Vertcal Fin design alternatives

[Head Turner]

It's back to tail rotors.

I have a theory which I would like some contructive opinions on.

The Vertical Fin (VF) operates in forward flight to provide anti torque and stability. In the hover the VF is redundant and the Tail Rotor (TR) is the anti-torque device.

Question 1. Does the VF add to the work load of thr TR in the hover?

Question 2. If the answer to Q1 is 'yes' then would a castoring VF be a feasable design feature to unload the TR. By castoring I would have a VF with a suitable bearing in the vertical plane which would allow normal anti-torque operation in the cruise and castor to the airflow (avoiding hitting the TR as it castors) in hovering siyuations.

Question 3. If a castoring VF is not feasable, then an alernative would be a 'Slatted@ VF which should be more efficient in the cruise and then allow air to pass through during hovering.

Or have I just a vivid imagination?

The benefit in my ideas are that the power demand of the TR would be reduced in this design benefitting either smallr TR or better power available to fenestron types/Notars.

[Shawn Coyle]

Question 1. Does the VF add to the work load of thr TR in the hover?

Yes - there is some blockage of the tail rotor thrust. Look at some conventional tail rotor aircraft such as the A109E or the Kaman Sea Sprite or the UH-60 to see some cut outs where the compromise between forward flight directional stability and hover tail rotor thrust were made.

Question 2. If the answer to Q1 is 'yes' then would a castoring VF be a feasable design feature to unload the TR. By castoring I would have a VF with a suitable bearing in the vertical plane which would allow normal anti-torque operation in the cruise and castor to the airflow (avoiding hitting the TR as it castors) in hovering siyuations.

Castoring would be an interesting idea. But let's think of how much it would castor and when it would have to made 'fixed'. If it castored all the time, it would be ineffective as a vertical stabilizer in forward flight. So maybe it would need to be powered to some position - but which position, and controlled by what?
Neat idea, but it would need a lot of work.
There's a good reason why something like this hasn't been developed or used by now- but I don't know all the reasons.

Question 3. If a castoring VF is not feasable, then an alernative would be a 'Slatted@ VF which should be more efficient in the cruise and then allow air to pass through during hovering.

I think something like this has just been done by NASA, but I don't have any details at the moment.

Aside from the fenestron, the only other attempt to do this was the Ring-Fin from the Bell 400 series, and it appeared to do both jobs quite well. Why it was not taken up on later machines is a mystery.

[Flight Safety]

HT, a somewhat similar system does exist. The H-60 has a horizontal stab that pivots. It has a cockpit position indicator, an automatic movement system, and a manual movement override.

I'm not sure of these figures, but I believe the stab moves to the level position as 40kts or higher, and moves to 40 degrees front edge up below 40kts. Nick can comment on this, but I believe the idea is to prevent the main rotor downwash from blowing the tail down at low speeds.

[Shawn Coyle]

Nick can probably answer the question about the horizontal stab better than I can, but it was not put there for tail rotor downwash or main rotor downwash effects.
But a similar system for the vertical stabilizer might be an interesting idea, if we had a good airspeed indicating system that worked in all azimuths below 40 KIAS....

[Lu Zuckerman]

Unloading the tail rotor.

If I remember correctly the vertical fin of the AH1-J Cobra was cambered like a wing in order to generate sidward lift at higher forward speed.

[NickLappos]

On most helos the vertical fin is a small contributer to forward flight anti-torque, if the TR is lost in cruise, virtually all helos will have to autorotate because the fin is not a substantial anti-torque device. The exception is the fenestron, which needs large fins because it is relatively poor in yaw stability at cruise flight.

That being said, the vertical fin is not a big penalty in hover, it does cost about 2% tail rotor thrust due to blockage if the fin is close to the rotor, nil if not.

To pivot the vertical fin would introduce a raft of problems, including the weight and cost of the pivot mechanism, the failure modes of the system (imagine the fun if it pivoted at Vne!), and the cockpit indications.

The stabilator on the Hawk family was installed mostly to help make things handle well for the crew, reason 5 was the compelling reason why it was used at all:

1) Pitch damping - The stabilizer is big enough to provide the pitch
damping needed to stabilize the aircraft in high speed cruise, where
it acts like any normal horizontal stabilizer.

2) Entry into autos - It has a slow motion angle of attack change
capability that allows it to also keep the nose from dropping during
entries into descents and autos, where most helicopters need a lot of
aft stick to hold things level.

