Castor Oil

This has been extracted from the newsletter of the Propstoppers RC Club for June 2000.

Fuel Technology – Castor oil From George Aldrich
Back in 1983 there was quite a controversy in Radio Control Modeler magazine about the tests that were necessary to measure the "lubricity" of various oils that might be useful in model engines. Castor oil was used as the benchmark, but it was obvious no one knew why this was so.

They apparently got a lot of info on various industry tests of lubricants, but these were really designed for other purposes.

This was my answer. I will remind you that I was a lubrication engineer and not a chemist, but I drew my chemical info from Bob Durr, the most experienced lubricant scientist in the labs at Conoco.

Bob worked with my group on many product development projects and I can tell you that he is one smart hombre! Small changes were made in the text, but surprisingly very little has really changed since this was originally written. Here goes with the answer:

"I thought I would answer your plea for more information on castor oil and its "film strength", which can be a very misleading term. I have never really seen a satisfactory way to measure the film strength of an oil like castor oil. We routinely use tests like the Falex test, theTimken test or the Shell 4-ball test, but these are primarily designed to measure the effect of chemical extreme pressure agents such as are used in gear oils.

These "EP" agents have no function in an IC engine, particularly the two-stroke model engine types. You really have to go back to the basics of lubrication to get a better handle on what happens in a model engine.

For any fluid to act as a lubricant, it must first be "polar" enough to wet the moving surfaces. Next, it must have a high resistance to surface boiling and vaporization at the temperatures encountered. Ideally the fluid should have "oiliness", which is difficult to measure but generally requires a rather large molecular structure. Even water can be a good lubricant under the right conditions.

Castor oil meets these rather simple requirements in an engine, with only one really severe drawback in that it is thermally unstable. This unusual instability is the thing that lets castor oil lubricate at temperatures well beyond those at which most synthetics will work. Castor oil is roughly 87% triglyceride ricinoleic acid, which is unique because there is a double bond in the 9th position and a hydroxyl in the 11th position. As the temperature goes up, it loses one molecule of water and becomes a “drying" oil. Castor oil has excellent storage stability at room temperatures, but it polymerizes rapidly as the temperature goes up. As it polymerizes, it forms ever-heavier "oils" that are rich in esters. These esters do not even begin to decompose until the temperature hits about 650 degrees F (350 deg.C).

Castor oil forms huge molecular structures at these elevated temperatures - in other words, as the temperature goes up, the castor oil exposed to these temperatures responds by becoming an even better lubricant! Unfortunately, the end byproduct of this process is what we refer to as "varnish." So, you can't have everything, but you can come close by running a mixture of castor oil with polyalkylene glycol like Union Carbide’s UCON, or their MA 731. This mixture has some synergistic properties, or better properties than either product had alone. As an interesting sidelight, castor oil can be stabilized to a degree by the addition of Vitamin E (Tocopherol) in small quantities, but if you make it too stable it would no longer offer the unusual high temperature protection that it did before.

Castor oil is not normally soluble in ordinary petroleum oils, but if you polymerize it for several hours at 300 degrees F (150 deg.C), the polymerized oil becomes soluble. Hydrogenation achieves somewhat the same effect. Castor oil has other unique properties. It is highly polar and has a great affinity for metal surfaces. It has a flash point of only 445 degrees F (230 deg.C), but its fire point is about 840 degrees F! (450 deg.C). This is very unusual behavior if you consider that polyalkylene glycols flash at about 350-400 degrees F (180-205 deg.C) and have a fire point of only about 550 degrees F (290 deg.C), or slightly higher. Nearly all of the common synthetics that we use burn in the combustion chamber if you run too lean. Castor oil does not, because it is busily forming more and more complex polymers as the temperature goes up. Most synthetics boil on the cylinder walls at temperatures slightly above their flash point. The same activity can take place in the wrist pin area, depending on engine design.

Synthetics also have another interesting feature - they would like to return to the materials from which they were made, usually things like ethylene oxide, complex alcohols, or other less suitable lubricants. This happens very rapidly when a critical temperature is reached. We call this phenomena "unzippering" for obvious reasons.

So, you have a choice. Run the engine too lean and it gets too hot. The synthetic burns or simply vaporizes, but castor oil decomposes into a soft varnish and a series of ester groups that still have powerful lubricity. Good reason for a mix of the two lubricants!

