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HomeTurbineAircraftHelicopterTurbine Helicopters › exhausts
01-15-2010 02:23 PM  8 years agoPost 1
Peter Wales

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Orlando Fl

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A hypothetical question.

Is it possible to get a turbine heli with a single stage engine and a straight out exhaust to hover absolutely horizontal by adding nose weight.

My instinct says no, but what happens if you keep on adding weight to the nose?

The reason for asking is that I now have my 212 with a PHT2 flying, but even with a ridiculous amount of lead in the nose, its not horizontal and the slightest breath of wind has it bobbing up and down like a cork in the water

Peter Wales
http://scalehelicopters.org

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01-15-2010 03:33 PM  8 years agoPost 2
PETER ROB

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Devon UK

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Peter To answer your question simply NO
As you, I know are aware, that the tail down is countering the thrust from the turbine
We had some success in getting one to hover(airframe)level, by tilting the mechanics backwards, but this was limited by the tail drive take off
Peter R

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01-15-2010 07:20 PM  8 years agoPost 3
Peter Wales

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Orlando Fl

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But if you put a load of weight in the nose, it would want to hover more level.

Peter Wales
http://scalehelicopters.org

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01-15-2010 08:22 PM  8 years agoPost 4
Havoc

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Ky.

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I think you would just cause it to require more aft cyclic to do the same thing?

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01-15-2010 08:31 PM  8 years agoPost 5
darrens

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United Kingdom

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Hi Peter

if your exhaust produces rearward thrust (pushing the model forward), you will have to trim the model with an equal opposite thrust, hence it hovers tail down but stationary. If you then add nose weight to level the machine, you are back where you started and it will begin to drift forward again and would require more rearward trim to counter the weight and so on and so on.
So my answer is in support of Peter Rob.........No, it won't work!

Regards
Darren

He who dies with the most toys is the winner!

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01-15-2010 08:34 PM  8 years agoPost 6
helibeli

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wales.uk

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It's about time these manufacturers came out with a "friendly" fitting two stage turbine to fit in scale fuselages which would solve the thrust question and not involve a lot of head scratching as how to fit the engine.Something similar to the Zealous Jetranger design but simpler maybe.

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01-15-2010 10:02 PM  8 years agoPost 7
Sparkhead

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Melbourne Australia

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just out of interest, how much thrust are we talking about with the single stage? Could some sort of deflector be put just inside the end of the exhaust to direct the thrust in a different direction. The 212 would probably be bad for that I suppose because you'd be pushing the hot gas onto the paintjob.


Mick.

Robbe & Futaba.

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01-15-2010 10:13 PM  8 years agoPost 8
PETER ROB

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Devon UK

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"friendly" fitting two stage
These 2 stage will fit in a lot of scale fuselages,
The problem that always needs addressing are the turbine outlets
Peter R

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01-15-2010 10:23 PM  8 years agoPost 9
PETER ROB

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Devon UK

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exhaust deflectors
Mick, the deflectors are always an add on so stick out further than the fuselage
The best way of eliminating the thrust efect is to deflect it up into the blades, a few full size military helicopters have this, I think to cool the exhaust gas and change the heat signature
I had the exhaust on my Vario single stage Cobra, deflected into the blades no thrust, and it sounded great
Peter R

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01-16-2010 07:42 AM  8 years agoPost 10
Darkstar1966

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Monaco

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As mentioned on scalerchelis, if you incorporate a lobe nozzel on the output you increase mixing of the aimbent air and reduce your residual thrust. Tested these designs and they have been very effective and also reduce dB output.
The first image was for a Jaka and the second one is made by a company in the US not my design and is made from a ceramic, which is very cool.

Regards,

Gregory

"Anything that screws its way into the sky flies according to unnatural principals."

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01-16-2010 01:00 PM  8 years agoPost 11
fredd

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UK - London

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Gregory,

very informative as always. I assume Jaka do not produce these and you has these made for your own usage ?

Frederic

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01-16-2010 01:18 PM  8 years agoPost 12
Darkstar1966

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Monaco

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I don't think any of them would do this on the smaller turbines, I know a few of them are playing more with Cheverons as they are more concerned with the noise and not a reduction of residual thrust.
It is easy to make you just get a section of 1~1.5mm tubing and then make a male and female mold of the nozzel design that the tubing fits over, then the tubing is heated then the two molds are pressed together and you have the nozzel design. It does not create issues with the EGT as the output area is bsically the same, also I designed them to be slightly larger.

Regards,

Gregory

"Anything that screws its way into the sky flies according to unnatural principals."

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01-16-2010 07:58 PM  8 years agoPost 13
helibeli

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wales.uk

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If you deflect the exhaust into the rotor blades,isn't there a danger of creating a hotspot on the disc which might cause deformation? unless you use M-blades.

