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Wobbly altimeter and ASI

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n7188u

Joined: 15 Nov 2015
Posts: 404

Posted: Thu Apr 01, 2021 7:46 am Post subject: Wobbly altimeter and ASI

Dear forum members,



One issue I had on my first flight yesterday was that my altimeter and ASI where oscillating up and down. The airspeed was quite fast, like a vibration, but the altimeter wobbled maybe 75 feet up and down more slowly.



This was particularly noticeable during initial climb and got better while level and almost disappearing toward the end of the flight.



I checked for leaks on the static line per Bud's recommendations a few postings ago (vacuum in line to 1000' and 135 KIAS) and the pressure held steady with no leaks.



Any ideas what may be causing this? I thought that with the fancy static probe we get with the kit the static pressure would be fairly stable. 



Best Regards,

Chris

rparigoris

Joined: 24 Nov 2009
Posts: 790

Posted: Thu Apr 01, 2021 12:55 pm Post subject: Re: Wobbly altimeter and ASI

Hi Chris



Kudos to you and your first flight! Curious, what motor and prop you have? If 914 were you at 100% power or 115%?



As far as your wobbly problem, I purchased a spare pitot - static from Europa because I wanted to fool with adding a AOA for Dynon D10A. Anyway the black static plug had a big burr inside and I trashed bullet when removing it. I machined a new one, let me tell you it's hard to machine without getting burr. At time Europa was out of business when I fabricated. Any careful inspect yours. Nev mentioned he could trim airspeed a little by putting an O-Ring on the pitot and move it fore and aft. Once he got it to his liking would CA it in place. 

http://www.europaowners.org/main.php?g2_itemId=28932



Good Luck

n7188u

Joined: 15 Nov 2015
Posts: 404

Posted: Thu Apr 01, 2021 7:15 pm Post subject: Re: Wobbly altimeter and ASI

Thanks for the reply Ron.



I have a 912 ULS, Woodcomp Klassic ground adjustable prop.



In case you wonder the plane as flown weighs 765 lbs. The airplane is unpainted, no interior (just Zolatine paint), no cushion on the right side, no carpets. Minimal wiring, no transponder (yet), single Becker radio, tablet for nav. I wanted to see what it did when light and boy does it leap of the ground. We have a short displaced threshold and didn't even made it to the end of the bars. And that is with only 5300 rpm. Lets see what happens when I lower the pitch a little . It was fun (may keep it like this for a little while). Well, the transponder/ADSB will go in soon if I want to go anywhere.



Thanks for the info on the probe. Will review. I purged the lines today just in case (but doubt water was in there). I suspect issues with the bullet tip. If problem persists will tape over the two vertical holes and only leave the horizontal ones. I wonder if at high angle of attack the two vertical ones play tricks with the static pressure.



Best,

Chris

n7188u

Joined: 15 Nov 2015
Posts: 404

Posted: Thu Apr 01, 2021 7:23 pm Post subject: Re: Wobbly altimeter and ASI

Sorry, I misled. 765# empty, add 157 for me and 12 gallons of fuel.

rparigoris

Joined: 24 Nov 2009
Posts: 790

Posted: Thu Apr 01, 2021 10:56 pm Post subject: Re: Wobbly altimeter and ASI

I spoke to my build partner today and he thought that you may want to try plumbing static to cabin and see if stable.

n7188u

Joined: 15 Nov 2015
Posts: 404

Posted: Fri Apr 02, 2021 11:32 am Post subject: Re: Wobbly altimeter and ASI

Thanks a good idea Ron.

Chris

Griffo

Joined: 18 Mar 2021
Posts: 70
Location: The Oaks, NSW, Australia

Posted: Sat Apr 03, 2021 2:28 pm Post subject: Re: Wobbly altimeter and ASI

Hi Chris,



I assume your engine is a Rotax 91



If your quoted take-off RPM 5300 is correct, be advised that Rotax recommend static RPM to be no less than 5200. This should easily give you over 5500 + RPM on climb out.

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n7188u

Joined: 15 Nov 2015
Posts: 404

Posted: Sat Apr 03, 2021 2:56 pm Post subject: Re: Wobbly altimeter and ASI

Griffo,



The static rpm is 5300. But during climb out it's about the same (which surprised me too). I am planning to reduce the pitch slightly but after I have a chance to let the airplane get up to cruise speed at a higher altitude and check the RPM/MP numbers.