In spite of all this, the synthetics are still excellent lubricants if you know their limitations and work within those limits. Used properly, engine life will be good with either product. Cooked on a lean run, castor oil will win every time. A mix of the two can give the best of both worlds. Most glow engines can get by with only a little castor oil in the oil mix, but diesels, with their higher cooling loads and heavier wrist pin pressures, thrive on more castor oil in the mix.

Like most things in this old life, lubricants are always a compromise of good and bad properties. We can and do get away with murder in our glo engines because they are "alcohol cooled" to a large degree. Diesels, though, can really stress the synthetics we use today and do better with a generous amount of castor oil in the lubricant mix. Synthetics yield a clean engine, while castor oil yields a dirty engine, but at least now you know why! "

Bert Striegler

Bert was the Sr. Research Engineer. (ret.) at Conoco Oil Co. He's a graduate in aeronautical engineering., and a long time modeler. I never understood how he wound up in the oil research business, but I guess it's because he's just very smart! I deserve no credit; Bert's the brain!

Thermal Soaring

This article comes from SAM Speaks #115 January-February 1994, no author was stated. It provides an interesting introduction to thermal soaring which is a skill that is fundamental to our hobby/sport.

THE ART OF R/C SOARING IN THERMALS

Not too long ago when a fellow who had been flying power for years became interested in soaring, he requested aid in the art of locating and riding up on thermals. He has since derived so much pleasure from soaring that he suggested expounding on a "thermalling primer".

What's a thermal? A thermal is, in the simplest language, a batch of hot rising air-an updraft. Damn its origin and all that unnecessary technical stuff; all we care about is what it "looks like" and how we can find it, recognize it when we do, and how we can make the most of its lift. Here's a method that works. Good pilots may have variations that differ but only slightly. This method has had me up in excess of one hour a number of times.

The basic skills required are the ability to launch to at least 300 feet, and to turn smoothly.

Proper preparation is important. Your sailplane should be balanced properly. The correct balance point will vary, not only with the design, but with the wind conditions and the skill of the pilot. That is, the balance point must be moved forward on "floater" type sailplanes to penetrate in wind, and the less skilled pilot will find that a rearward C.G. makes a smooth turn sheer luck. I have found that most pilots have the controls set too sensitively, thus magnifying the natural tendency to over-control. If you are having trouble making smooth turns, DESENSITIZE YOUR CONTROLS. Where you go in a thermal is not nearly so important as how smoothly you get there. The trim should be set for the optimum glide angle when not turning. Then, when in a thermal, the pilot will move the rudder trim for the turn radius desired and feed in up-elevator trim to maintain the proper glide angle.

The Search is not just a matter of luck. Unless someone else is going up like a homesick Martian, or if you have some other good reason to head for a known spot, you must perform a logical search pattern to reduce the chance of bad luck. The search pattern should take into account the recognition problem.

It is easier to recognize lift when the flight path is perpendicular to a line between the observer and the aircraft. A favorite method is to fly a pattern as shown below:

Because of altitude limitations, the downwind leg will usually not be as long as shown above, but the basic idea is to keep the model on a straight, smooth course, perpendicular to the pilot's line of sight. Another excellent method, especially when the wind is up, is to fly a series of left-to-right-to-left zig zags upwind, being careful not to cover the same "ground" (air) again.

You may develop your own, maybe better, search pattern, but keep the salient points above in mind.

Recognition is the most difficult part of thermalling. The real difference between being able to go up in thermals and just getting umpteen 3-minute (or less) flights every Sunday, is the simple (so it may seem) ability to recognize lift when it happens. I've got to say it again! Nobody can recognize lift when he's jerking the elevator up and down. Keep your hands off the stick!

Picture in your mind's eye the normal sink rate of your machine. Now-when you see that downward line become zero, or better yet, an upward line, you're in LIFT. Even if the sink rate only becomes zero, you're in lift. Many times I have seen expert flyers max out for 10 minutes, never getting any higher than the launch. ZERO SINK! On occasion I have been in zero sink for 2 or 3 minutes, only to have the embryonic thermal develop into the fable of the week, taking my model to the limits of visibility. Don't throw away the "zero sinkers".

