While today's NOTAR system uses a stand-alone fan, Bell's propulsive anti-torque system (PATS) concept, proposed for UCAR and being considered for MAPL, would use PT airflow. PATS would add an LP fan to 'wrap' the PT stream with a cooler airflow, resulting in higher mass flow and lower exhaust temps.

I/C

Call me crazy but . . .
 

Another crazy thought,

Mr. Lappos said the S76 engines produce from the exhaust about 100 pounds of thrust a HIGH SPEED,

but the thrust the engine is producing to give 100 pounds of thrust at HIGH SPEED, I imagine could produce a lot more thrust in a hover, a LOT more.

I'm not really sure how to explain this, when I learned to fly jets (business jets) It was explained to me that suppose you were standing on the brakes of a Lear on a runway, then went to TAKEOFF power setting of say 97% N1, the engine would be producing a "jet" of air that had a velocity of say 50 ft/sec, since the air surrounding the engine exhaust was near zero the, the difference between the 50 ft/sec "jet" of air and the surrounding relatively still air is what gave you thrust.

Then as you let go of the brakes and started to accelerate down the runway, with THE SAME POWER SETTING and N1 speed, what would happen when the aircraft reached an equivalent speed to 50 ft/sec?

If the velocity of the "jet" of air coming out of the turbofan would equal that of the surrounding air, you would theoretically stop having thrust and thus accelerating.

To solve this I know as the aircraft accelerates, more air is pushed inside the engine, more air goes thru the engine and thus the thrust increases, without moving the power setting, so in reality when the aircraft is already accelerated to 50 ft/sec, the engine is now producing a "jet" of air much higher than 50 ft/sec to keep producing thrust, say now 100 ft/sec, BUT THE INTERESTING PART IS THAT IT'S WITH THE SAME 97% N1 POWER SETTING!

So now a turbofan engine that was producing a STATIC 4,000 lbs. of thrust is now producing twice that because of the all the air that is being rammed into the inlet at high speed, a turbofan's engine inlet has the benefit of being exposed to the relative wind, and its easy to understand how this happens, but a helicopter with its particle separators, or ducting is not subject to same, or is it? Think of the Bell 212/412 inlets that actually have to suck air away from the particle separator duct?

So my point is that if an S76 produces 100 pounds of thrust at HIGH SPEED, with a power setting of say 90% N1, without the benefit of much ram air (remember particle separators, and indirect inlet) increasing considerably the amount of air going try the engine, and thus exhaust. Then the amount of thrust produced at a hover with the same 90% N1 power setting would be a lot more than 100 pounds according to my uneducated guess, remember you are a hover and the air surrounding the engine outlet is almost perpendicular to the exhaust due to downwash.

I'm sorry if my explanation isn't good but English is a second language to me and its sometimes had to explain myself, feel free to tell me I am crazy for thinking this.

Ian, Hilife,

Thanks so much for the reply, I will need some time to digest that information.

Blender Pilot

Remember "high speed" for that S76 is a bit different then the high speed of a jet. Vne is only about 150kts (Ha only...I'm used to flying helicopters where Vne is about 100kts lol). Also the inlets on the S76 are not that bad...they are fairly direct and there is no particle seperators.

Max

Five from memory, all flown in cammo by the Royal Marines (which is part of the Navy?) So it was prob. a AH1 rather than an HT2 or 3.


Thanks for clarifying what drives the fan. I've never seen a diagram that shows the linkage but I assumed it was engine powered rather than an electric motor ...

i think from memory the marine ones are a ht4 or ah4

a link to notar explained
http://www.kulikovair.com/Notar2.htm



steve

You know, when you look at the diagram, you can see how a chap can get confused. It looks like an jet outlet pointing down the tail cone.

;)

BlenderPilot

When you next see your business jet instructor, kick him in the Christmas Crackers if you would.

It is a common misconception that thrust in a jet engine is caused by the reaction of the exhaust gas stream on the ambient air, it is not. If this were true, then how do rockets propel themselves in space where there is no atmosphere?

There are a multitude of thrust forces acting within the engine at every stage, for arguments sake let’s stick to the exhaust/nozzle section and subsonic air flow.

The best basic example I have seen to explain thrust is the classic party balloon.

Imagine if you will blowing up a balloon and tying a knot around the teet so no air escapes. The higher pressure air within the balloon trying to escape is acting with equal force on the entire inner surface of the balloon and therefore opposing forces cancel each other out and the balloon remains stationary.

Untie the knot and let go of the balloon and it shoots off in a direction OPPOSITE the opening at the teet. NOT because the escaping air is pushing on the ambient air outside but because the forces exerted on the inner wall OPPOSITE the now open teet are greater than at the exit hole where there is not longer a surface for the pressure to act upon. (Newton’s third law of motion)

A jet propulsion engine works on exactly the same principle. The jet engine expels high velocity higher mass air rearwards thereby exerting a force not on the ambient air but on the area OPPOSITE the open exhaust outlet, this being the rear of the turbine.

The corporate jet you learnt to fly in would have embodied a pitot style inlet and by moving forward through the air you create a ram effect increasing inlet air pressure further still. The higher inlet pressure corresponds to higher pressure air through the entire engine cycle therefore engines can benefit from this (think supercharging). But I think I am correct in saying that without this inlet ram effect, thrust actually decreases with increased forward speed so this benefit merely maintains thrust levels and would not double them as your instructor suggests, but I may be wrong here.

Regarding S-76 thrust I’m really rattling my brain here. Firstly, the A and C Series have the inlet at the front in the inlet plenum; the B has its inlet at the rear and does not have a particle separator as such but bypass doors which are usually extended when hovering close to the ground. I would hazard a guess that neither design has any noticeable inlet ram effect even at high forward speeds.

You have referred to an N1 (NG Gas Generator speed) of 90% for both scenarios – hover and forward flight. Assuming that there were no gas losses in the compressor side (bleed valves closed and cabin services off), the volume of air passing through the engine would be the same, therefore I would assume that exhaust gas thrust would be the same.

However, if both scenarios resulted in different Tq settings, then it is fair to suggest that NG/Nf and ITT readings would differ and therefore exhaust gas thrust would vary accordingly.

As a result of my thoughts above, I too may well deserve a kick in the Christmas Crackers, but lets hope not, it is Sunday after all.

I think the off-set on the NOTAR exhaust was more to do with the airflow around the tail-boom pushing the hot ehaust down on to the ground (fire hazard), during early testing with the standard 500 setup.

BM

Regarding the 76.

The intake and particle separator on the B model directs the air into the intake with the EAPS off and effectively slightly pressurises the intake area.

As I understand it, the intake area of the A & C models becomes pressurised above 40 - 50kts.

NOTAR confusion alert
 

I hope this tidies up any loose ends;

http://www.geocities.com/pprunessilsoesid/notar.jpg

Using the natural characteristics of helicopter aerodynamics, the NOTAR® anti-torque system provides safe, quiet, responsive, FOD-resistant directional control. The enclosed variable- pitch composite blade fan produces a low pressure, high volume of ambient air to pressurize the composite tailboom. The air is expelled through two slots which run the length of the tailboom on the starboard (right) side, causing a boundary-layer control called the “Coanda Effect.” The result is that the tailboom becomes a “wing,” flying in the downwash of the rotor system, producing up to 70 percent of the anti-torque required in a hover.
The balance of the directional control is accomplished by a rotating thruster.
In forward flight, the vertical stabilizers provide the majority of the anti-torque, however directional control remains a function of the jet thruster.

Source

http://www.geocities.com/pprunessils...toID533238.jpg


:confused:
SS