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Grob 103 ASI Very Sensitive in Yaw?



 
 
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  #1  
Old September 17th 16, 04:05 PM posted to rec.aviation.soaring
Papa3[_2_]
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Default Grob 103 ASI Very Sensitive in Yaw?

I was checking someone out yesterday in a Grob 103 and we were experimenting with various things including yaw sensitivity. We noticed that the ASI is completely useless with anything more than about 10 degrees of yaw. Like, it goes to almost zero and flutters around. Given how poor Grob rudders are, this is a real consideration. I don't recall our club's Twin Astir behaving like this.

I'm going to check on the statics and pitot installation today, but I'm wondering if anyone else has experience with this. I'm assuming it's not just this one ship...

Erik Mann (P3)
  #2  
Old September 17th 16, 07:59 PM posted to rec.aviation.soaring
Charlie M. (UH & 002 owner/pilot)
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Default Grob 103 ASI Very Sensitive in Yaw?

Recent rain/wash?
Recent wax job?

Sounds like a clogged port or line, nose or tail boom ports?
  #3  
Old September 18th 16, 04:48 AM posted to rec.aviation.soaring
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Default Grob 103 ASI Very Sensitive in Yaw?

We've had both a 1978 retractable gear G-103 Twin Astir and a 1986 G-103 Twin II Acro (well, not really "acro" since the AD) in the fleet. The old one had fuselage side statics and a short tail mounted pitot while the newer one has the pitot head, static ports and T.E. venturi on a single multi-probe on the leading edge of the vertical stabilizer. They were both better at maintaining an airspeed reading in a slip than gliders with nose pot pitots I've flown (L-13, L-23, ASW-15b etc.) Sounds like it may be just that one ship. What setup does your ship use for the pitot/static system?
  #4  
Old September 18th 16, 04:46 PM posted to rec.aviation.soaring
Papa3[_2_]
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Default Grob 103 ASI Very Sensitive in Yaw?

Ours is a fairly early 103 (has the trim tab on the elevator). The pitot is a short brass tube in the leading edge of the vertical stab. I need to figure out which statics are connected.

I've flown a lot of different Grobs (I owned a single-place Astir, used to teach in a bunch of other 103s), and this particular one has always (since we got it) had this issue. So, it's not something recent.

I have to dig up a maintenance manual, but in the meantime, anyone know if there are two sets of statics? One set is in the forward fuselage. I think the other is in the tail boom (if there is another set). I assume one set was used for the initial certification, so I'd like to make sure we are on those for the ASI.

FWIW - one ASI was sent back to Winter for repair and calibration last year, so it's a "known good" instrument.

P3

  #5  
Old September 19th 16, 02:05 PM posted to rec.aviation.soaring
BG[_4_]
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Default Grob 103 ASI Very Sensitive in Yaw?

There are a total of no less than 6 static ports on that vintage of Grob. The 4 on the tail boom are connected together and there 2 on each side of the cockpit area as well. By design if any of these ports are clogged, a yaw will give a problem as you describe. Use a wire and probe the ports. You can blow carefully on each one and watch for a response on ASI.

By design a yaw will give some ram pressure to one side with a compensating negative on the opposite side , together should be close to true static.

Design must not have been great as it was replaced with a triple prob on the tail in the later models.

Buzz
  #6  
Old September 19th 16, 02:17 PM posted to rec.aviation.soaring
Dave Nadler
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Default Grob 103 ASI Very Sensitive in Yaw?

On Monday, September 19, 2016 at 9:05:26 AM UTC-4, BG wrote:
...You can blow carefully on each one and watch for a response on ASI.


Do NOT blow into the statics.
Gently (GENTLY !) suck on the static...
  #7  
Old September 19th 16, 03:37 PM posted to rec.aviation.soaring
Papa3[_2_]
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Default Grob 103 ASI Very Sensitive in Yaw?

On Monday, September 19, 2016 at 9:05:26 AM UTC-4, BG wrote:
There are a total of no less than 6 static ports on that vintage of Grob. The 4 on the tail boom are connected together and there 2 on each side of the cockpit area as well. By design if any of these ports are clogged, a yaw will give a problem as you describe. Use a wire and probe the ports. You can blow carefully on each one and watch for a response on ASI.

By design a yaw will give some ram pressure to one side with a compensating negative on the opposite side , together should be close to true static..

Design must not have been great as it was replaced with a triple prob on the tail in the later models.

Buzz


Thanks - this is exactly what I was looking for/figuring. I have a lot of time in tracing pneumatic systems issues on other gliders, so I'm comfortable with the process. Interestingly, the fuselage on this glider was refinished before we bought it. Makes me wonder if they got a lot of sanding dust, filler, and paint in the system.

On the good news side, the new USB Boroscope I bought will come in handy for inspection the tubing from the inside of the fuselage to see if there are any obvious problems (like tubes dangling or open T-fittings).

 




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