Measure the diameter of the thermal to get the most out of it. This is an important aspect that many otherwise good pilots miss. Because the normal sink rate of any glider goes up as the radius of turn goes down, it is a superior technique to fly the largest circle that lets you remain inside the thermal. Thermals vary in diameter, not only from thermal to thermal, but within the thermal. As the height increases, so does the diameter.

Do not turn the instant you recognize lift. Continue straight until the lift has been passed. Now do a 180 deg. At the previously determined center turn 90 deg. and fly to no lift. Turn 180 deg. and repeat. Now you know its depth, width, diameter and exact location. You know how big a circle you can fly and where its center should be. The knowledge thus gained is worth hundreds of feet, and will have cost you less than 50 feet. Many thermals are lost because the pilot never quite knew exactly where they were. Many feet of altitude are lost by turning in a tight spiral in a big thermal. I have frequently noticed another pilot in a thermal, joined him, measured it and then by flying with this knowledge (which he never bothered to get) flew right up past him. When someone flies up through me it embarrasses the hell out of me. I won't let it happen if I can help it-will you?

Fly smoothly! Second only to recognition, smoothness is the most important aspect of flying thermals well. Learn to turn without losing altitude. Learn just how sharp you can turn your particular model without tip stalling. Practice this until you can turn as tightly as possible without diving or tip stalling.

Make the largest circle you can and still stay in the thermal. This will result in the lowest relative sink rate and therefore the greatest net rising velocity.

Drift with the lift. Did you ever notice a "whirlwind" or a "dust-devil"? They move DOWNWIND! So does a thermal, but generally, not as fast. Therefore, it is nice to find a thermal upwind and stay in it drifting downward until you feel it is wise to return upwind and find another. To fly an hour, you are likely to fly in 10 or more thermals, yet never move from the launch area.

When is the best time? I have seen days when all the best lift was over before 10:00 AM, and have flown in good lift when it was too dark to fly a block away, but generally, the best lift will be between 10 AM and 3 PM, mean sun time. Generally speaking, before 10 AM, there is insufficient heating of the ground by the sun for good lift, and by 3 PM, the air has heated to the point where good lift is less likely.

Where? Everywhere south of 90th parallel, except over water and sometimes even over water. Ever see a flat-bottom cloud? It's sitting "on-top" of a thermal. Ever see a "dust-devil" or a "whirlwind"? Those were thermals. I flew a Drifter clean out of sight in a "dust-devil" once. Having spent most of my life in the East, I can tell you the lift is good from coast to coast.

Trimming Models

This article by Ron St Jean appeared in SAM Speaks #132 November-December 1996. Ron is probably best known as the designer of the "Ramrod" series of models in the 1950's which are nostalgia legal models. I have been informed that they do not translate to R/C very well, certainly there is not a great deal of space for R/C gear. Ron is a great experimenter in the field of model aerodynamics and builds and flies special models to test out his theories.

PRINCIPLES OF STABILITY AND PERFORMANCE
Hi-Speed vs. Low-Speed Forces by Ron St. Jean

The basic idea is that some adjustments or offsets are mainly effective at low speeds, while others have a greater effect at high speeds. When we understand which is which, and the interaction between forces, it not only allows us to explain why we may be having a problem in flight, but this knowledge permits us to take effective corrective measures. Adjustments and offsets may be categorized into the two types. This list is not necessarily all-inclusive:

High Speed - rudder offset, wing wash in/out, decalage (relative inci dence) and wing cocking (changes wash-in/out).

Low Speed - stab tilt, downthrust, side thrust, spiral prop wash effect, CG location, weighted wing tip, drag flap and engine torque.

A few examples should serve to provide the understanding needed to utilize this principle of high speed and low speed adjustments. Although they are probably most useful in free flight power events because of the vast speed difference between climb and glide speeds, they should be of equal use in HLG and some use in rubber events, as well as in RC. The following il lustrations generally assume a power model:

1. Condition: Model tends to dive when first launched, but noses up as speed is gained.

Explanation: At low air speeds either downthrust or a nose heavy condition (low-speed) will nose the model down. Incidence, being high-speed, noses the model up, overcoming the low-speed factor as speed is gained.

