Update On ThePoorman'sAutopilot

By Donald E. Hewes (EAA 32101)Aero Research Engineer

12 Meadow Dr.Newport News, VA 23606

S,INCE THE PRELIMINARY report on my homebuiltautopilot appeared in the April issue of SPORT AVIA-TION, I have received letters from and had conversa-tions with a large number of enthusiasts about instal-ling a similar system in their own airplanes. At the Fri-day Forum that I conducted with the help of DougGarner this year at Oshkosh, just about everyone of theabout 250 attendees indicated that they wanted to builda unit. (The majority of attendees were active builders ofthe VariEze, the KR-1 and KR-2, and the Thorp T-18.)Because of this very enthusiastic response, I have de-voted several hundred hours in refining and testing thesystem so that it should be fairly easy to build, alignand operate.

There are literally hundreds of possible circuit ar-rangements and design details and I doubt that I havecovered them all and found the very best combination.However, I think that the arrangement covered in thisarticle represents a practical and effective system. In theJune 1978 issue of SPORT AVIATION. Doug Garnerpresented recent refinements to his wing-leveler systemwhich is the basis of my system. The rate sensor andpulse width modulator (PWM) circuits are essentiallythe same in both systems; however, other details of thecircuits are different.

The system can be used in (1) the manual-trim modewhich provides roll trim while the autopilot is not beingused, (2) the wing-leveler mode which holds the wingslevel and prevents the airplane from entering a spiraldive, and (3) the heading-hold mode which locks theairplane onto the desired compass heading. An addi-tional feature is a left-right command switch whichpermits the pilot to command a steady turn rate or asmall heading change depending on which mode is beingused.

In the flight tests with my BD-4, I have found thatthe wing-leveler mode would recover the airplane from a45° banked attitude in a spiral to a wing level attitudein 8 to 10 seconds. Also I found that the heading holdmode would hold the desired heading within ±1° infairly calm air and ±2 or 3 degrees in fairly turbulentconditions.

When I wrote the original article about this au-topilot, I had planned to provide complete constructionand operating instructions in this present article. As Istarted preparing this material it was quickly apparentthat there was a very large quantity of details whichwould be needed by some of those desiring to build thesystem but lacking a general knowledge of electrical cir-cuits. Because this material would require a great dealmore of my time to prepare, I have elected to limit theinformation covered in this article to those details which

describe the changes to the system and how the fin;system works in general.

The information concerning circuit layout, compo-nent arrangement for easy assembly and compact pack-aging, and initial balancing of the circuit for optimumoperation are not included in this article. Those readerswho wish to obtain more information, especially theseparticular steps, should refer to the note at the end ofthis article.

I believe that for someone who is capable of buildingtheir own airplane the system is not too difficult tobuild, install and operate. The use of only simple handtools (drill press, disc sander, etc.) is required. If you in-tend to build the system, you should know how to solderand you should know a little bit about the fundamentalsof electrical circuits. Such information is readily availa-ble in do-it-yourself type books at most electronic supplystores. You will need a 0-10 volt scale voltmeter andwill find an oscilloscope very helpful.

Inasmuch as I have had access only to my BD-4airplane for installation and flight testing, I cannotguarantee that the system will work as well in someother airplane design. However, as an aeronautical en-gineer, I see no reason why the system cannot be madeto perform equally as well in any other design with verylit t le modificat ion. The only modification requiredshould be the sizing and installation details of the con-trol tabs used to produce the aerodynamic rolling mo-ment.

System DesignA block diagram of the improved system is given in

Figure 1. This diagram is similar to that given in the

FIGURE 1

original article except that several features have beenadded, as noted by the asterisks. In addition several de-tails of the circuit have been revised and a couple errorsin the original circuit have been corrected. The followinggives a review of these changes and additions.

Heading Sensor — Figure 2 presents sketches andnotes concerning the heading sensor which detectschanges in heading of the airplane from the desiredheading. This unit has been changed so that it now hastwo photo transistors rather than one. This changeminimizes the heading errors caused by changes in am-bient lighting conditions and greatly increases the rangeof heading errors over which the autopilot can "capture"the heading.