3) Sideslip stability - It also keeps the nose from pitching up or
down when sideslips are made at speed, because it detects lateral acceleration and puts some tail up or down to flatten the pitch attitude response.

4) Maneuvering stability - It also helps by detecting the bank angle in a turn and putting the nose down a bit, forcing the pilot to pull back on the stick during high bank angle turns. This gives the aircraft a pleasant aft stick pull to build g's in a turn, so-called
positive maneuvering stability.

5) Hover mode - It moves to the hover position (about 40 degrees
trailing edge down) when decelerating to a hover, so the rotor
downwash on it does not cause the nose to pitch up. This makes the approaches quite pleasant, with the nose staying flat, allowing fast decels and good visibility. At full aft CG, if the stab is not in hover position, the pilot should not make fast quick stops, to avoid running out of forward stick.

[Dave_Jackson]

Quotation from 'Letter to the Editor' ~ VERTIFLITE - Spring 2003

I remembered having recently greeted a Sikorsky engineer with a rather blank stare when he recommended that NACA/NASA study the tail rotor problem. I responded with the equivalent of "Tilt your head 90 degrees and use what we already know."

Never again would I think such thoughts! I now think of the tail rotor as immersed in a flow field which, at times, would make a gnat dizzy. But with a time averaged flow field having some resemblance to a vortex, said vortex rotating in a direction matching that of the individual blade-tip vortices????

Well, anyway, errrr, in certain flight conditions ........

I'm certainly glad we already know that the tail rotor is a practical device!!!

Frederic B. Gustafson
Hampton, Virginia

[Head Turner]

I have been experimenting with a modle with suitable designed slatted vertical fin and engine power as low as possible to compare performance with a solid section vertical fin. But without the computer data iit seems to fly in the cruise with a slatted fin of about 60% size of the cabbered profile design, and lighter. The hover performance can't really be judged.
The castoring vertical fin idea was a simple system linked into a similar mechanical design as the Agusta horizontal stabiliser of the A model and worked fine, The complexity appears to be no great headache as the MD Notars have adjustable VF's
Thank you for all your ideas.

[Dave_Jackson]

Little known bit of rotorcraft history.

Once upon a time, some rotor rooters spent many evenings drinking beer and discussing rotorcraft ideas. For years the only outputs from this group were 'letting off gas' and mad dashes to the urinal. Finally, their ultimate craft evolved and, quite naturally, they called it the 'Fart-n-Dart'.

Head Turner ~ Seriously, one of the features might be of interest.

They designed the fuselage so that it was aerodynamically in balance about its vertical axis. By doing this, a wind from any direction would not yaw the craft. This allowed the yaw control to be handled by a very small tail rotor [The smaller the better. Infinitely small being infinitely better ~ but that's another story. ]

In forward flight they need more "drag' at the rear of the craft to hold the course, of course. To achieve this they had a vertical stabilizer with a small chord. The trailing edge of the vertical stabilizer had a slat, similar to a rudder but this slat only slide forward and aft. It did not pivot. As the craft's forward velocity increased, this trailing edge slat extended back. This increased the vertical stabilizer's chord and resisted unwanted yaw. During rearward flight the chord decreased.
_______________________________________

The helicopter was not a big seller. Some say it was because too much time was spent in the engineering department and too little time in the marketing and legal departments. A few suggest that it might have been the name.

Anyone have additional information on the 'Fart-n-Dart'?



Edited to record publication of concept ~ OTHER: Aerodynamics - Rotorcraft Concept - Variable Vertical Fin

[NickLappos]

The idea that the size of the vertical fin somehow slows the yaw rate in a hover is flawed to start with. Then to make a fancy contraption to reduce this non=loss is folly, then to make another contraption to fix the poor yaw handling at cruise is further folly.

Why not look at the initial problem that you think exists - remove the vertical fin, run the tail rotor as is and measure the "improvement" in hover performance and yaw rate. Report the findings here, and prevent Dave from spending all those billions of electrons here in pprune fixing fictional problems!

At a yaw rate of 30 degrees per second, a vertical fin on an S76 has a velocity of 13 feet per second. Its drag coefficient at 90 degrees is probably 1.3, so it resists the yaw with a grand force of about 8 1/2 pounds. To chase this 8 1/2 pounds (at a 30 degree per second hover!) Dave would redesign the whole ass end of the machine!