Uses of Condition: The combination of downthrust and incidence produces a stabilizing force in the longitudinal mode. Should an under powered model tend to power stall (e.g. a 1 /2 A Texaco) the downthrust would predominate at low speeds approaching the stall, lowering the nose before the stall is encountered. The nose lowering would then cause speed to increase, and eventually the incidence would take over. Oscillations would eventually be replaced by a condi tion of constant speed and angle of climb. Once equilibrium is established, power changes will mainly change climb angle.

In the VTO era, vertical takeoffs were facilitated by downthrust working against incidence. With 10° down and normal incidence on the moderately powered models of the era, models could be launched straight up against a moderate breeze without looping in. In those days, the model had to stand on three pegs for takeoff. The low speed effect of the downthrust would cause the nose to pull into the wind while speed was increasing.

Incidence and CG location are balanced for each airplane to produce the same kind of longitudinal glide stability dis cussed above in regards to a low powered model in the climb mode. Here, however, a forward CG is the force opposing incidence so as to establish equilibrium. But as the CG is moved aft and the incidence simultaneously reduced to produce a slower glide and less drag due to incidence, a limit is reached where incidence is no longer effective. Stability is then totally lost, and the model "zeroes out." For each design, then, there is an optimum CG location. At such point, there will be sufficient decalage to provide minimum stability and maximum performance. Moving the CG farther aft of this point will require decalage reductions in order to maintain a stall-less glide, and thus increase performance at the sacrifice of stability. Conversely, a forward CG shift will require more incidence to compensate, and stability will be increased, but at a sacrifice of performance. Models that are too nose heavy need excess incidence in order to glide, and therefore will loop under power.

2. Condition: Model tends to go strongly to the right when launched (perhaps crashing), but assumes normal climb turn
after picking up speed.

Explanation: Two low-speed factors can cause a model to bank sharply to the right upon being launched-right thrust and the effects of spiral prop wash. In the case of the latter, the rotating prop wash impinges upon forward fuselage areas creating both roll and yaw forces to the right. It continues, though somewhat diminished, to similarly impinge upon the fin, creating a compensating left yaw force.

Uses of Condition: In some cases a model will loop before going into a normal climb turn. When this happens side thrust may be added to help establish the turn before the model loops. Or the design may be changed to utilize spiral prop wash and thus avoid side thrust:
Assuming single engine tractor models with normal prop direction of rotation, a stronger right hand tendency may be had by
(a) deepening the forward part of the fuselage (or raising the pylon),
(b) moving the thrust line toward an extreme position (either very high or very low),
(c) reducing fin area,
(d) moving the fin forward to make its area less effective,
(e) moving part or all of the fin area to the stabilizer tips to free it from prop wash impingement, and
(f) increasing dihedral, which works similarly to decreasing fin area.
Conversely, doing the opposite of any of these will reduce an excessive low-speed right hand tendency.
One of the major reasons for the success of the Ramrod design in the 50s was its strong natural right-hand tendency. It was so strong that left rudder and left wing warp (right wing washed in) was required to compensate and cause a normal climb turn to the right. This produced what is called top rudder by full-scale pilots. That is, the left rudder helps hold the tail down in a right turn, thus preventing spiral dives. These left offsets then aided longitudinal stability by forcing the model into a tight left glide circle at the bottom of a stall recovery. The high speed encountered at the bottom of the stall makes rudder and wing offsets strong enough to result in about a 45° bank to the left. This prevents further stalls. At the same time, the stability provided by ample dihedral and incidence prevented spiral dives in the glide.

3. Condition: Glide circle is nearly independent of rudder or wing offsets.

Explanation: A low-speed factor is causing glide circle.

Use of Condition: Prior to the discovery of low-speed adjustments with which to control glide circle, adjusting a power model often required much dangerous trial and error if there was no natural glide circle. One method of achieving the needed circle was to add rudder offset while compensat ing with opposite side thrust. Quite often the two adjust ments were not balanced, and a crash resulted.

Today we can control power and glide circles almost independently of each other. Rudder is typically used for the power pattern, while one of several low-speed adjustments may be used for glide, stabilizer tilt being the favorite. But drag flaps and weighted wing tips will work just as well.

4. Condition: A typical full scale light plane requires right rudder offsets during climb and left rudder offsets in the glide, in order to maintain straight flight.

