Thursday, September 26, 2013

Pitot fixation flange evolution

For some time now, as per previous posts, I've been working on the new fixation flange for the 8 mm pitot tube, results at last are good and the website  will be soon updated will all the information needed for the DIY maker.

The production piece is those depicted here below, in figure F12.1.
Figure F12.1 Pitot fixation flange , JLJ courtesy

Although it is only an accessory it's so comfortable to use that it worth some of our time. You can view a video of the whole pitot test assembly here below

Video P12.1 Pitot , pitot assembly test

With this accessory flange the piece count, the weight and the size are optimized further and this is a contribution to a very fast overall construction, calibration and mounting process.

Fixation to frame is achieved with four M6 screws. The four M6 screws warrant a really good fixation and it's also possible to use only 2 holes and trim the other unused fixation tab. Such a flexibility is valuable when the pitot is mounted into the wings.

The flange dimension allow to mount the pitot inside the wing when at lest a thickness of 25mm is available, indeed the wing preparation and the mounting is quite similar to aileron servos mounting.

Regarding mounting I think also a “frontal like” mounting can be useful in some situations so I'm evaluating a possible modification of the original design. Refer to the below rendered figure, here you can view a fixation flange with two extra fixation tabs.

Figure P12.2 Experimental Pitot fixation flange with more fixation tabs, rendered image
Eventually the unused tab can be cut out with an abrasive disc, probably some scars will remain but they can be polish with waterproof abrasive paper and abrasive paste.

The impact of this new design on dimensions and weight is limited, eventually the physical prototype test will highlight any unforeseen issue and will give the opportunity to try a better design.

Have fun,

Tuesday, September 17, 2013

Drilling DIY pitot static annulus pressure taps

During manufacturing operations of DIY pitot a sensible as important phase is the drilling of pressure taps, I introduce some common issue for the 8mm pitot probe.

Making of the DIY pitot tube is straightforward, anyway special care should be taken in the annulus static pressure port drilling operations. As the involved dimensions are reduced some manufacturing problem can arise, have a look to a the pitot probe static ports here below
Figure P10.1 Finished annulus static ports, JLJ used pitot.
Holes are 1 mm diameter and spaced 45° to have eight holes total on an annulus shape. To drill that holes by hand can be really thrilling and sometimes the result is unusable so a lot of swarf crap should be expected. To contrast the fate a more strong approach can be used, the drilling jig. Here below an example of well proven and comfortable setup
Figure P10.2 Pitot tube on a DIY dead center drilling jig in action, the tube is wrapped into masking tape to avoid scratches during drilling

In this drilling jig two main components are assembled on a railed rigid frame, an adjustable dead center and a mandrel. Dead center will be pushed tight against the pitot tube to ensure that the component will not bend when the drill bit plunges into the piece. The used mandrel have the possibility to rotate with the clamps locked on the tube; when turning the tube by hand a dial provide angle so it's possible to work fast and precisely. Usually this kind of mandrels can by bought in hardware stores as they are used, for example, for lathe or milling operations. With such a kind of setup you can simply put the jig on your vertical drill or lathe table vice, drill the first hole on the center line of the tube and go ahead simply rotating the mandrel of 45° per time. If your equipment is correctly aligned every hole will be perpendicular to the tube surface, with next to zero effort.

Consider the following figure depicting the air path around mechanical burrs.

Figure P10.3 Possible streamlines around burrs

Recalling the Navier-Stokes equations and in particular the equations for incompressible fluid flow it's evident the strictly correlation between machining burrs, that cause a local change in stream velocity, and pressure tap error in pressure readings. Given V as the current flow speed, \(\rho\) as the fluid density and \(p\)as the stream line pressure the Euler equation is \(dp=-rhoVdv \) , that correlate directly the change in stream speed and pressure.

Look figure 10.3 that shows a cut view of a pressure tap with two different kind of idealized burrs, one upstream the other downstream. Both the defects leads to a variation in the stream speed so a variation in pressure at the tap point. So the pressure tap must be deburred but the tap opening on the tube shall be flush to the outside diameter.

Figure P10.4 Pressure tap deburred in a wrong way, unwanted radius introduced

Reaming a hole, as per figure 10.4 is to avoid in fact the correct hole intersection with the tube must me sharp, no radius allowed.

The use of a tapered reamer is common in mechanical work but inappropriate for pressure tap making. If the probe ports are not properly prepared the calibration factor will be notably dependent on Reynolds number. If speed increase also the reading error will increase. By many authors the static pressure error is the main source for speed reading error, total pressure port is often considerate as ideal, my experience lead to agree this approach.