Although the sensor will work using the letters atthe cardinal points of the compass, as discussed in theoriginal article, I have added a special "target" on thecenter of the compass card face which the sensor nowtracks. The target is aligned with the cardinal pointsbut the sensor does not actually "see" the letters. Thetarget is much wider than the letters, is closer to theaxis of rotation of the compass card, and has uniformcontrast (solid white on the black background). Use ofthe target permitted the sensor to be moved closer to thecenter of the compass so that the view of the compassmarkings is less obscured.

SPORT AVIATION 41

With these changes, it is now possible to simply turnthe sensor to a new heading for changes up to about 30degrees and the autopilot will automatically fly theairplane to a new heading without losing capture. Thiscapability also minimizes the possibility of losing cap-ture in turbulent flying conditions. With the originalsystem, capture could be maintained only within about±5° of the heading setting and was frequently lost whenmore than light turbulent conditions were encountered.

It was found that the compass target could be addedquite easily by carefully removing the compass frontcover assembly and gluing the target in place on therotating compass card.

Rate Sensor and Servoed Vac-uum Pump — Figure 3 presentssome sketches and notes for the ratesensor. A small vacuum pump hasbeen built directly into the body ofthe rate sensor. Originally I usedthe regulated vacuum system whichI have in the BD-4 for the gyro com-pass, as a vacuum source for the jet.However, changes in power settingsof the engine caused fairly signific-ant zero shifts in the rate sensoroutput. Tests revealed that the nor-mal flow velocity in the jet is gener-ally in the range of about one-footper second and that changes in thisvelocity of only a few percent canproduce a full-scale shift in the out-put of the sensor.

Earlier use of a small vacuumpump, similar to that described byDoug Garner in his article in theJune 1978 issue of SPORT AVIA-TION, was unsuccessful because ofpoor open-loop speed regulation ofthe electric motor I was using. Themotor that Doug uses apparentlyhas very good speed regulation butis rather expensive and was notreadily available to me. I used onewhich was taken out of one of theseveral radio-controlled model servosthat I had available. This motor isvery rugged and well built and costsabout $9.00 when bought separately.

After some discussion of this prob-lem of jet velocity changes withDoug and also with Dale Walker ofEosmira, Houston, Texas (Dale isalso working on development of thissystem and has encountered thesame difficulty), I decided to use a MOTORpif tM-closed-loop system employing twofeedback signals. One comes directlyfrom the input voltage to the pumpmotor and the other from two resis-tors that sum the output signalsfrom the two amplifiers tied directlyto the thermistors in the rate sensor.Dale has used this latter signal inanother manner and suggested thatit was not of much use for the pumpbecause of the very low gain in-volved. However, I have found thatthe system appears to have a littlebetter long-term stability and, con-sequently, have retained it.

With this signal in the system, the speed of themotor changes automatically to compensate for changesin air jet velocity. A small potentiometer (pot) permitsthe speed of the motor to be adjusted to provide theproper velocity of the air jet thereby eliminating theneed for a needle valve to adjust the flow as suggestedin Reference 2.

Command Switch — This optional feature was addedas a convenience to provide the ability either to com-mand a small heading change, such as required to com-pensate for wind drift, or to command a standard turnrate, such as might be used in an instrument holdingpattern. The particular type of command provided is de-termined by the mode in which the autopilot is beingoperated. This feature was obtained merely by adding

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42 MARCH 1979

two adjustable resistors and a single-pole double-throwswitch.

Control Tab — I indicated in the first article that Iintended to install a second identical control tab (Foot-note 1) on the other wing tip so that the roll controlpower could be doubled. I have done this and the resultsshowed a very marked improvement. With the controlpower increased to about 20% of that for the ailerons,the wing-leveler mode is now able to recover theairplane from a steep spiral in about 8 to 10 secondsafter the autopilot is switched on. (The controls are leftfree throughout the test.) Furthermore, the heading-holdmode is able to maintain the desired heading in heavyturbulent conditions.

I have performed numerous hard-over tests in whichthe control tabs were held at their maximum deflectionwhile all normal flight maneuvers, including landingsand take-offs, were performed. These tests showed thatthe handling characteristics and control responses of myBD-4 were essentially unaffected even though 20% ofthe aileron control travel was used to counter-balancethe hard-over tabs.