[low height bug]

Doesn’t the vertical surface become an issue in azimuth manoeuvres? I remember seeing data from a Brand X a/c that showed that the pedals were close to fsd a much lower speed when the a/c was travelling at 300 to 330 deg azimuth in comparison to the other azimuths. I concluded (possibly erroneously) that the relative direction of the airflow over the vertical surface was causing a horizontal “lift” force requiring the tail rotor thrust to be reduced to maintain the a/c heading. Again the beer induced assumption was that any increase in speed would have resulted in the a/c turning towards the direction of flight.

lhb

[Dave_Jackson]

Hi Nick,

You have provided me with as much information as anyone has. For that, I am truly appreciative.

But, for the pleasure of pushing the envelope, or pushing buttons, the following is offered.

Your 13-feet per second represents a crosswind of 9-miles per hour. Drag is based on the square of the wind's velocity, therefor a crosswind of 25-miles per hour will exert a force of 68 pounds.

Assuming that the tail-rotor is capable of delivering 4 pounds of thrust per horsepower, then the required power is ( 68 lbs / 4lbs ) = 17 hp.

The craft's gross weight to power is (11,700 lbs / 1600 hp) = 7.3 lbs per hp, therefor the weight loss is (17 hp * 7.3 lbs) = 124 lbs.

The craft's useful load is 4627 lbs., therefor the loss of payload is (124 lbs / 4627 lbs) = 2.7%.


Margin of error; +/- 2000%.
Sources of information; ~ Sikorsky. S76 ~ Drag.

Perhaps the advantage may not be significant for a single rotor helicopter; particularly if part of this VS is necessary to 'balance' the drag at front of the craft. For a helicopter, with dual laterally mounted rotors, which are in close proximity to each other, it might be a big advantage. This will be particularly true if the yaw is by opposed longitudinal cyclic.

[SASless]

Nick,

The "A" model Chinook had a "sharp" tail fin.....and the subsequent models have all had the "square" butt design. I have been led to believe that simple change equalled the effect adding 37 feet of aft vertical fin to the aircraft. I know the stability increase betweent he "A" model and the others is tremendous thus the change was most effective. Did ya'll at Sikorsky ever play with that concept on any of your machines.....squaring off the fin to enhance stability in forward flight?

[NickLappos]

Dave,
Your logic is impeccable, if the fin were the only thing flying, but the body and rotor are there too. The total power needed in the side flight goes down by an enormous factor as the wind speed increases, usually the first 10 knots equals about 10% increase in hover gross weight. The main rotor does not care where the wind is from, it starts gaining performance at 8 knots or so, even in a tail wind. Since nobody allows increase in gross weight to account for wind, the net gain with the fin is over 7.5% (2.5ish from the 10) in hover, of course because of the wind!

Sasless, the Chinook family is a hurtin configuration in yaw, because the twin rotor provides no natural yaw stability. the tail rotor on a single rotor config is powerfully stabilizing, the rotor area is equal to a fin of 4 to 8 times that area, so little vertical fin is needed for stability. The exception is a fan-in-fin, which has little addition to the yaw stability, and so needs extra fin area 9see the end plates on most EC machines with fenestrons, and now on Comanche).

[[email protected]]

Lowheightbug, I think the helicopter you describe probably had an anti clockwise (when viewed from above) rotor so the powerful retreating blade vortices are on the port side of the aircraft. If you now blow these vortices by introducing a wind from the 10 o'clock or by hover taxiing left and forward, the vortices get shed into the tail rotor, thus reducing it's efficiency and therefore requiring more left pedal to produce the correct anti torque thrust.

[Dave_Jackson]

Nick,

"The total power needed in the side flight goes down by an enormous factor as the wind speed increases"

Strong argument! The main rotor will provide more thrust during 'transitional' flight.

Unfortunately, the same cannot be said for the tail rotor.

[NickLappos]

Dave,
It is interesting that the tail rotor typically needs no more power at side flight conditions, all the way out to very high speeds (35 knots anyway). The extra pitch range needed and the extra pedal travel that we see are needed to create the angle of attack needed for the thrust, the actual torque needed by the tail rotor usually drops as side flight speed is increased, after about 10 to 12 knots or so.

[MaxNg]

Nick

Is this apparent reduction in tq needed ( above 10 kts sideways) by the tail rotor a result of the main rotor translating and therfore nesesitating in a reduction in tq (pitch) and consequently less tail rotor pitch (t/r tq) ?

[NickLappos]

Max,
The effect you describe is quite important to side flight, since the tail rotor has to work less as speed increases and main torque drops. In fact, the tail rotor itself sees less power need as it moves into the free stream, much as the main rotor needs less power in vertical climbs.