To get good measuring results do not use a tapered reamer or drill bits to remove the burrs. A most precise way to attain flush deburred holes is to drill a preliminary hole of about 0,8 mm dia and after use a flat end mill to enlarge the hole to nominal 1mm dia. A pretty result can be attained, on carbon pitot tubes, directly drilling the 1 mm dia holes and using a wood piece to remove by hand the burrs.
Have fun,

Monday, September 9, 2013

DIY Mechanical angle of attack windvanes part 3

This post is the continuation of Angle of attack windvanes posts miniserie

DIY air data boom with angle of attack and angle of sideslip mechanical vanes

An example numeric design will be here presented. Refer to the following scilab file, it's a simple script file to evaluate the windvane parameters. For the sake of simplicity, when possible, the nomenclature is those of the main, freely available, reference “Wieringa (1967),Evaluation and Design of Wind Vanes, Royal Netherlands Meteorological Institute, De Bilt” By the end of the post a compact set of equations and a basic procedure for windvane sizing will be exposed.

Design process is recursive and require to go forth and back through the specification, 3D design and performance evaluation phases; formally to find the best windvane design can be seen as a multivariable, multiobjective optimization problem. To avoid a complexity explosion only a basic approach to design evaluation is presented in figure F9,3. Note that our fast preliminary design is possible thanks to the availability of a closed form math model for the vane, results and performance evaluation can be eventually refined further during a successive simulation phase.

Executing the Scilab file will return the following output, that summarize the supplied example windvane design parameters

Vane parameters

Windvane weight grams 8.00

rv lenght 25.00 mm

rw lenght 6.25 mm

Vane calculated parameters

Fin surface 380.95 mm^2

Vane inertia 0.000006 kgm^2

Natural frequency 72.63 rad/s

Damped frequency 72.60 rad/S

Damping ratio zeta 0.030

Decay distance 92.3 [m]

Shaft at test condition with no friction

Relative wind speed 30.00 m/s

Alfa value 0.10 degrees

Aerodynamic Torque 5.75e-05 Nm

Shaft at test condition with viscous friction

Viscous friction term Dm 2.000e-05 Nms/rad

Damping ratio increase due to viscous friction 0.022

Total damping ratio 0.052

Table T9.1 Scilab script output

Consider the possible model for the main shaft bearing friction in figure P9.1, let's pretend that the curve is approximate by excess so the torque values are conservative, to be more general consider also that torque values are the sum of the torque generated by all the frictions on the shaft including those derived by the position sensor.
Figure F9.1 General bearing friction model including stiction, to carry out a general analysis consider this curve comprehensive of all the friction caused by bearing and sensors.

In table 9.1 is reported the approximate torque generated by the lift of the fin at the shaft, 5.75e-05 Nm for a wind blowing at 30 m/s;. The aerodynamic torque that our vane generate for an alfa of 0,1° at 30 m/s is greater than the torque of 0,02e-3 necessary to the shaft to begin to rotate. At 30 m/s the vane will hopefully correctly rotate to 0,1° alfa. differently if the speed is reduced to the lower bound of required speed range, in our case10 m/s, the aerodynamic torque is 0,006e-3Nm so the windvane will not rotate to 0,1°; at the former speed the static error cannot be smaller than 0,02/0,006*0.1=3,3e-1.

Given storque as the static torque required to move the shaft ,statico as the max static error and N as the vane torque coefficient the following constrain should be satisfied by a healthy vane

Nstatico>>storque, where N is speed dependent
With our example numeric values 0.0036633*0,1/360*6,28>>storque, storque <<0.0064e-3
In those cases when it's non possible to reduce anymore the friction it will be necessary to increase the windvane fin area.

Table 9.1 shown a low damping ratio, quite different from our initial requirement of a value greater than 0,15. Recalling that our current model neglects the bearing damping some considerations should be added for a correct result interpretation.

Note that when wind speed is very high, as friction torque contribution is little compared to the torque generated by the fin, the behavior of the vane will be essentially those predicted by the model, so a very low damped response should be expected. At lower speeds the contribution of friction are not negligible and lead to an increase in the response damping.

Wieringa Eq.30 add a viscous term proportional to rotation speed to the ideal case model, no static friction is considered. Other authors for example 1974, JAMIES T.KARAM, JR. TECNICAL REPORT AFIT TR 74-8” page 6 have considered stiction.

Again according to Wieringa the damping ratio variation against relative wind speed is plotted on the following figure valid for the example vane.
Figure F9.2 Damping ratio against relative wind speed, Dm assumed to be 20e-6 Nms/rad
To add damping at higher speeds some mechanism, aerodynamic or mechanical based, can be employed, the example probe do not use any mean to compensate for relative wind speed.

Commonly the choice of the sensor for vane angle is oriented to the use of low friction devices, hall type sensors have been successfully employed, see a commercial available typical rotary sensor at this link.

The whole design process is somewhat recursive, the following figure pictures at glance a possible design development path.

Figure F9.3 Possible design process development path

Inspecting the figure quickly jump to attention the fact that post design simulation and testing should be carried out on the vane, although not treated here the design validation is to be considered as a primary design task.

Nice designs,