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01-16-2010 09:00 PM  8 years agoPost 14
PETER ROB

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Devon UK

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hotspot on the disc
There is always the possibility of over cooking the blades, it has only happened to me once, and that was because I left one blade too close to the exhaust on start up, that was with an EC135, 4 blades and twin exhaust, with the Cobra, 2 blade and single exhaust never a problem, never had any problems with all the other multi blades
The tip is to spin the blades by hand on start up, or to hold the blades until the turbine starts then give it a spin
Peter R

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01-17-2010 11:23 AM  8 years agoPost 15
Héli Suisse

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Geneva Switzerland

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Peter Rob is fully right it happend to me too once in the past, and the 2 solutions Peter R gave are the ones I'm presently using, the best (most practicable) being holding the X blades out of the hot gas while starting up (gas procedure). Were you starting with kero sequence then spinning the head by hand would be the most efficient.

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01-18-2010 01:08 PM  8 years agoPost 16
WIRLYBIRD

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CAPE TOWN / SOUTH AFRICA.

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Just came across this now , my two NH90's , although the outlets exhaust into the blades , I've been standing on my head for almost four years now. I've just had to get used to it , something like the difference between a two bladed , or five bladed head , you do get used to it , and learn to counter it's nonsense , as said worse in the wind!!
Dave.

WHAT GOES UP MUST SURELY COME DOWN.

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01-20-2010 12:03 AM  8 years agoPost 17
seattle_helo

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Seattle, WA USA

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Residual thrust is a big problem on a turbine 500 with a single, scale outlet. Folks I spoke with who had built 500's before me had lamented the issue of a non-level hover. Based on what I have heard and read as well as tried myself I think adding weight to the nose does help somewhat. I'm not sure though if adding enough weight to equal the amount of thrust is practical, especially given the "moment arm" of where the outlet is relative to the typically reduced leverage in front of the main shaft. I think you would need even more weight than the output of the thrust. The other issue would be that as the thrust changed slightly (as the governor varied RPM) it would probably make the machine sway as the CG shifted. I borrowed an idea to angle the exhaust downward from some guys in Germany. Except I changed the design somewhat to give it the maximum downward angle possible as constrained by the fuse.

Basically, by rerouting the thrust relative to the pivot point it puts the burden on the main rotor disc and might even provide a small amount of ancillary lift.

The machine stays perfectly level with this pipe but the downside is that anything under the outlet is toast.

nick

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01-20-2010 07:35 AM  8 years agoPost 18
PETER ROB

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Devon UK

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anything under the outlet is toast.
The issue of burnt grass and fire risk, became a concern at the club field
These are two of the solutions I came up with, my favorite is thw blanked exhaust with the holes in the side, an idea based on seeing Peter Wale's Lama at Kevil a few years ago,
The military have various exhaust's on their 500's, the bent end is one of them
Peter R

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01-21-2010 04:44 AM  8 years agoPost 19
gonzalom

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Northern California

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Peter,

I had long talks with Vario about this subject for my turbine Lama...
As you know at the beginning I had a modified exhaust to avoid this issue on my Lama and my exhaust had the 2 holes on the side to move the flow in to another direction (inspiration from your Lama).... yes she hover 100% level.... but very noisy and the scale look was not what I want.

Talked again to Vario and they insisted to me that I didn't need to do any changes to my exhaust and by adding some weight in the nose I will achieve the almost level hover that I was looking. I purchased a new exhaust and I installed as it is.....add some more weight in the nose.

To my surprise the Lama hover almost level, the noise was reduced dramatically, temp on the turbine went down by almost 100 C and she looks very scale..... I was very very happy with the results. I do believe this can be achieve in bigger models as I did due to the massive 9ft rotor wash my Lama produced and the 49 lbs on the ship... here is a video for the audience:

http://www.youtube.com/watch?v=QGzL50OXh6A

As you always love to experiment! give it a try with the load in the nose and see what happens! is just lead? you are always my top experimental pilot

Gonzalo

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01-21-2010 09:28 AM  8 years agoPost 20
PETER ROB

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Devon UK

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Gonzalo, Nice flight, I have seen Ian Bishop,s Turbine Lama fly a couple of times they are really impressive heli's in the air
A long wait before mine fly's
I have tried the lead on a number of occasions, it does not completely solve the thrust issue, but is essential to counter the weight that you get from behind the mast, to get the CofG correct
With the turbine 500,if you just have a straight exhaust, you will over heat the tail boom, to a point where it would go soft and spongy,
Vario's exhaust have 2 diverters out the sides of the fuselage, which work but to the purist do not look correct, hence the reason for my own exhaust
I have just sent a twin wall, with offset end to Colin in NZ, he said he put over 2 Kgs in the nose for his maidem flight, by the time he finishes building that will be reduced
I have been scratch building(along with a load of other scratch projects)a set of Turbine mechanics for the Lama/JetCat SPH5, this will mean I will have no thrust out of the rear
I am waiting for the Cine-Scale Gazelle to arrive, as I plan to make a more scale like exhaust for that, that should be interesting
Just an observation,with a full size helicopter, when it is in forward flight, cruising, the heli is level, when it comes back to the hover, most times the tail droops,
Peter R

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