Interference affects on Pitot probe

[JQCF1]

Hi all,
We are considering minor exterior (aerodynamic) changes to a nose radome. Any leads as to how this will/might affect the pitot (total pressure) probe that is slightly aft of the radome. We are talking Vne appx 250kts. Any reasonable measure of probe standoff distance to fuselage diameters or whatever ratios??
It will not be convenient to use a chase plane over a measured course and I do not have CFD capabilities.

Any ideas, thanks. John

 

[rb1957]

this Could affect the readings of the probe; i'm assuming the change isn't so minor that anyone looking at it would say the effect would be negligible (otherwise you wouldn't've asked the question).

that said, i'd expect to see a flight test that would compare the new probe with an unaffected probe (a flight test probe on a wing tip maybe?) ... it'd probably be best to first fly the unmodified plane, to calibrate the flight test probe.

good luck !

 

[FieldTeam]

I've seen examples of what rb1957 stated on USAF aircraft, where the probes, which normally would be positioned on the fuselage aft of the radome (707), are mounted on the wing tips due to nose radome modifications.

 

[JQCF1]

Hi Rb and Field Team,
We do not propose changing the pitot probes, just the radome configuration; and that not drastically. The static ports are on the aft fuselage and will not be disturbed.
This TC'd a/c is currently operating on a Special/Experimental A/W cert (8130-7) so we can play games but adding a wingtip probe is not easy considering the plumbing needs.
Turns out at a meeting today there may be several minor aero mods to the radome in this program and maybe the best way is to develop an alternate location for the total pressure probe to use on the airspeed system. Calibrate that alternate system and forget the original until we go back to standard A/W category. This may be the way to go since the a/c has dual total probes at the same fuselage station (Rt and Left) for pilot/copilot seperate systems
Your comments are welcome and appreciated; any more??
Thanks, John

 

[berkshire]

JQCF1
If you feel the modification may mess up the pitot pressure, see if you can borrow a trailing bomb pitot/ static unit with its own calibrated ASI.
Use this to see if the installed unit has significant errors.
B.E.

 

[rb1957]

i understood that you were changing the airplane contours near the probe, thus affecting the airflow around the probe.

if it is "only" the total pressure probe ('cause static pressure is being read from somewhere else), then maybe the effects are secondary (changes in dynamic pressure are reflected in static pressure so that total pressure is constant).

it may be safe to install a temporay probe (for flight test purposes) to verify the system. i think this should be calibrated in the against the unmodified fuselage (benchmarked) then the flight test probe would become the calibration standard for the modified probe.

 

[JQCF1]

RB and Berkshire.
Neat ideas. I like the trailing probe. This a/c is pressurized. How do we trail the p/s probe? ie, How do we release it to trail after take-off and retrieve it before landing?
I've never used this scheme but it sounds like it will work. Are there kits available or do we build one? Would a single flight to calibrate be sufficient to rely on the original installed system with a calibration chart like a wet compass correction card?
We're getting there but like they say the devil is in the details!! Thanks, John

 

[berkshire]

jqcfi
I found this company that makes cones and trailing bombs

http://www.spaceagecontrol.com/trailing-cone.htm

The last time I used one of these things, was at Grumman American,in the 1970's and I think it was something the company made.
B.E.

 

[JQCF1]

Hi Berkshire,

We must be on the same frequency!! I also found Space Age Control (via Google) and some of their components look quite doable. Neat website.
I forwarded their URL to my client and we'll see what he wants to do. He's holding the purse strings.
I'm also proposing a Pitot probe at a different azimuth position than the two that exist now just behind the radome.
Thanks, John

 

[berkshire]

JQCF1 (Structural)
If you have two pitot heads on opposite sides of the aircraft, most often they are connected by a tee , or small chamber, behind the instrument panel. The idea of this is to reduce errors caused by yawing of the airframe.
If you think your modified radome is going to produce standing waves that will affect both at once. you may be better off taking Field Teams advice and putting a pitot head on the wing. You can always tape it up there with 600mph tape, the same goes for the vinyl tube to the cabin.
That is if you cannot use a trailing bomb due to pressurisation problems.
The other thing you can do is fly a triangular course with a GPS , average the results to remove wind effects, then prepare a deviation chart from the results.
B.E.

 

[davidjh]

Forget the trailing bomb instl; trust me, you do NOT want to get involved with those. Get a "kiel" pitot - it is a small device with a ringed shroud around the pitot probe inlet face. It provides 100% recovery over lage flow angles, but is not ice protected, but that is something you can handle in flt test.

Find some place up around the nose, preferably on the centerline to mount it. Many airplanes have an access hatch on the lower fuse CL that is a good loccation. You arn't particularly concerned about the high speed regime - the faster the speed the more likely a pitot probe will have 100% recovery - its the slow speed regime where the higher local flow angles "may" cause the production probe to "break down" - the production probes have sharp leading edges which can result in break down with small changes in local flow - I've seen a sudden drop in the production airspeed from a 90knot realm down to 45 knots on a 100,000 lb transport at airplane stall angles of attack, but the old kiel pitot remained valid.

The comment about sideslip causing pitot errors is valid.
But the airspeed system need be accurate or have measured errors only throughout a defined part of the envelope - depends on the regulations the airplane was TC'd to. The safety concern is that the pitots do not "break down" at airplane angles of attack coupled with sideslip angles that are reasonably expected to be experienced - such as a cross wind landing, or engine out flight at the takeoff safety speed.

The comments re doing baseline/unmodified airplane tests with a "reference" pitot, and then repeating them on the modified airplane are certainly the pure way. But a good reference pitot instl should permit testing only on the modified airplane.

Re the comment on the pitot probes being manifolded together - my expereince is that most regulations (mil or civl)require completely independant airspeed systems for the pilot and copilot. With the sideslip comment being even more relevant in this situation.

Good luck - God forbid we know how fast/slow we are flying.

 

[rb1957]

a question to you guys who obviously know more about this than i do ... how does the local profile affect total pressure ? obvoiusly it'll affect static pressure (but these ports are remote from the change). i guess, just maybe, it could develop a shock (which would be really bad for the rest of the plane).