nose down attitude.

 

2) the "propeller" governor ! ! was nothing more than a "hammer" that you could feel in the throttle that let you know "back off! you dumb-a*^"

 

3)??? we will leave it up to you 300 experts out there--(hint) was also a military mod.

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I'm a bit confused on the purpose of the horizontal stabilizer. From my understanding it's purpose is to keep the fuselage more or less straight and level during forward flight.

 

If that is true then why does the fuselage follow the cyclic inputs so closely rather than moving just the rotor disc?

 

Also, I'm curious on how stable helicopters like the Bell 47 or Lama are in forward flight because they lack vertical stabilizers. Does this mean that the pilot must constantly use pedal input to keep the nose straight?

 

The purpose of a horizontal and vertical stabilizer is to enhance the helicopter’s pitch and yaw stability. The helicopter is aerodynamically unstable being a streamlined body with a large cross-section area forward of the CG. The main rotor is another factor, even though having speed stability, it exhibits angle of attack instability with respect to pitch. These pitch and yaw deviations add up to an increased workload for the pilot.

 

Since the helicopter is designed with cyclic control, the pilot can apply control moments about the helicopter’s CG to overcome any inherent Instability. As long as the pilots margin of control is greater than the margin of instability, the pilot can learn to control an unstable helicopter, just as he first learned to hover.

 

Early Sikorsky helicopters did not incorporate horizontal or vertical stabilizers. The R-4, H-5, and R-6 were flown successfully without horizontal or vertical stabilizers. The pilot via cyclic control tamed the aircraft’s inherent instability. With respect to the lack of a vertical stabilizer, the conventional tail rotor took up the slack, producing excellent yaw control, damping, and directional stability on Its own accord.

 

However, over the years, manufacturers realized that the horizontal and vertical stabilizer were essential to enhanced stability and improved flying qualities. As always there are trade-offs, increased stability often means decreased controllability. In general, pilots can deal with some instability; therefore, most prefer controllability and maneuverability over more rigid stability. There’s also some aerodynamic concerns with respect to the size and placement of the horizontal stabilizer. These concern the sudden changes in download and upload on the horizontal stabilizer due to the main rotor wakes impingement during the transition from hover into forward flight and the relative airflows accompanying high rates of climb and descent.

 

As an example, the Bell 206 has the horizontal stabilizer farther forward on the tail boom to place the stabilizer in the main rotor wake during the hover. This makes for a smoother transition as the helicopter transitions from a hover into forward flight, thereby avoiding any abrupt trim changes as the rotor wake moves aft of the horizontal stabilizer. The trade-off with the Bell solution (placing the stabilizer in the main rotor wake) is a reduction in hover performance which is minimized by its low disk load. The UH-60 has a so-called processor controlled stabilator (combination stabilizer elevator) that more or less aligns itself with the rotor wake during the hover and moves to a more horizontal position as the helicopter moves into forward flight.

 

So we have lots of configurations between manufacturers with horizontal stabilizers in many different sizes, angles and positions, forward mounted, aft mounted, T-tail, half tail, V-tail, free-floating, cyclic linkage, processor controlled, etc. With any of these configurations, one of the main objectives is to avoid abrupt trim changes by managing the rotor wakes transition across the horizontal stabilizer.

 

The horizontal stabilizer can have an adverse aerodynamic effect during autorotation. We not only have to deal with main rotor wake impingement, but we also need to deal with the relative air flow due to high rates of climb and descent. In the case of autorotation, an upload due to the rate of descent can cause an unwanted nose down pitch moment. The Bell designs use an inverted airfoil for the horizontal stabilizer which creates a download in forward flight to keep the fuselage at an angle of attack for lowest drag. It is also designed to stall when the helicopter is in steep autorotation to avoid an excessive nose down pitch moment. In contrast, the UH-60 eludes this autorotation pitfall via its processor controlled repositioning of the stabilator’s angle of incidence based on airspeed, collective position, and pitch rate.

 

As mentioned in Pokey’s post, The TH-55 had a similar autorotation problem, excessive nose-down pitching and left rolling motion. It was handled with a configuration change that reduced the span of the horizontal stabilizer in addition to an upper-leading-edge spoiler which improved the nose down pitching characteristic without degrading other handling qualities.

Edited by iChris
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