Explanation: Torque is an in significant factor in most FF models because it is over powered by the effects of spiral prop wash.But because of vast airframe differ ences, most full scale air-craft are especially suscep tible to the effects of engine torque.

Some manufacturers have compensated for this low-speed torque factor by introducing fin offset, a high-speed factor. But it is normally only in the cruise mode that the two are balanced to produce hands-off straight flight. During climb, speed is reduced, but torque is increased due to added RPM. In this condition the torque easily overpowers the small fin offset, requiring right rudder to compensate. In a similar manner, torque disappears in the glide, the fin offset takes over, and left rudder is required.

Problem Solution: If right thrust were used to compensate for torque instead of right fin offset, the effect of one low-speed factor could be balanced with another. In addition, the effect of the thrust offset would grow and diminish with the torque, as both are functions of RPM. The Beech Bonanza for example has had right and down thrust since 1964, when the horsepower was increased from 225 to 285.

Ron St. Jean, P.O. Box 149, Yerrington, NV 89447.

A Dozen Aeronautical Myths

One often overhears comments about the behavior of model aircraft at the field, and while the remarks are about models, they would also apply to full size machines.
Some of these expressed ideas reflect common myths, so I dug into my memory from 40 plus years ago for the explanations of why they are wrong, and present them for discussion.

Myth 1:
A model will tend to weather cock into wind during flight.

Assuming a steady (non gust) wind, the aircraft can do no such thing, short of being anchored to the ground in some way. A fuller explanation follows below at myth 4.

Myth 2:
A model will stall if it flies too slow.

This one can be correct, but not necessarily so. It is not speed that causes a stall, but separation of the airflow from the wing. This means an aircraft can stall at high speed, e.g. a snap roll, or not stall at zero speed, e.g. the top of a hammer head (sometimes called a stall turn despite no separation burble). Think of stalling as an angle of attack (around 16 degrees), rather than speed.

(By the way, a 60 degree level turn increases your stall speed by 41%). It is the higher angle of attack in a turn, that can cause a stall, leading into a low speed snap roll

If you lose power, you are unlikely to stall if you get the nose down below level flight. Warning signs to get the nose lower come from having a lot of up elevator applied. It means you are approaching 16 degrees.

Myth 3:
The more stable an aircraft, the better it is at aerobatics.

Quite the opposite. A stable aircraft wants to keep doing what it is designed to do, normally regain level flight if disturbed from it. You don't need this stability fighting you if you are trying to make the aircraft follow your commands. A good trainer should be stable, but few are these days.
Neutral stability best serves the aerobatic pilot since the machine keeps doing what was last commanded to, with no deviation.
Unstable aircraft are usually beyond human control, as they increase any deviation input given to them. Too far back a C.G. can make a simple up command turn into an unwanted loop for example. Modern fighter jets are unstable in order to attain rapid response, but need a computer to fly them.

Myth 4:
Turns down wind are more dangerous than turns upwind.

In some ways this is true, but not for the usually given reasons. In a turn downwind, a gust will tend to roll you on your back since the high wing usually presents more under wing area to the gust, than the low wing.

Secondly, your increased ground speed downwind makes a prang take place at higher ground speed than turning up wind.

Thirdly, the increase in ground speed gives the illusion of a higher airspeed, tricking one into slowing the airspeed, perhaps to the stall.

That said, the aircraft has no way of keeping track of it’s relationship to the earth below it. This is an important point to remember and this enlarges on the explanation on Myth 1. Assuming a non changing wind, once the machine is airborne, the ground relationships cease. It is in a river of air, and the motion of the river over the earth, doesn't affect it's flight characteristics.

One way to get this clear, is to imagine you are in a free floating balloon watching a model circle around you. The balloon may be doing 100 kph over ground, but you won't fell a breath of wind. Neither will the model circling you, other than it’s own airspeed.

Only when a machine reemerges from the "river" of air, and touches the shore do we need to worry about the earthly relationship.

Another example: Imagine you are on a train moving at speed. As you walk in the direction of travel, your speed over the ground is increased by the speed of your walk, and conversely, if you walk to the back of the train, your speed over the ground is reduced. The speed of your walk in the train, is not affected by the speed of the train. If you bump into someone, your momentum is relative to the train, and it doesn’t matter which way you walk.