Based on these tests, I am recommending that thecontrol tabs be sized to produce a total rolling momentfrom 15 to 25% of the maximum produced by the ailer-ons and that in no case should it exceed a 30% value.The relative magnitude of the rolling moment producedby the control tab is determined by measuring theamount of stick or wheel travel required to maintainstraight and level flight with the control tab deflected toits maximum travel. (Be sure to maintain zero sideslipthroughout by keeping the ball centered.) Compare thismeasurement with one-half of the total lateral travel ofthe stick. The following formula can be used to roughlyestimate the size of the tab needed to produce the de-sired moments for any airplane equipped with standardailerons:

ST = .20 SA x lWhere Sj = total tab areas, SA = total area of both ai-lerons, YA = spanwise distance from centerline of theairplane to the mid-span of one aileron and Yf = thecorresponding spanwise distance for one of the controltabs, as illustrated in Figure 4. The total tab area canbe provided by using either one or two tabs.

7

FIGURE 4

Maximum control power can be adjusted, once thetabs have been sized and installed, by altering the armlength of the control-tab torque arms. These arm lengthsshould be adjusted so that maximum travel of the tabsis produced by a 90° rotation of the servo from its neu-tral or centered position. In no case should the tabs de-flect more than ± 25° from their neutral position becauseof loss of control effectiveness and large increases in ad-verse effects. The method for determining the neutralposition for each tab is discussed subsequently.

These comments apply primarily to the type of ex-ternally mounted tab I have used and I do not know howwell they will apply to a tab of some other geometry.Tabs applied directly to the ailerons may be very muchsmaller than those I have used and the above formulawill not apply.

I have had a number of discussions with variouspeople who are building the VariEze and want to usethe roll-trim tab on the right wing for the autopilot. Ihave discouraged this use because this tab is hinged atits leading edge and, therefore, has rather large hingemoments which will impose high loads on the servos.Furthermore, the single tab probably will not be suffi-cient to provide the necessary rolling moment to bothtrim the airplane and control it for the autopilot func-tions. Another factor was recently brought to my atten-tion by Burt Rutan who pointed out that the single tabproduces a pitching moment along with the rolling mo-ment because of its placement aft of the center gravity.My recommendation for the VariEze, therefore, is toeliminate the original trim tab and install two auxiliarytabs, one on each wing, in a manner similar to that Ihave used on my airplane. I recommend, however, thatthe tabs be mounted below the trailing edge rather thanabove it. This will keep the tabs out of the low-energywake which trails behind the wing. Use of the two tabswill eliminate the pitching problem and provide suffi-cient rolling moment to handle both trim and controlfunctions.

Voltage Regulators and Control Servo — A secondservo was added to drive the added control tab, con-sequently a second separate 5-volt regulator was alsoadded to handle the additional current load which wouldhave over-loaded the single regulator. There was noneed to add a second 8-volt regulator. The voltage reg-ulator for the servos has been changed from an LM340Tto a LM340K which is the same basic regulator butmounted in a TO-3 case that is larger and has greaterheat dissipation capabilities than the other unit. It wasfound that the LM340T regulators ran quite hot whenused with the Heathkit high-torque servos. It was desir-able to make the change because the regulators had atendency to cut off occasionally when they became over-heated. This is a built-in feature of the regulators thatprevents regulator failure but it also puts the autopilotout of commission until the units cool down.

Although the servos work almost constantly whenthe system is on, I have had no servo failure to date inmany hours of operating (probably a hundred or more).Several months ago I was experiencing what I thoughtwas a servo failure and I consulted with the HeathCompany about the problem. However, subsequently Ifound that the problem was due to several other sources.In the course of my correspondence with Heath, they in-dicated that the high torque unit (GDA 1205-8) was notintended for continuous duty and recommended that thestandard units, which are intended for continuous use(GDA 1205-4 or -5), be employed. In view of the factthat the high torque unit appears to have more thanenough power to drive the control tab, I believe thateither of these other two servos or similar units by othermanufacturers will be adequate, provided theaerodynamically balanced tabs (Footnote 2) are used. Infact, I believe that if these units are used, it may be pos-sible to use the original LM340T voltage regulatorswithout overloading them, however I have not made anytests to verify this point. The "T" version is considerablysmaller than the "K" version and probably is easier toobtain. (Radio Shack does not carry the "K" series butother regular electronic supply houses generally handleeither type.)

