---------------------------------------------------------- KIS-List Digest Archive --- Total Messages Posted Wed 09/24/14: 2 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 05:51 AM - Static Port (Owen Baker) 2. 05:52 AM - Static port (Owen Baker) ________________________________ Message 1 _____________________________________ Time: 05:51:19 AM PST US From: "Owen Baker " Subject: KIS-List: Static Port 9/24/2014 Reposted From: Owen Baker Sent: Tuesday, October 22, 2013 4:46 PM Subject: Re: KIS-List: TAS Error with my new EFIS 10/22/2013 Hello KIS Listers, As mentioned earlier attached are copies of pictures of the two static ports on the forward fuselage sides of my KIS TR-1. One port has a home made wedge to create an increased static pressure at the static port, 100_602. The other one, 100_599 does not. The wedge has the effect of the airspeed indicator seeing a slightly increased static air stream pressure which results in a slower airspeed indication on the airspeed indicator needle as balanced against the dynamic air stream pressure coming from the pitot tube into the airspeed indicator. This slight change in static airstream pressure causes me to speed up a bit to see a specific airspeed indication number on the instrument. This wedge ' no wedge combination on the two static ports gives me the airspeed and altitude information that I want to see in the cockpit. OC ________________________________ Message 2 _____________________________________ Time: 05:52:08 AM PST US From: "Owen Baker " Subject: KIS-List: Static port 9/24/2014 Hello KIS Listers, MIke Pienaar wrote: =93Where is a good place to put the static port on the TR4?=94 and Galin wrote (in part) =93..... make sure you verify the accuracy at different airspeeds.=94 See the copied material below for further elaboration on this subject: OC ====================== From: Owen Baker Sent: Tuesday, October 22, 2013 8:17 AM Subject: TAS Error with my new EFIS 10/22/2013 Hello Galin, On this subject you wrote (see copied below): =93Pardon the long write-up.=94 No apology needed ' your post was very detailed and valuable. I thank you for both the testing and the reminder write up that you provided. I also posted on this subject back in April 15, 2010 in Matronics KIS list message #21700 under the subject =93Re: Here=92s an odd question=94. I=92ll copy that posting below, but I don=92t know how well it will come through. I also have some pictures of my KIS TR-1 static ports with and without wedge someplace in my computer. If I can find the pictures I will post them separately. Thank you again for your work on our behalf. OC =============== Match: #27 Message: #21700 From: Subject: Re: Here's an odd question Date: Apr 15, 2010 4/15/2010 Hello Mike McCann, You wrote: "Not sure why this (altitude variation with airspeed) would be, or more importantly, how to fix it." Hello Bernie, You wrote: "There didn't seem to be any theory behind it." This is not a very rare or mysterious phenomena. Inaccurate airspeed indications can be caused by inaccurate dynamic and static air pressure forces. Inaccurate altitude indications can be caused by inaccurate static pressure forces. AIRSPEED. Let's talk about inaccurate airspeed measurements first. An airspeed indicator is a balancing mechanism. It balances the difference between a force created by dynamic air pressure and a force created by static air pressure. The force from the dynamic air pressure is the result of the forward movement of the airplane. The faster the airplane moves the greater the force exerted. In order to get an accurate measure of that dynamic force one needs to accurately sense the free stream dynamic air pressure and send it to the airspeed indicator via leak and kink free tubing. That means sensing the air movement in the actual direction that the airplane is moving and having that air movement not affected by some local air flow direction change caused by the airframe itself. This is why you sometimes see flight test airplanes with a long boom sticking out forward with a small vane mechanism on the front of it. This boom and vane mechanism, along with connecting tubing, is a pitot tube system intended to accurately measure the force from the free stream dynamic air pressure without any inaccuracies introduced by local airframe air flow. See Note One below. So much for the dynamic force side of the airspeed indicator balancing act, what about the static force side? An accurate static force is provided by a static port ideally located somewhere on the airframe such that it is measuring the true static air pressure. But finding that ideal location and making the perfect static port that does not introduce static air pressure errors is not always so quick and easy. And what do we do if we discover that the static port that we have installed is not producing accurate static air pressure? See Note Two below. ALTITUDE. An altimeter is also a balancing mechanism. It measures the difference between the initial altitude setting of the altimeter mechanism as compared to the static air pressure encountered by that same mechanism while in flight and displays that difference in some lineal measurement (usually in feet in our part of the world). If the static pressure provided to the altimeter via the static port and the connecting tubing is in error or changes with the airplane's airspeed, when the actual altitude is constant, then the altimeter's altitude indication will be in error. And what do we do if we discover that the static port that we have installed is not producing accurate static air pressure? See Note Two below. 