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HelicopterTurbine Helicopters › NH-90 & Jetcat Turbine Problems(again!)
06-01-2004 08:06 PM  13 years agoPost 1
edg

rrApprentice

San Francisco, CA

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Hi All,

Well, I gave my NH-90 it's first test hover yesterday after getting my turbine back from JetCat. So, all should be good with the turbine.

There's still something wrong. I hovered it for a bit and it seemed somewhat OK, but control was very wobbly. It just didn't feel right. I knew I was pretty tail heavy, so I thought maybe this was due to balance. I added some weight up front to balance it out. I should also add that the Max Temp reading of my 1st hover was 850C. It still seems on the high side.

With the balance better (but now at least a pound heavier) I tried again. Control was slightly better. Still didn't feel right. But, now in addition to the wobble, after a couple of seconds in the air the turbine just seemed to sag and would no longer provide enough power to lift it.

I now think it's a head speed issue. I'm not sure what my head speed is, but I' m using the same gearing that was in it from the JetCopter SX pod & boom (1250 RPM?). Does this seem like the likely problem? Has anyone else run into this issue when going from a pod & boom to a heavier fuse? The wobbly control and sagging sure seem like head speed to me.

I'm still concerned with temp. Both hovers, the max temp hit 850. But, this time the outside of the fuse just felt a bit warm rather than hot as it did prior to sending the turbine in.

Thanks for any help!

-edg-

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06-01-2004 08:24 PM  13 years agoPost 2
Doug

rrElite Veteran

Port Saint Luice Florida....

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Any chance you are injesting hot exhaust back into the intake? As a rule of thumb each deg C increase in intake temp will produce 3 deg C higher exhaust temp.

First member of Member of Bearings Anonymous

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06-02-2004 01:28 AM  13 years agoPost 3
heliVoY

rrVeteran

NJ

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1400 rpm head speed would probably feel better, but the heli should fly just fine on 1200 ... sounds like you are still not getting up to your propper head speed ..... do you have the PC link for the turbine ? I would be curious to see your RPM/time/TXset graphs ...

Wojtek

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06-02-2004 02:14 AM  13 years agoPost 4
wolfdad

rrKey Veteran

Southern Maryland

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I would say Woj is probably right on the money. What's the weight on your bird? You might also want to take a look at your pitch curve. Once I revamped my pod and boom to the -4° and +14° (hovering at mid stick vice 3/4 stick), I saw a noticeable improvement and bringing the head speed up to the 1420 mark makes even a more dramatic change, however you should still be able to hold a stable, comfortable hover at the 1270 head speed. Also, that 850° temp sounds a bit high. Is there any chance that your intake is blocked or that you ingested something? I have noticed that mine picks up an incredible amount of crud on the intake screen...enough that cleaning the screen between flights is mandatory.

The sag in power during the hover is also indicative of reduced air flow through the turbine.

wolfdad sends.....

"There are those who have...and, those who will" IRCHA #2117, AMA #70068, Turbine Waiver #105

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06-02-2004 02:28 AM  13 years agoPost 5
heliVoY

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NJ

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850 is definately high !! especially in a hover !! something else has to be wrong ....

hee is what it took my 30lbs (dry) bell 230 to get overheated (and thats with a make shift pipe that is less than eficient at keeping things cool) !!! at 1400 head speed, i in an instant cranked in a full 12 deg of positive pitch ! (these are wide chord blades , with a lot more resistance than the graupner blades for the NH90)

as im landing the heli, the turbine actually spools back up to the full rpm ... (when overheated, a hp5 will go to 80% power to try to restabilize temp)

http://www.voy-tech.net/images/bell3.wmv


as for keeping the temp inside the heli cool (intake air temp), try wrapping the pipe itself in heat blanket (the kind Sandy recomended you use) ..... you can get it at Jegs.com

Wojtek

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06-02-2004 10:41 AM  13 years agoPost 6
F.Pock

rrNovice

Solingen west germany

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Hello

excuse my bad english,

we fly also a Jet Cat PHT3 in a graupner NH90. We have the original exhaust, all windows closed. We run with 94.000rpm on the turbine and 1280rpm on rotorhead. Our max. exhaust temp. is 680 grad. So i also think there is something more wrong. Have you any pictures of your bird? maybe we can see there something. In the moment i can not help, but our NH flys very well without problems.

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06-02-2004 04:38 PM  13 years agoPost 7
edg

rrApprentice

San Francisco, CA

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OK, So it looks like it's not the head speed.

I just got the turbine back from JetCat -- I'm pretty damn sure the screen's not blocked. At least it wasn't on my first hover which registered 850C. I also have cut out some extra holes in the back for cooling.

My last hover, where I added weight up front, I packed in with foam. I think my bad... I thiink I may be blocking the intake of air! That would explain the sagging after a couple secs in the air. That's totally consistent with Woz's description of it cutting back to 80% power.

However, that still doesn't explain my first hover. Temp still is high. Plus the control just doesn't feel right. I had a very hard time holding it in a hover. My pitch curve is set to hover mid-stick. I flattened it out a bit last time since it was leaping off the ground to quickly.

Sigh. I just don't trust this thing very much anymore. I may stick it back in a pod & boom and fly it that way for a while. It flew just great pod & boom. As far as I can tell, I didn't do anything wrong in the build. Tail drive seems solid w/ nearly no vibration. And the mechs were totally checked out at JetCat....

-edg-

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06-02-2004 04:44 PM  13 years agoPost 8
Doug

rrElite Veteran

Port Saint Luice Florida....

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Get yourself a small electronic outdoor thermometer (the ones with the remote probe) and stick it where you can see it through the window. Put the probe within about 1" of the intake and hover it, monitoring the inlet temp It will tell the tale.

First member of Member of Bearings Anonymous

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HelicopterTurbine Helicopters › NH-90 & Jetcat Turbine Problems(again!)
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