Circuit Details — A diagram of the revised systemcircuit is given in Figure 5. Aside from the features al-

SPORT AVIATION 43

FIGURE 5

ready discussed, several details have been changed. Themanual trim has been moved to the input of the finalrate amplifier and the switching has been changed sothat outputs of the rate and heading sensors can be in-dividually switched on or off. This change eliminatedthe need for two of the meters used originally. The gainsfor the rate and heading output amplifiers are now in-puted separately to the sensing amplifier. The limiternow consists of two pots coupled to the input of a bufferamplifier. This eliminated the use of the light emittingdiodes which were found to be the source of much elec-trical noise and frequent failures. Finally the singlemicroammeter was connected in parallel with the pulsewidth modulator rather than in series so as to be lesscritical as to the type of meter used and to facilitatescaling of the meter.

The control panel now incorporates the single manualtrim pot (Tj^j), vacuum pump speed pot (Ty), final out-put meter, wing leveler switch (Sjj), heading-hold switch(Sjj), command switch (SQ), and servo power switch (Sg).All other pots and trimming or adjustable resistors re-quire only occasional ground adjustments and do notneed to be on the control panel.

How The System Controls The AirplaneThe flight path of an airplane in level flight is com-

posed basically of a series of straight and curved linesand an airplane should be made to fly these linesprimarily by controlling the bank angle. Although somepeople think differently, this should be done primarilyby use of the ailerons and not the rudder. For mostairplanes, the rudder will do the job but it is not as ef-fective as the ailerons. In some cases, however, the rud-der will not work because of the lack of sufficient effec-tive dihedral to roll the plane. (The plane must slip orskid to the right in order to bank to the left, and viceversa, when rudder-only is used.) Thus the ailerons oran equivalent roll control system, such as the tabs usedfor my airplane, are the logical methods of controllingthe direction of flight with the autopilot.

The wing-leveler system functions by sensing thepresence of a turning or yawing rate and applying a rol-ling moment, through deflection of the auxiliary rollcontrol tab, to oppose the turn. On the surface thissounds very simple and logical, but just sensing theturning rate is not sufficient. If a rolling rate is not44 MARCH 1979

sensed at the same time that the yawing rate is sensed,the airplane will be brought back to a zero turn rate butthe airplane will be rolling at the same time. Con-sequently, the airplane will overshoot and go into a"dutch roll" type oscillation about the wing-level at-titude.

This problem is solved by tilting the rate sensor toabout a 45° attitude so that it will sense both rollingand yawing rates (a rate to the right in either or bothyaw and roll should produce a roll moment to the left).The wing leveler will now bring both rates to zero at orclose to the same time.

The wing-leveler system will tend to hold a steadyheading in calm air for a few minutes so that you cansafely pick up your charts and read them without hold-ing the stick or wheel (assuming you have trimmed theplane first, of course). This system, however, is charac-teristically incapable of holding a steady heading in-definitely and any amount of turbulence can cause theheading to change significantly in a short time. Furth-ermore, changes in lateral and directional trim of theairplane due to power changes, to fuel useage, or to in-advertent displacement of the ailerons and rudder willinduce a constant turn rate in spite of the wing leveler.

This turn rate can be overcome only by the pilotoffsetting the rate sensor zero in the direction oppositeto the turn or by retrimming the airplane. Thus it isseen that the wing leveler is not an "autopilot" in thesense that a true autopilot will hold a given courseheading indefinitely.

The heading-hold mode brings this system closer tobeing a true autopilot by sensing the change in headingand adding a corrective roll command as the headingerror builds. If the airplane is in perfect trim, the sys-tem will hold the airplane on the desired heading andwill return to it if a gust disturbs the airplane. How-ever, if the airplane is not trimmed properly, the out-of-trim moments acting on the airplane will cause a smallamount of heading error.

If the out-of-trim moment is about the roll axis, thesystem will balance this moment directly without dis-turbing the airplane in yaw or sideslip. However, if themoment is about the yaw axis, the airplane will sideslipso that the directional stability of the airplane will de-velop the balancing moment required about the yawaxis. The roll control actually will be opposing the rol-ling moment caused by the sideslip and the effective di-hedral of the airplane. Generally, this sideslipping willnot be very large but it may be objectionable.