'OC' Says: "The best investment we can make is the effort to gather and understand knowledge." Note One: Why don't we use these long boom and vane type pitot tubes on our experimental amateur built aircraft? Because the boom and vane would take a terrific beating from people walking into them on the ground and because the dynamic force errors introduced by the type of pitot tubes that we commonly use are just not great enough to cause us concern. There is not very much we can do about adjusting the force coming from dynamic air pressure other than using a pitot tube located a sufficient distance from a disturbing piece of airframe (usually the bottom surface of a wing) and avoiding a leak or kink in the tubing going from the pitot tube to the airspeed indicator. Note Two: So we have built our airplane, installed our static port(s), and discovered that we are getting inaccurate airspeed and / or altitude indications and decide to do something about it. We could just go on installing new static ports in different locations, but that is a lot of work and we are not assured of better results. So we should do just what the big boy aerodynamic types do, we fudge or bandaid as needed to get the air to give us the results that we want. Let's say that the airspeed indicator is reading too high -- it says the airplane is going faster than it really is. (See Note Three). The dynamic air pressure side of the airspeed indicator is providing too much force in the desired balancing act. How can we counteract that excess dynamic force? We increase the static force being fed to the airspeed instrument by the static port by installing a small wedge just aft of the opening on the static port (thin edge of the wedge facing forward towards the hole). This small wedge causes air to pile up and increase the static air pressure going to the airspeed indicator (and also to the altimeter unless you have provided separate static ports for the two instruments) and give us the accurate force balance measurement that we want. Let's say that the altimeter reading goes down 200 feet when you speed up 60 miles per hour (Mike, you did not say which direction your altitude was changing with the changes in airspeed). This means that the static port is feeding greater than static pressure as your airplane flies faster. How can we reduce that undesired increase in static air pressure? We install a small wedge in front of the hole in the static port (thin edge of the wedge facing aft towards the hole) to shield it a bit from dynamic air pressure coming from the front in order to get a more stable and accurate altitude reading. How do we get the right size wedge facing the correct direction to get the airspeed and altitude results that we want? The same way the big boys do: TRIAL AND ERROR and MORE TRIAL AND ERROR. Because our airplanes are experimental, amateur built we are permitted to do just that -- experiment. Note Three: So now we know how to tweak our static port(s) to give us accurate airspeed and altitude information, but how do we know that the airspeed and altitude information is inaccurate in the first place and during our trial and error efforts how do we know when we have achieved the accuracy that we are seeking? The answer to those two questions is not as simple as one might first expect. I won't attempt to answer them here because the answers are too big and complex to answer in this forum. What I will suggest is that the reader google "accurate airspeed" and "accurate altitude" and delve into those subjects to the level desired. Here is just one source you will find: http://gpsinformation.net/main/altitude.htm Note Four: This is a personal view point. There are several methods available for determining accurate true airspeed. Some rather elaborate -- some use GPS. Just google "accurate airspeed using GPS". One thing that I've never quite understood regarding these methods is the focus on precisely determining airspeed accuracy indication in the cruising airspeed range. If I determine that my airspeed indicator shows 150 knots indicated airspeed and I determine through some elaborate scheme that I am actually only going 145 knots through the air what do I do with that information? Being 5 nautical miles short of my destination after a one hour flight is a trivial navigation error contribution compared to all the other error sources (such as heading, wind, and climb airspeed) that I have to contend with and should overcome anyway by some means of real time enroute navigation. I think that if I were going to invest a lot of time and effort in determining my exact airspeed error I would be inclined to do that determination in the approach airspeed arena, not the cruising airspeed arena. And even then I would not be obsessed with absolute airspeed accuracy, I'd just want to know what number on the indicator gives me the right kind of safe approach and landing time after time. ====================== ----- Original Message ----- From: Cc: "mike mccann" ; "Pulsar builders" Sent: Thursday, April 15, 2010 9:18 AM Subject: Re: Here's an odd question >I noticed that the back half of the round washer where the static exits on > the SR22 had the back half of it filed down so there was in effect a > little shield protecting the static exit hole. ((Sorry about this > description.) > > I asked the people at their booth why they did that. I think I attended > four air shows before I found someone who was involved with the > engineering of the system. The answer - - - - "That is what we had to do > to make the system work right". There didn't seem to be any theory behind > it. They indicated that they had to fiddle around to get it to behave the > way they wanted. Maybe they were just trying to get rid of me. > > In any event, I did the same with mine and it works fine. > > Bernie Wilder =================== >> >> ----- Original Message ----- >> From: mike mccann >> To: Pulsar builders >> Sent: Thursday, April 15, 2010 5:06 AM >> Subject: Here's an odd question >> >> >> All, >> >> Utilizing a GPS, I've found that