StanFoster
Active Member
- Joined
- Nov 16, 2003
- Messages
- 17,139
- Location
- Paxton, Il
- Aircraft
- Helicycle N360SF
- Total Flight Time
- 1250
There was some talk this morning in another thread about ASI reading wrong.
I chimed in and mentioned I would post a thread on a bleed air adjustment fitting I came up with to make my ASI read much closer to the correct airspeed.
Every rotorcraft I have owned had my ASI reading too fast. My SparrowHawk was the worst....it would read 8-10 mph too fast at cruise, and as much as 10-12 mph too fast at high speed runs.
I came up with the idea of making a valve that would bleed off just enough of the excess pitot tube pressure...making the ASI read accurately.
I know the whole problem stems from the static port not being located at neutral pressure....plus some other things I dont understand enough to fix. But I do know that bleeding off a little pitot tube pressure works great. This may be considered a 'bandaid' fix instead of properly configuring the static port.....and pitot tube....but it works for me. I hear so many times figures of gyros going way over 100 mph....and much higher figures...and I have always wondered what their true averaged GPS groundspeeds really are. My experience has had every one of mine read too fast.
I got fussy about my ASI being off in my SparrowHawk to do something about it. I would average my GPS groundspeed runs after flying straight and level into the wind...and then fly 180 the other way....noting the two groundspeeds...and averaging. Say my GPS showed me going 90 mph with the wind....and 70 mph against...I want my ASI to say 80 mph. My SparrowHawk would read 88-90....not acceptable! So...after my GPS run...I would just keep flying on the 2nd leg....and simply turn the screw on my airbleed valve...and watch the airspeed slowly go down till it read 80 mph. My experience with this varified that the ASI stayed accurate until there was a big change in temperature towards winter...then it was only off 2-3 mph...and a simple partial turn of the screw dialed her in again. I found that I only adjusted this thing basically twice a year, and I was always checking it just for the fun of it...and the pleasure knowing that ASI was giving me actual airspeed readings.
Here are some pictures. I took a wellnut....and took the screw out.....put a sloping notch in it with a hacksaw. One end of this wellnut was just the right size for a vinly tube to go into it.....then I T'd it into the pitot line.
The slotted screw was inserted in the back end of the wellnut...and when it was all the way screwed in....it was like before...the ASI was reading way too high. But as you back out the screw, there is a point when the slot just starts releasing a little air.....then the ASI starts to read less. As you turn the screw CCW, more of the slot gets exposed and more air gets bled out. This goes on until the ASI reading matches the GPS averaged groundspeed...and you are done adjusting.
My Helicycle has its static port positioned as the plans show...and when I start flying it...I will be conducting some GPS groundspeed runs. If my ASI doesnt read correctly, I will try a static port as Chuck Beaty suggested. But if that doesnt give me correct results...I will be having a bleed air adjustment screw in my instrument pod.
The last picture is a tee made just for the picture..its just a butt joint..whereas of course a real T would be inserted.
Stan
I chimed in and mentioned I would post a thread on a bleed air adjustment fitting I came up with to make my ASI read much closer to the correct airspeed.
Every rotorcraft I have owned had my ASI reading too fast. My SparrowHawk was the worst....it would read 8-10 mph too fast at cruise, and as much as 10-12 mph too fast at high speed runs.
I came up with the idea of making a valve that would bleed off just enough of the excess pitot tube pressure...making the ASI read accurately.
I know the whole problem stems from the static port not being located at neutral pressure....plus some other things I dont understand enough to fix. But I do know that bleeding off a little pitot tube pressure works great. This may be considered a 'bandaid' fix instead of properly configuring the static port.....and pitot tube....but it works for me. I hear so many times figures of gyros going way over 100 mph....and much higher figures...and I have always wondered what their true averaged GPS groundspeeds really are. My experience has had every one of mine read too fast.
I got fussy about my ASI being off in my SparrowHawk to do something about it. I would average my GPS groundspeed runs after flying straight and level into the wind...and then fly 180 the other way....noting the two groundspeeds...and averaging. Say my GPS showed me going 90 mph with the wind....and 70 mph against...I want my ASI to say 80 mph. My SparrowHawk would read 88-90....not acceptable! So...after my GPS run...I would just keep flying on the 2nd leg....and simply turn the screw on my airbleed valve...and watch the airspeed slowly go down till it read 80 mph. My experience with this varified that the ASI stayed accurate until there was a big change in temperature towards winter...then it was only off 2-3 mph...and a simple partial turn of the screw dialed her in again. I found that I only adjusted this thing basically twice a year, and I was always checking it just for the fun of it...and the pleasure knowing that ASI was giving me actual airspeed readings.
Here are some pictures. I took a wellnut....and took the screw out.....put a sloping notch in it with a hacksaw. One end of this wellnut was just the right size for a vinly tube to go into it.....then I T'd it into the pitot line.
The slotted screw was inserted in the back end of the wellnut...and when it was all the way screwed in....it was like before...the ASI was reading way too high. But as you back out the screw, there is a point when the slot just starts releasing a little air.....then the ASI starts to read less. As you turn the screw CCW, more of the slot gets exposed and more air gets bled out. This goes on until the ASI reading matches the GPS averaged groundspeed...and you are done adjusting.
My Helicycle has its static port positioned as the plans show...and when I start flying it...I will be conducting some GPS groundspeed runs. If my ASI doesnt read correctly, I will try a static port as Chuck Beaty suggested. But if that doesnt give me correct results...I will be having a bleed air adjustment screw in my instrument pod.
The last picture is a tee made just for the picture..its just a butt joint..whereas of course a real T would be inserted.
Stan