Thus it can be seen that even the heading-hold modedoes not provide a true autopilot capability and thatsome pilot attention may be required, at least occasion-ally, to maintain the airplane in near-trim conditions.Have heart, though, the heading-hold mode will do avery good job of substituting for one of those verysophisticated commercially made (and expensive) au-topilots.

Speaking of sophistication, I added the optional fea-ture of the command switch SQ which merely adds asmall out-of-trim signal to the system so that it is biasedin one direction or the other. If the system is in the wing-leveler mode, the switch will cause the system to per-form a specific banked turn, depending on the setting ofthe adjustable resistor, as long as the switch is held inthe desired direction. The turn is not necessarily coor-dinated and some sideslip will develop. As soon as theswitch is neutralized, the system will return the airplaneto the wing level attitude. If the system is in the heading-hold mode, the same switch will cause the system tochange heading by a specific amount, and so on.

One peculiarity of my airplane is that it has essen-tially no geometric dihedral, and as a result, the un-

coordinated turn produced by this system causes the fuelin the partially filled wing tanks to shift in the directionof the turn. This out-of-trim shift tends to hold theairplane in the turn after the turn command switch isreturned to zero, especially if the turn is held for morethan a few seconds. This is not a problem with theheading-hold mode because the turns involved are verysmall and are short in duration. An airplane with onlyfuselage tanks or with some geometric dihedral in thewings should not have this minor problem.

Installation and Flight AdjustmentsFollowing bench checkout of the system to adjust it

for proper alignment, the system should be installed inthe plane with the rate sensor mounted right side upwith the appropriate end of the sensor facing aft and tilteddownward about 45° (not critical). The proper sensingof the heading sensor also should be checked by rotatingthe compass card away from the neutral position in whichthe target is directly beneath the two photo transistors.With the heading-hold switch (Sj-j) on, the meter nee-dle should deflect in the direction of the bank that wouldbe required to recenter the compass card if the airplanewere flying. (Be careful, this takes a little bit of thoughtdoing it on the ground. It's easier doing it in the air, butthen you may have to land to reverse the heading out-put.) If the output is backwards, it can be reversed byswitching the output of D through the 50 KOHM resis-tor to the + terminal of E.

Roll Trim — The final adjustment steps are done inflight in smooth air after short lengths of coarse threador fine yarn have been attached to the trailing edge ofeach tab. Establish straight and level flight at cruiseconditions and with the system operating in themanual- t r im mode, note the angle of the trai l ingthreads relative to the angle of each tab. Use the flightcontrols to maintain straight and level flight if needed.With the meter needle centered (consequently the servocentered), the threads should be aligned with the tabs. Ifthey are not, land and re-adjust the pushrod lengths andrepeat the flight test until "eyeball" alignment isachieved. Be sure the servos stay in their neutral posi-tion throughout this operation.

Next, release the controls with the airplane controlstrimmed and the meter centered to check hands-off trim.Retrim the airplane if necessary but do not move theservo control tabs. If the airplane has ground adjustabletrim tabs and is not in trim at this point, land and re-adjust them until trim is achieved. If aileron trim islacking, the airplane will have to be trimmed in roll byusing rudder trim alone and accepting the resultantsideslip. An alternate to this sideslip condition is tooffset the system using the manual trim control of theautopilot system. However, this will make the systemunsymmetrical with possible loss of effectiveness in onedirection. Of course, a compromise of a little bit of boththe two trimming methods may be the best solution tothis problem.

Wing Leveler — The next step is to switch to the wing-leveler mode and check first to see that the meter is cen-tered as a straight flight path is maintained. Retrim withTJ^J, if necessary, until the airplane will maintain essen-tially straight flight with hands off the controls. If straightand level flight is achieved but the needle is not centered,the airplane is out of trim in roll. Retrim the ailerons,if possible. If straight flight is achieved (no turn rate)but the wings are not level (ball not centered), the air-plane is out of trim in yaw and perhaps roll as well. Re-trim rudder and then ailerons, if needed.

At this point, the airplane may be oscillating in rollat a constant amplitude, and if so, the gain GR of the

rate amplifier C should be slowly reduced until the oscil-lation gradually disappears. This point represents themaximum usable gain for the rate signal. If the oscilla-tion is not evident, the gain should be increased untilthe oscillation is evident and then the gain reduced asbefore.