Chris

Griffo

Joined: 18 Mar 2021
Posts: 70
Location: The Oaks, NSW, Australia

Posted: Sat Apr 03, 2021 4:16 pm Post subject: Re: Wobbly altimeter and ASI

My ATEC Zephyr/912 ULS, 2 blade ground adjustable Fiti prop, with static 5200 rpm, goes to well over 5500 within its 100 m ground role and is doing its rated 5700-5800 on climb out (at)70 knots / 1500+ ft/min (one pilot full fuel). 



Hoping to purchase a Europa Classic/912 ULS in very near future. As Europa considerably heavier with shorter wing span - expecting longer ground run and slower climb out but much higher economy cruise (wanted for long trips away)

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Last edited by Griffo on Sat Apr 03, 2021 9:39 pm; edited 1 time in total

Peter pender

Joined: 15 Jan 2021
Posts: 16

Posted: Sat Apr 03, 2021 9:08 pm Post subject: Re: Wobbly altimeter and ASI

Griffo, suggest you look at SL-912-016R1 and report back on the recommended revs. cheers Peter

Griffo

Joined: 18 Mar 2021
Posts: 70
Location: The Oaks, NSW, Australia

Posted: Sat Apr 03, 2021 9:38 pm Post subject: Re: Wobbly altimeter and ASI

Hi Peter,



Which part of SL - 912 - 016R1 do you feel I should take note of ?



3.1.2 Performance Recommendations -  seems to support my (above reported) setting.



As does



3.1.3 Fixed Pitch and Ground Adjustable Propellers - relevant to Chris's aircraft - also supports my observations in my Zephyr

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Griffo

Joined: 18 Mar 2021
Posts: 70
Location: The Oaks, NSW, Australia

Posted: Mon Apr 05, 2021 2:08 am Post subject: Re: Wobbly altimeter and ASI

Chris -I have been thinking on this 5300 rpm Static and your claimed 5300 rpm on Climb out. 

With a fixed pitch propeller and constant throttle: 

If the engine is properly tuned (carburettor balance) and everything is as expected and you have correctly performed your Static run up engine rpm check - you must get an rpm rise, as the aircraft gathers speed, on ground role and Climb out.

The only exception to this would be an abnormally high Climb out angle (above best climb angle), which may have the effect of limiting rpm (you should still have some rpm increase in ground role.)

Is your tacho working correctly?

If you are not getting an rpm rise, something serious may be wrong – get your local Europa or LSA aircraft guru to help diagnose the problem.

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n7188u

Joined: 15 Nov 2015
Posts: 404

Posted: Mon Apr 05, 2021 10:24 am Post subject: Re: Wobbly altimeter and ASI

Griffo,



Now that I flown the airplane a little more I can provide additional info. The RPM does stay somewhat constant at between 5200-5400 in my plane during initial climb (around 60 kias). As I retract the flaps and increase speed to around 80 kias the RPM will go up I think around 200-300 rpm but at that point I am throttling back because the climb rate and deck angle are a little steep for my taste at this point in flight testing. 



Is the tach accurate? don't know. You got me curious so I will check. But the blade angle is set to what is recommended for this prop and the 912 ULS (17.5°) so it shouldn't be too far off. The SportCruiser LSA uses this prop with the 912 ULS and the manual calls for 5000±100 RPM during static RPM test so I don't think my tach is off. 



Props are not all created the same. I remember reading some Europa docs that said the warp drive tended to have a lower RPM than static as the airplane starts to move as the prop starts to "bite" better (I tried to find it to provide as refence but couldn't find it). I think the DUC propellers claim a similar effect. In my LongEZ it is the "normal" way, I get around 2200 static and then 2500 as soon as I start the takeoff roll. It's a big change.



Take all I said with a grain of salt. It is based on my observations and what I remember reading (if I find where I read the info from Europa about the Warp drive props I'lll post it). 



Best Regards,

Chris

Griffo

Joined: 18 Mar 2021
Posts: 70
Location: The Oaks, NSW, Australia

Posted: Mon Apr 05, 2021 2:10 pm Post subject: Re: Wobbly altimeter and ASI

Chris,

My experience of this class of aircraft is limited to 12 years and some 900 hrs in the one LSA, an ATEC Zephyr /Rotax 912 ULS.  From this I would say that 5200 to 5400 is a significant increase in rpm. In my view not quite enough for best take-off performance but getting there – some “tweaking” required.

You are absolutely correct when you say different props/airframes will influence engine performance, as will your particular take-off strategy. Personally, I use full throttle on start, only reducing when a good safe (turn back) altitude has been achieved and then only to maximum continuous rpm (high speed climb) until cruise altitude & leveling off, when I go to my preferred economy cruise of 5000-5200 rpm. 

Observed by many – Rotax 91 engines love revs – so as long as you are staying below the 5 minute max power and above 5200 under load, everything should just be peachy.

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