my altimeter in my Pulsar varies with >> changes in airspeed (altitude will change 100-200 feet with speed >> changes of 60 mph). >> >> Has anyone ever heard of this. Not sure why this would be, or more >> importantly, how to fix it. >> >> Many thanks, >> >> Mike >> >> Pulsar 1 >> N116Km End Msg: #27 === From: Galin Hernandez Sent: Tuesday, October 22, 2013 2:03 AM Subject: KIS-List: TAS Error with my new EFIS Pardon the long write-up. After buying my airplane several years ago I did a series of maneuvers at different power settings and, by using GPS groundspeeds, I made a fairly accurate calibrated airspeed chart. For the next few years I used the CAS chart often and found that, at normal power settings, I consistently cruised at 143KTAS. Fast forward to this year when I installed a complete DYNON Skyview EFIS system which automatically calculates TAS. But, at normal cruise power settings, the Skyview consistently showed me cruising at 156KTAS. The Skyview also showed that no matter which direction I was flying, I always had a headwind. Knowing that the Skyview did not change the aerodynamics of the airplane and the system had just passed an IFR check, something was not right. On subsequent flights I calculated my TAS using my old CAS chart and came back to 143KTAS not the 156KTAS the Skyview was displaying. The plots on my CAS chart shows that as my airspeed increases, IAS and CAS spread apart with IAS being almost 20KT faster at full power. Since the Skyview does not have a method of factoring in a CAS, it uses IAS for the calculations. With the significantly higher IAS used by the Skyview it would display a higher TAS than actual matching what I was seeing. Talking with the engineers at DYNON, they told me that in a =93well designed=94 system the difference between IAS and CAS should not be more than a few knots. If my IAS and CAS was that far apart, there is a problem with either the pitot or static source position. Having IAS significantly higher than CAS meant that either the pitot tube is in an area of =93higher=94 pressure or the static source is in an area of =93lower=94 pressure. If the pitot is in an area of =93higher=94 pressure the error would only be in airspeed. But if the static source is in an area of =93lower=94 pressure, altitude indications will show me flying at a higher altitude than I really am. This is a potentially deadly situation when flying real IMC, which I do. To isolate if the error was in the pitot or static, I flew to different airports with ILS approaches over the next few months. I found that when at the FAF on the glideslope of each airport, the Skyview displayed a consistent 100-110=92 higher than what the approach plate showed. In order to confirm what I found, I did several high speed passes, over different airports, trying to stay about 10=92 above the runway. At each airport the Skyview displayed a consistent 100-110=92 higher than what the runway really was . This matched what I found with the FAF experiment. Since my CAS chart showed the IAS and CAS getting further apart the faster I flew, the data obtained from the test flights was consistent with the static source being in an area where pressure drops as airspeed increases. So I needed to correct the static source placement. The dual static source I have was installed by Rich Trickel back in 2008 and I really didn=92t want to change where they were located. So in order to correct for a possible changing static pressure situation I experimented by placing a piece of Gorilla tape just behind each static port hole and doing the flight checks again. With one piece of tape, the FAF and high speed runway checks altitude error dropped to about 40=92. My displayed TAS error, based on my CAS chart, had also dropped to about 10kts. I figured I was on the right track so I added a second piece of tape effectively doubling the thickness behind the static port hole. The next series of flights showed my FAF and high speed runway checks had dropped to within a few feet. My TAS error had also dropped to within 2Kts at normal cruise. A third piece of tape caused the FAF and high speed runway checks to show the altitude difference was about 40=92 but in the opposite direction. Now the TAS error was back to about 10Kts, also in the opposite direction than before. Armed with this information, I went back to using two pieces of Gorilla tape just behind the static source hole. During the next flights the FAF and high speed runway checks showed the altitude error was within 10 feet of what I was expecting. Additionally I did a series of maneuvers at different power settings and, by using GPS groundspeeds, calculated that my TAS was within a few knots of what the Skyview was displaying. During my final test flight (October 21, 2013) I found that normal approach speed (90Kts) and normal cruise (143Kts) speeds were within 2Kts of calculated. I also found that as I approached to land, the wind aloft display matched very closely to what the tower was reporting. Now I know what I have to do to correct my static source position error. I can complete the work by making a piece of aluminum the thickness of two pieces of Gorilla tape and permanently attach it just behind to the static source hole. So, if your EFIS show an airspeed higher than what the =93book=94 shows your airplane should be doing, check it out. You may have a pitot/static source error that can be corrected. Overall I think I corrected a possibly serious situation and now completely trust my Skyview is displaying correct information. ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message kis-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/KIS-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/kis-list Browse Digests http://www.matronics.com/digest/kis-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.