[rebel-builders] Rebel - HEEEEeeeeeeeeeeeeeeelp ! (Airspeed

[Dave Ricker]

Rick

I don't know a lot about this (any aerodynamicists in here??) but this article
(http://www.unitedsensorcorp.com/pitot.html) is indicating a pitot-static probe (has the
static port built into the probe neck) can offer +/- 2% accuracy up to 30 deg of angle. They
call this a Prandtl type pitot probe. It has the normal pitot probe opening but it also has
the (several) static ports drilled radially into a (separate) chamber around the pitot port.
I suspect the several ports help balance the static pressure with angle of attack.

The important thing is that in theory the pitot port is supposed to face directly into the
flow to measure the dynamic+static pressure and the static ports perpendicular to the flow
(and with no chamfering) to measure the static pressure and the difference between the two
shows up on the ASI as the correct airspeed.

Knowing what you can make when you put your mind to it you could make up a test unit with a
little time on the lathe and try it out. I'm not sure about the idea of making the opening
larger, what you're trying to capture is the "stagnated" air in front of the tube opening so
I'm sceptical that making it bigger would help. I'm suspecting the static part of the
present setup may be putting the pitot part off and putting them on the same probe may clear
the air so to speak. Wayne's (?) comment about taking the static in the cabin may give an
indication. If you're going to make a test probe I'd think about making the probe end
spherical so that the flow around it isn't changed much by pitch, see the diagram in the
article.

You could test out the unit on the ground by hanging it out the window/sunroof of the car &
see what happens even if you use a manometer to check the performance. It wouldn't be too
hard to make up a rudimentary yaw scale & take data especially if you conned Wendy into
driving ;-)

Well, hope this is useful, I may want to go into the back shop and make something up to try
out myself if it pans out.

Cheers,

Dave

Bob Patterson wrote:

Hi Rick !

No problem going to 1/2" if you want - you do still have to
reduce to size of tubing to connect to the ASI at some point,
though.... :-)

As for static - just disconnect the tube at the back of the
ASI - that's what most of us end up doing ! :-)

--
......bobp
http://www.prosumers.ca
http://bpatterson.qhealthbeauty.com
http://apatterson2.qhealthzone.com

-------------------------------orig.-------------------------
On Tuesday 20 February 2007 03:00, Rick Harper wrote:

G'day Bob !

Yeah - I mentioned the "shuttlecock pitot" idea to my designer

(aircraft ) mate .... but I might remake the pitot tubes to 3/8" as you
suggest ....

(do you reckon there would be any problem if I made 'em 1/2"

even ?!?!?!?!?! ....... I can't think of any - other than a rain collector
of course)

I'll have a go at repositioning the static as well .....

It's just a pisser - after building & repairing all this time - now I can't

figure out the full advantage of the new cuffs & VG's until I get the ASI
sorted out properly :o(

Thanks for your thoughts though !!!

Biggus
541R
----- Original Message -----
From: Bob Patterson
To: rebel-builders@dcsol.com
Sent: Tuesday, February 20, 2007 3:46 AM
Subject: Re: [rebel-builders] Rebel - HEEEEeeeeeeeeeeeeeeelp ! (Airspeed

Indicator)

Hi Rick !

Congrats on getting flying again !! Great, isn't it ?!!!

Welcome to the Rebel ! The ASI problem you have is
NORMAL !!!! Almost ALL Rebel ASI's tend to read low on the high
end, and high on the low end ..... most likely because of the
huge range of angles achievable.

One solution is to go to a much larger diameter pitot tube -
say 3/8" +. This gives a wider area for pressure, and a better
chance of closer readings. Best location is about 3-6" below
the wing, about 1/3 of the way back - plain tube mounted on the
inboard side of the strut, fastened to the wing by thin, wrap-around
straps worked for me on 2 Rebels.... Most just use cabin air
for static - it's VERY difficult to find a good position for source.

You could play around with the angle of mounting the
pitot tube - likely it's different now than original. The cuffs
will likely allow cruise at a more nose-down attitude - but
unfortunately, also allow a ridiculously high attitude for
landing ! Test aircraft have the end of the tube mounted
on a pivotting shuttle-cock, so it's always aligned to the
airflow !

--
......bobp
http://www.prosumers.ca
http://bpatterson.qhealthbeauty.com
http://apatterson2.qhealthzone.com

-------------------------------orig.-------------------------
On Monday 19 February 2007 05:09, Rick Harper wrote:

G'day Everyone ....

Well ,..... it's been 18 (or19) months since I damaged her & pulled

her

apart ...... but .............

VH - REB ( Rebel 541R ) now FLYS AGAIN Woo Woo !!!!!

(and the stall with my home made cuffs and VG's is REALLY benign &

docile

---------------------------------------------------------------------------------------

Now ...... I need your help people ....

My Airspeed Indicator was just checked (in situ') and is reading 98 -

99%

acurate over it's entire range - ( 20 to 160 knots ) ..... BUT ...

When I test flew her - it consistantly read 12 ish Knots UNDER range in

the

air at high speed ...... and at least 10 ish knots OVER as I was rounding
out to land ?!?!?

(I did a bunch of flying on Saturday & Sunday - and, using the GPS over

a

multitude of headings & altitudes - acertained that the cruise speed is
actually 100 knots - and

I have BOTH the pitot and the static probe hanging off the front of the

leading edge of the port wing - by about 18" - and about 5 feet out from

the

root

My own suspicions are :

1) Problem with the static air side of the system - somewhere ?!?

2) False readings from the high angle of attack she now can achieve

3) pixie dust caught in my pitot tubes ... (just kidding)

Anyone got any suggestions about WHERE to / how to set up the static

system / inlet etc, etc .... ???

Thanks

Rick "Biggus" Harper
541R
( Airborne again

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--
David A. Ricker
Fall River, Nova Scotia
Canada





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