H ANDS ON HOMEBUILDER’S HANDBOOK
Use a vernier control when fine adjustments are needed.
Controls for Your Engine Choosing your type BY DICK KOEHLER
SOMETIMES PILOTS ARE REFERRED to as “stick and throttle jockeys” because there are two distinctly different control systems on the plane. Stick, of course, refers to the aircraft’s primary flight controls, usually for the elevator, ailerons, and rudder. These are critical for flight, and most people build them with the best materials. Throttle refers to the engine controls, and in my experience, many builders tend to skimp a bit here. I have seen everything from engine controls taken off certificated aircraft to parts that were shaky on lawn mowers being used. So, let’s look at the options that are available for the homebuilder to ensure proper—and safe—control of the engine. This will be a two-part column; I’ll cover the cockpit side in this issue and the engine connection side next month. Basic engine controls are usually of the push/pull type, made by moving a solid or multi-strand inner wire within a solid-wire, spiralwound outer sleeve. In the simplest designs there is no inner or outer material to help reduce friction or keep out contaminants. Better units have polyethylene liners, and the best have Teflon
TYPE
NOTES
GLIDE FREE
$26
Typically not used for engine controls; used for towrope release, heat ducts, etc.
RATCHET
$27
Prevents creep due to vibration
BUTTON LOCK
$47
Common for mixture control
FRICTION LOCK
$54-$63
VERNIER
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APPROXIMATE COST
casing liners for smooth operation, even in high-heat engine areas. Better units will also have a polyolefin outer protective cover. This outer cover will usually add about $1.50 per foot to the cost of the control (an extra $10 on average) but will add to the life of the control. Standard controls have an outside diameter of either 3/16 inch or 1/4 inch and are held in place with AN742-D3 or -D4 clamps. Controls with the outer cover seem to lock in place better in the clamp and resist slipping. Now that you know the basic design, let’s move on. There are five control designs, in order of secure action, known as: glide free, ratchet, button lock, friction lock, and vernier.
$70
Universal standard for general aviation aircraft Good for fine adjustments
PHOTOGRAPHY BY JIM KOEPNICK
A typical engine will have two controls (throttle and mixture) or three (prop control for a constant-speed prop). So, how does the pilot tell them apart? Glide free is, as the name implies, free to move with no locking device, only the inherent friction in the control to hold it in any particular position. For this reason glide free is usually not used for primary engine controls, but it might be used for heater/defroster ducts, towrope release, etc. A typical price for a 6-foot glide free control is about $26. Six feet may be a bit long for most homebuilts, but I will use that length to compare the costs of the various basic designs. Ratchet type controls have a catch or clicker in the handle to hold it at various steps. This is usually the preferred control for heater/defroster controls since they will not creep during the vibration of flight. A typical 6-foot Ratchet control is $27. Some older Cessna 150s have this control for the mixture. The button lock type typically has a push button on its center that, when released, will lock the control in place. To move the control the pilot must first push the button and hold it in while repositioning the control. In a panic, simply pushing on the button and the control simultaneously will allow it to go to the full-in position. This control is fairly common for the mixture control on many certificated aircraft. A typical 6-foot button lock cable will cost about $47. The first three controls we covered usually only come with a 0.050- or .062inch solid inner wire. This makes it important to not kink them at any time, and they require special attachment items at the engine component. These attachments will be covered next time. A friction lock control comes with either a solid inner wire or a 0.078 multistrand cable with a solid male-threaded end in 10-32 or 1/4-28. Friction lock controls are the universal standard across most general aviation and military aircraft for all engine systems. The control
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H ANDS ON HOMEBUILDER’S HANDBOOK
A ratchet control.
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will have a knurled inner knob that, when tightened clockwise, will squeeze a plastic cone onto the inner control shaft and lock it in position. The pilot must overtly twist the knob to lock and twist again to unlock. Alternatively, the friction can be set high enough that the control will not creep, but the pilot can still move the control. A 6-foot friction lock control will run you about $54 for the solid-wire type and $63 for the threaded end. Besides individual controls, a throttle quadrant will usually be of the friction lock type, with one friction mechanism for all controls, squeezing them to hold them in place. Lastly is the vernier control. It usually has a larger push button in its center rather than a button lock, but it also has a fine adjustment twist or rotation mode. For large or rapid movements, the button must be pushed, but for fine or slow adjustments, simply rotate the knob, clockwise for in and counterclockwise for out. This allows for
PHOTOGRAPHY BY JIM KOEPNICK
very fine adjustment and is therefore usually found on mixture and prop controls. A typical engine will have two controls (throttle and mixture) or three (prop control for a constant-speed prop). So, how does the pilot tell them apart? Back in 1986, after a series of accidents by pilots grabbing the wrong engine control at the wrong moment, the FAA standardized the controls by color and shape. The FAA throttle is black and has a smooth round shape. Some folks like to use a white throttle, and you will probably get your airworthiness certificate with it, but it is not FAA standard. The mixture control is red and has six longitudinal raised pointed ridges on it, whereas the prop control is blue and has six rounded longitudinal grooves. Therefore, a pilot can differentiate between the controls both visually and by tactile feel. Most of the preceding discussion covered individual controls. The homebuilder has the option to install a throttle quadrant.
There are no locking devices on glide free controls.
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The Boeing 777’s Great Great Great Great Granddaddy
HOMEBUILDER’S HANDBOOK
1928 Boeing 40C
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The button lock control is used commonly on certificated aircraft.
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With a friction lock, the user must twist the knob to lock and twist again to unlock.
These range in price from about $60 to several hundred dollars. I strongly suggest that you pay the extra for adjustable friction, if available. Along with the quadrant you will need the actual cables, which will run an extra $40 each, approximately. There are also various options available for tandem seat applications both for single controls and quadrants. Side-by-siders usually share, with the controls mounted in the middle. This should get you a long way down the road of becoming a throttle connoisseur. Richard Koehler, EAA 161427, is an active airframe and powerplant mechanic with inspection authorization, a commercial pilot with instrument and multi-engine ratings, and a technical counselor and flight advisor. Watch a companion video to this article at www.EAA.org.
104 Sport Aviation July 2010
PHOTOGRAPHY BY JIM KOEPNICK
