Technology Cloud Cap - Unmanned Aircraft & Drones

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Cloud CapTechnologyPO Box 1500, No 8 Fourth StreetHood River, OR 97031(541) 387-2120 ph (541) 387-2030fax• The pitot tube must not be in the propeller wake (a thrusting propeller adds total pressureto the flow and will corrupt the reading).• There should be no leaks or obstructions in the path between the pitot port inlet and thesensor in the avionics.The last point brings up an interesting question: what about flying in the rain? The pitot tubeshown in Fig. 9 is suitable for flying in the rain, due to the large opening (1/4 in.) which preventsthe bridging of a drop of water. If a water droplet bridges the pathways of the Pitot tube, it wouldobstruct the total pressure path. For slow speed vehicles the pitot tube needs be on the order of¼” in diameter to prevent water droplets from bridging across it.4.2 Static SystemAn accurate static pressure system is more complicated than the pitot system. Many small UAVsdo not include a static pressure reference, instead using the internal fuselage pressure as the staticpressure. This is a bad idea because the fuselage is not typically vented in any consistent wayand may host pressures which are far from static.Typically full size aircraft go through a test and evaluation period in which a position on thefuselage exterior is found that is close to static pressure, and that is used for the static source.Until such an evaluation can be done we recommend that you use a static reference similar toFig. 10.Figure 10 – Example static tube on the right wingtipOur location of the Static tube on the wing was based on a location that; wouldn’t be influencedby the fuselage pressure disturbance, and far enough in from the wingtip to prevent damageduring take off or landing if the wing tip touched the ground. The static system shown in Fig. 10is made of a ¼” carbon tubing. The end of the tube is plugged and rounded off to minimize thedisturbance to the air flowing over the tube. Back from the tip, at least 8 tube diameters, a ringof four small holes are drilled symmetrically around the shaft. The holes are the static pressuresource. In the wing the back side of the shaft is sealed and connected to soft Tygon tubing whichruns back to the static port on the avionics. Some things to watch out for when using this type ofstatic source:• A squared off or rough tip will cause disturbances that will affect the static pressure.This can be mitigated by moving the static pressure holes further back along the tube.Piccolo Aircraft Integration Guidelines Page 12 of 20
Cloud CapTechnologyPO Box 1500, No 8 Fourth StreetHood River, OR 97031(541) 387-2120 ph (541) 387-2030fax• The mount structure behind the tube can influence the static pressure field (assuming thevehicle is subsonic!). If the structure is large (like a fat fuselage) the effect can besignificant. While the exact effect is difficult to predict without some form ofcomputational fluid dynamics; a rough rule of thumb is that the static pressure holesshould be in front of the structure a distance of twice the structure diameter (assuming ablunt axisymmetric structure like a UAV fuselage).5 Wiring HarnessYou will need to build, or purchase from us, a flight harness that supports your aircraft servoconfiguration. If you decide to build your own flight harnesses follow these guidelines:• Make all leads as short as possible (minimize extra wire length); this will help with RFcoupling from onboard transmitter which can interfere with RC hobby servos.• Avoid using servo lead extensions, the extra wire and extra connectors all will addpotential failure mode.• When routing your servo and RF leads try to keep them separated as much as possible,cleanly routed and tied down. Never run coax in parallel with unshielded servo wires. Ifyou have to troubleshoot RF interference problems working with a neat and cleaninstallation will make your job much easier. If you are working with a rat’s nest of wiresand connectors with extra leads everywhere the likelihood of success in troubleshooting,and the repeatability from vehicle to vehicle, will be limited.• Use crimp pin and socket connectors, do not use solder cup type connectors. Solderjoints on wiring harnesses will fail due to vibration unless proper strain relief is provided.5.1 Typical Flight HarnessBelow is the schematic for a typical flight harness with ailerons, for a printable version go to thedownload page on our website (www.uavautopilots.com). We also stock these as shown if youdon’t want to assemble them yourself.Piccolo Aircraft Integration Guidelines Page 13 of 20
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