Just as you are in the moving train, the aircraft is in the moving air mass. In the example above, the aircraft speed is equivalent to your walk speed, and the air mass (wind speed) is like the train moving over the ground.

Consider a 180 degree turn in still air. It involves reversing ground speed from say + 100 North to - 100 South, a relative speed change of 200 within the time of turn.

Now imagine you are flying into a headwind of 100 going North,. Your ground speed is now zero. You now again do a 180 degree turn, in the same time frame. Your final ground speed is 200. Once again, a relative speed change of 200 within the time of turn.

Notice, the aircraft undergoes the same accelerations within the time of the turn. It make no difference because the wind is blowing. As you turn down wind, it may help you to visualise the wind is helping carry the aircraft down wind and accelerate it over the ground.

Myth 5:
The model can make a tighter turn if it slows down.

Look at our pylon champ for an answer to that one. It again comes down to angle of attack and it is the stalling angle of your machine that determines it's minimum radius turn. You can turn at the minimum radius at more than one speed, but the faster the speed, the more bank is required which in turn means you increase angle of attack. If one stalls at these speeds, a snap roll usually results. The increased angle of attack required in a level turn will slow you down if you don’t add power.

Myth 6:
A high wing gives pendulum stability.

This is misleading, because a pendulum is fixed to a support, whereas the aircraft is not fixed in any way. What happens is that as the aircraft banks, It sideslips towards the low wing, and it is the retarding effect of this relative airflow on the top wing that rights the plane (see myth 7).

Myth 7:
Dihedral works because the horizontal lift component of the lower wing is greater than the other.

Yes, partly but more is involved. Imagine you can slide the model along a wire through it's C.G., and it's not hard to see that while the above effect will slow a rotation, it won't stop it, and certainly won't bring the model upright. Once again, it is the sideslip that increases the lift on the lower wing and levels it. As the wing drops, the model slides in that direction, causing a greater relative angle of attack and lift on the lower wing. The opposite wing has a lesser angle to the relative airflow.

Myth 8:
"Dual servo rate should be low for strong winds and high for light winds". (After hearing this, I assume the proponent thinks that strong winds give more airflow over the controls and less control deflection is thus required.)

If one realizes that an airplanes "wind" is due to it's motion, and not the wind speed (when airborne, remember it is in a river of air), then different rates of throw are not involved with wind speed although they are with airplane speed through the air (relative wind).

Myth 9: Big vertical stabilizer (fin) means directional stability.

If we remember that an aircraft will sideslip towards the lower wing in a bank, the relative air stream creates side forces either side of the C.G. These can either yaw the model into the slip or out of it. Too large fin acts like a weather cock during the slip, and tightens the turn. The end result is the nose dropping and a spiral dive. The perfect size fin will balance the area ahead of the C.G., so that the yaw is appropriate for the sideslip involved. Too small a fin will yaw a machine out of the turn.
It is the rudder effect that turns (yaws) the airplane (else it would crab in a straight line but wouldn't turn), despite the fact you may only use aileron input. The bank causes the machine to sideslip, which brings in the rudder yaw effect.

Myth 10:
A model will glide farther if it is light.

Assuming no wind, the angle of glide relates to the lift and drag of the machine. Without drag you would glide horizontal indefinitely and without lift, you have a vertical descent (don’t we know). The actual glide angle is thus a ratio of these two (Lift to Drag), and not related to weight. Wind has an effect on the angle over the ground, but not through the "river of" air. The heavy model will reach the ground sooner, since it glides at a higher speed, but both will glide the same distance in still air.

The minimum sink speed of a model is flown slower, near the maximum angle of attack (not the faster best distance speed), since one is more concerned with lift than speed. Here a light model will glide a longer period than a heavy one.

Myth 11:
A model will gain the most height in a given time, if we climb it at the best angle of climb.

There is a difference between angle and rate of climb. The first compares altitude to forward distance and is affected by wind, the second compares altitude to time and is not influenced by wind. Best rate of climb is done faster than best angle of climb.

Myth 12:

A headwind will slow a model more than an airliner.

If we go back to the "river of air" concept, one will find it easier to grasp the picture that everything in the "river" is carried along at the same speed. Hence a ten knot headwind will take 10 knots off the model and airliner speed, equally.

Now let the arguments begin.

Arnold