Now switch to the manual-trim mode and let theplane diverge in the direction of greater spiral tendency.As a moderate spiral develops, take hands off the con-trols, switch the wing leveler on and observe the au-tomatic recovery which should take only a few secondsto return the airplane to the wing level condition. If itdoesn't actually return it to full level conditions, itshould at least keep the spiral from diverging further. Ifit doesn't even do that, would you consider building anew airplane?

Heading Hold — The next adjustment is the headinggain which can be adjusted after switching on the heading-hold mode and setting the sensor index a few degreesoff from the current position of the N or S letters on thecompass card. The autopilot should automatically bankthe airplane in the direction of the setting, that is, inthe direction to place the target directly under the sensor(the N or S should be aligned with the sensor index). Ifthe airplane banks in the other direction, you goofedin the ground check (you see, I told you it was tricky);so land and switch the connection at amplifier E.

Okay, if you didn't goof, the airplane will automaticallyturn out of the bank as it approaches the new heading, orit may overshoot and bank back again and then continuein that good old dutch-roll fashion. No, don't tilt the gyrosome more; just back off on the gain G j-j at amplifier Duntil the oscillation ceases. On repeated attempts, theplane should bank smoothly to the new heading and thenlevel its wings as the heading is reached without an over-shoot. If no oscillation is experienced at first, increasethe gain Gpj until the oscillation does show up and thenback off as before. If it doesn't show up by the time youget to full gain, forget it and leave it there. You got yourmoney's worth already!

Command Switch — The final adjustments have todo with the resistors attached to switch SQ. You havethe choice of adjusting these for either a desired turn rateor a heading change. You can't have both, or at least notvery easily (you might if you want to go back and changethe gains, but I don't think you really want to do that).Anyway, simply go to the mode you want and throw SQin one direction. Adjust the appropriate resistor untilthe desired turn rate or heading change is obtained. Thenswitch to the other direction and repeat with the otherresistor.

You now have a single-axis roll-command electro-fluidic autopilot. HAPPY FLYING!

PROBLEM AREASThere are three basic problem areas that have not as

yet been solved. The first of these has to do with zerodrift in the rate sensor circuitry. This is believed to bedue to thermal changes either within the sensor itself orthe associated amplifiers. The amplifiers are operatingat a large gain and therefore are very sensitive to mi-nute changes in the circuitry due to thermal effects.

The second problem also has to do with temperatureand was brought to my attention by Dale Walker. Hefound that the rate sensor would not operate for about15 to 30 minutes at room temperature after leaving it inthe freezer of a refrigerator until it was cold soaked. Indiscussing the problem subsequently with Doug Garner,it was decided that the problem had to do with the factthat the air and the interior of the sensor body have to

SPORT AVIATION 45

be above a certain temperature for the thermistors tooperate correctly. Otherwise the thermistors dissipatetheir heat too rapidly and saturate the amplifiers A andB.

A solution perhaps to both problems would appear tobe the use of insulation around the sensor and a smallregulated heater. However, I have not pursued either ofthese problems as yet because they have not seemed tobe at all serious in actual operation. I have flown insome freezing as well as torrid weather without notingserious adverse effects of temperature, however, thecabin has usually been at fairly comfortable tempera-tures at these times so the rate sensor was not actuallyexposed to the extreme temperature.

The third problem has to do with radio-frequency in-duced voltages whenever the communications transmitteris keyed. These voltages cause the amplifiers to saturateand drive the servo hardover as long as the mike keyis pressed. The system resumes normal operations when-ever the transmission is completed. I have been merelyswitching off the servo switch, Sg, whenever I use thetransmitter for anything other than a very brief message.The solution to the problem obviously is to provide shield-ing which has not been done. However, I have not beenbothered very much by this problem and have been toobusy to correct it.

POSSIBLE FUTURE DEVELOPMENTSThere are several intriguing avenues for future de-

velopment of this general system. The first is the de-velopment of a simple magnetometer which will elimi-nate the need for a compass and the heading sensor.This magnetometer currently is under development byDoug Garner, who fathered the whole idea of the simplefluidic autopilot in the first place. Doug says that he hasseveral approaches going and I expect you will be hear-ing from him directly sometime in the near future.

