Static Port Location

[Utah-Jay]

I see several have located their static ports behind the cargo area, I am wondering why? My build mentor has his static ports on the boot cowl of his Carbon Cub, it seems it would be a lot easier to plumb the ports close to the panel/avionics. Certainly locating the static ports so far aft requires a longer plumbing run.

I am not trying to reinvent the wheel, just checking/wondering if the BH design dictates the aft static port location.

All help and ideas are welcomed

Jay

[Battson]

Jay - this has been discussed for decades, including the old "Yahoo group" long before the forum existed.

Many Bearhawk builders have had trouble with getting inaccurate readings from static ports located on the boot cowl. I know some other aircraft types have made it work, and some Bearhawks made it work too, but not all.

IIRC, the Bearhawk community's collective experience has shown that static ports work reliably, in practically all cases, when located on the sides of the fuselage at the next station behind the aft bulkhead (station numbers not on hand sorry).

Hope this helps, someone will correct me if I misremembered that.

[RobinDeMarco]

Perhaps that location has the least amount of airflow variation? From what I’ve seen from you tube videos and builder blogs, there are a number of different ways to do the boot cowl. Maybe each of those methods has a different effect on the airflow and causes the result to vary build to build. The section aft of the baggage area is pretty constant.

[spinningwrench]

Turbulence is a bad thing for static ports. Behind the baggage compartment is as smooth as it gets. Look at the location on a Citabria or Husky.

[Chris Werner]

I plan to install my static ports on the side of the fuselage behind the bagage area. This is very similar to where they are on my piper comanche.

[Utah-Jay]

OK, it seems although a few have done otherwise, I will put my static ports behind the baggage area.

Thanks everyone

[Nev]

It's actually pretty straight forward to do it that way, then run one line forward. I queried it too at the time. It's one area that I've had no further issues with.

[alaskabearhawk]

From an aeronautical engineer on the old Yahoo group:

A pitot-static system works by measuring the difference in pressure created by the forward speed of the aircraft (pitot pressure at the inlet to the pitot tube) vs. ambient pressure at the static ports (a pressure as close to ambient as is possible). The pitot system is relatively insensitive as long as the pitot tube is pointing somewhat forward (+/- 15 degrees into the relative wind).

The static system is very sensitive to the local environment and at best is a compromise. The shape of the fuselage has areas that have higher than ambient pressure and other areas that have lower than ambient pressure and the pressure in these areas changes with flight conditions, vent opening/closing, cabin leaks etc. The internal cabin pressure is almost always lower pressure than ambient due to the shape of the fuselage. If I remember correctly, the Cessna manual for the 172 says that a 200 ft error occurs when changing to the alternate static which opens inside of the cabin

When an airplane manufacturer certifies a plane for IFR the static must be accurate (+/- 25 ft total error) over the entire range of speed and control positions. This requires a survey of the aircraft static port locations vs a reference static system. (usually done with reference to a trailing cone system that follows the airplane by several hundred feet). It can take several tries to get a static port located so that it meets requirements. Another way to obtain an approximate check of the static system is to fly over a known elevation (field, lake etc.) at several speeds and see what the altimeter shows. You have to set the local barometric pressure in the kolsman window of the altimeter. You will need a ground observer to observe your true height above the reference so you will have a comparison to the altimeter reading. This second method only gives approximate data but is better than nothing.

For a homebuilt aircraft, usually the best static location is obtained by copying a production aircraft static system and location from an airplane that is as similar as possible to the homebuilt. This location still needs to be checked by one of the above methods.

Static error can ruin your whole day. I have a professional test pilot friend that was testing a factory prototype. Tested stall speed on the aircraft was 58 IAS. After an avionics change was made which also affected the static system, the airplane stalled on final with an indicated air speed of 84 mph on the first flight after the changes. Luckily he survived but the airplane was scrap.

[kestrel]

I was having some trouble with my wing mounted pitot/static and decided to install a new static system in the fuselage. I milled a surface mounted static port that I could tape to the surface of the airplane and test different locations. I tried several locations on the boot cowl. All of them showed significant altitude error as soon as a pushed the throttle forward. When I tried behind the baggage as I saw a number of other people had done, I was bout +/- 15 feet. I installed my new static system behind the rear baggage.

[rkennell]

I have read the 2022 year thread as I am considering the placement of the static port. The posts are helpful but I have a clarification question. The Companion fuselage is based on the four place and has similar dimensions, however the baggage compartment is moved up to the station for the second row of seats, station D-N, and the engine mount is about 6" shorter. If I place the port behind the baggage area for the Companion it will be at the baggage area of the Bearhawk four, station E-M. To place the port in the suggest position I would need to go back to station F-L. Am I thinking correctly or is placement dependent on the specific build?

[Battson]

I have read the 2022 year thread as I am considering the placement of the static port. The posts are helpful but I have a clarification question. The Companion fuselage is based on the four place and has similar dimensions, however the baggage compartment is moved up to the station for the second row of seats, station D-N, and the engine mount is about 6" shorter. If I place the port behind the baggage area for the Companion it will be at the baggage area of the Bearhawk four, station E-M. To place the port in the suggest position I would need to go back to station F-L. Am I thinking correctly or is placement dependent on the specific build?

I would put them in the same location relative to the fuselage exterior geometry, as has been proven to work on the 4-place. The interior structure of the fuselage doesn't concern the airflow around the outside of the aircraft - and that's what drives the performance of the static ports, airflow around the fuselage. It's unhelpful that I referenced the aft bulkhead in the above post, it would have been more relevant if I referred to the exterior geometry of the fuselage.

I don't have Companion plans to hand, but if you put the static ports about 1 station ahead of the LE of the horisontal stabiliser, then that would be a location which is known to work pretty well.

[rkennell]

Thanks for the clarification.

[svyolo]

I strongly concur with Johnathon's answer.