Next, there is the use of a sensor to detect sideslipand apply a corrective yawing moment through a smallauxiliary rudder tab. This will help overcome some ofthe out-of-trim conditions caused by changes in powersettings and shift or usage of fuel in the wing tanks.

It is very easy to see how this single-axis system canbe applied almost directly to the pitch axis so as to pro-vide a form of speed holding capabilities. My plane hasstable operation in pitch and can fly indefinitely withhands off in non-turbulent air. However, it can be easilydisturbed when turbulence is encountered. Surprisinglyenough, having the single-axis autopilot operatingmakes me more conscious of pitch difficulties, even incalm air.

The addition of a VOR-holding mode is a very likelydevelopment which I already have looked into to someextent. All that is needed is an effective way of couplinginto the OBS drive signals. I believe that the VOR sig-nal can merely be added to the heading-hold mode so asto provide a heading bias signal.

Different forms of roll, yaw, and pitch control devicesis one area which needs to be developed further. The useof the small auxiliary tab suited my airplane very well,but it is not as applicable to other designs. Certainly,the direct coupling of the tabs onto the existing aileron,rudder and elevator needs to be pursued because it tendsto be more universally applicable. However, the use ofvarious types of spoilers seems to have some merit. Oneof the prime considerations in the selection of a controlsystem is the requirement for low torque in order toutilize the low cost model servos. Some types of spoilerswill operate with essentially no aerodynamic hinge mo-ments.

Finally, there are some alternate types of servos thatshould be explored. The auto industry uses a servo in46 MARCH 1979

the speed control system that may be adaptable. Theremay be some types of small pneumatic or hydraulic ser-vos which can be obtained inexpensively and adapted tothis use.

CONCLUSIONIn closing I merely want to comment that this effort

has been a hobby activity. Without receiving anymonetary compensation, I have derived much enjoymentand satisfaction from it and have received a liberalself-education in electronics and opera.ion of autopilotsystems. But most of all, I have experienced the greatpleasure of meeting and conversing with hundreds offine and interesting people whom I, otherwise, wouldnever have encountered. This is one of the fine benefitsof this country of ours and being a part of the EAA.

NOTEAdditional information concerning the simple

electro-fluidic autopilot can be obtained from the two ar-ticles mentioned in this article. Also, there are twoknown suppliers of kits for the autopilot: Eos mira, 1108Hedwig Green, Houston, TX 77024 and Omnics Corpora-tion, P. O. Box 371, Ansonville, NC 28007.

As noted in the introduction of this article, I havecollected a large amount of detailed information dealingwith construction and operation of the system. I electedto leave out a large portion due to the sheer quantity ofmaterial and the size of the effort required to assembleit in a suitable form for publication. Furthermore, I hadno way of knowing just what demand there would be forthis information. I am prepared to expend the additionaleffort and money required to compile a complete bookletwith detailed construction information with sketchesand photos, covering the sensors, circuit board designand fabrication, component layout and assembly, wiringprocedures and alignment or balancing procedures. Alsoincluded will be the information presented in this andmy previous article so that the booklet will be a com-plete compilation. Before doing this, however, it isnecessary for me to know that there will be a sufficientdemand of at least 200 copies to warrant the effort. I es-timate that the cost of this booklet, which would beprinted on high quality paper and bound with a heavy-duty cover, should be between $8 and $12, with cost ofhandling and postage included, depending on the finalnumber of orders.

If you are sincerely interested in this booklet, pleasewrite to me with an enclosed self-addressed and stampedenvelope so that I can contact you subsequently. Pleaseinclude a check for $5 as a deposit, as well as any com-ments on the information you would like to have in-cluded. I will try to answer any specific detailed ques-tions that you may have at the time but would prefer tohandle most of these in the booklet. I will return a letterindicating the final details of the booklet. I expect thatit will take about a month to determine the demand andto return the letter, and about 3 to 4 months to preparethe booklet. The deposit will be returned if the bookletis not published.

Footnote 1 — A more technically correct terminology isauxiliary roll control surface but I prefer to use "tab" forconvenience.

Footnote 2 — An aerodynamic balanced tab is onewhich has some of its area ahead of its hinge line. Theamount of area, generally ranging from about 10% to30"# of the total area, depends on the amount of balancedesired and the specific geometry of the tab and its loca-tion on the wing.