It is sometimes said that taxiing the DC-3 is one of the most important aspects of its operation. When one learns to taxi the aircraft properly he should have very little problem with the take-off and landing roll.
TAXIING THE DC-3 IS NOT DIFFICULT Basic understanding of using differential power, very modest use of the brakes, the tail wheel lock, appropriate power settings and use of the rudder, will insure that the pilot can smoothly, accurately and safely taxi the DC-3. Differential Power: Differential Power is probably the most important aspect of taxiing the DC-3 or, any multi-engine, tail-wheel airplane. Understanding the use of differential power is basic to safe taxiing. Differential power is exactly what it says. The pilot uses increasing or decreasing power on the engines to maintain straight-ahead taxiing, normal turns, turns in tight areas and control in windy/gusty conditions. The concept is simple: When you increase power on one engine without a like increase on the other engine, the aircraft will turn (all other factors being equal i.e., tail wheel unlocked, brakes not being used, etc.) in the direction of the engine with less power. A corresponding reduction of power on the low power side will increase the rate of turn in that direction. Normally very modest amounts of power input will suffice in any situation. Occasionally, (very tight turns, high winds conditions) will require a moderate amount of power input on the high power side.
REMEMBER: DIFFERENTIAL POWER IS THE BASIS FOR ALL PROPER TAXIING OF THE DC-3. Hand Placement on the Throttles: Each pilot will determine the best hand placement for his/her own comfort. Some will have their hand on top of both throttles; others will have their hand lower on the levers with less pressure on the friction lock. It is up to you to determine the most comfortable position for your hands on the throttle levers to be able to manipulate them.
NOTE: While taxiing, ALWAYS cross check your RPM. Normally, you will only need 800 - 1000 RPM for taxi. Uphill, turns, start from dead stop, etc. will require more. Normally, you will only need to adjust the throttles 100 - 200 RPM at most to start or stop a movement to the right or left. CROSS-CHECK your tachometer !!!
TAIL WHEEL LOCK: The tail wheel lock is probably the least important factor in properly taxiing the DC-3. It is, however, useful if it is (l) properly adjusted so the aircraft goes straight and, (2) appropriately used so that the aircraft is not attempted to turn against it while locked.
CAUTION: THE TAIL WHEEL LOCK MUST BE LOCKED DURING TAKE OFF AND LANDING. We are talking only about taxiing at this time. The tail wheel lock will only lock while the tail wheel is in a straight ahead position. It would not lock if the tail wheel were turned completely around 180 degrees. The tail wheel can be put in locked position at any time a turn is being made so long as the aircraft is intended to be taxied straight immediately following the turn. The lock will fall into place as soon as the tail wheel is straight in line with the aircraft. This means that you cannot "over shoot" your turn and attempt to taxi back to the center line of the runway if you have put the tail wheel lock in place during the turn.
WARNING: DO NOT ATTEMPT TO PULL THE TAIL WHEEL LOCK HANDLE IF THERE IS VIRTUALLY ANY PRESSURE ON IT AT ALL. YOU MUST WIGGLE THE TAIL OF THE AIRCRAFT WITH POWER TO REDUCE ANY PRESSURE ON THE TAIL WHEEL LOCK. PULLING THE TAIL WHEEL HANDLE WITH PRESSURE ON IT WILL STRETCH OR BREAK THE TAIL WHEEL LOCK CABLE.
If the tail wheel lock is in place and the handle will not move freely when the aircraft is parked, it is a very simple procedure to 'wiggle' the tail with the power as you begin your taxi. Again, do not pull hard on the tail wheel lock handle. A good practice habit to get into while you are driving your car on the street is to drive straight and when you come to a corner where you are going to turn, reach your right hand down and simulate that you are unlocking the tail wheel by pulling your hand backwards. After you have completed your 90-degree turn, then again reach down and 'lock' the make believe tail wheel by releasing the tail wheel lock. You will find that doing this repeatedly will ingrain the procedure and hand movement in your mind so that it will become much more familiar to you when you are actually taxiing the DC-3.
BRAKES: The use of brakes is a necessary but troublesome control in taxiing the DC-3. They are necessary because, obviously, you must be able to stop the aircraft since we do not have reverse thrusts on the propellers. Also, they are helpful in making turns. While not absolutely necessary, brakes are helpful in taxiing straight. However, this use of the brakes causes the most problem with people learning to taxi the DC-3.
THE BRAKES ARE NOT TO BE USED AS AN INAPPROPRIATE CRUTCH FOR THE IMPROPER USE OF DIFFERENTIAL POWER IN TAXIING THE DC-3.
FOOTNOTE: Props are full forward - low pitch, high RPM. Throttles adjust the RPM. Mixtures are Auto Rich on our a/c as it makes no difference for fuel metering on the ground - some will disagree here saying Auto Lean while taxiing because of the possibility of fouling the plugs. That really does not happen - Auto Rich on ground operations is just fine. The rudder may or may not be used (I proabably should revise my lesson plan !!). In calm wind conditions very negligible effect from propeller thrust. In very windy conditions they should be used because if there is a cross-wind or tail wind then the a/c will tend to be blown around the rudder.
Big problem with a really strong quartering tail wind. Usually the pilot (student) will try to work the rudder against the wind. Very wrong. The rudder should be held in the position that it is being blown as this is easier on the pilot(s) - I have been in situations that you physically could not hold the rudder against the wind - and it will protect the rudder from inadvertant blowing over and damaging the supports if the pilot gets forgetful and releases foot pressure. I allow students - and do it myself - to use the rudder all of the time because it feels un-natural to hold it in the centered position while taxiing. John Pappas "DreanFlight"
Here is an input based upon personal operating experience over 3,000 hours of flight operations in USAF C-47s in the early 60s:
Normal taxiing was done with the tail wheel unlocked using primarily differential braking with minor assist from the rudder. However, our C-47 brakes were notorious for fade if used excessively and allowed to get hot. So once the airplane was pointed in the right direction, using brakes, the remaining taxi was primarily with rudder steering and occasional differential power assist, until another significant turn was needed. For long straight taxi during substantial crosswinds, we often would lock the tail wheel after getting aligned.
For tight turns out of the chocks, or during adverse cross wind taxiing, usually the rudder was displaced in the direction of the turn (to ease weathervaning tendancies) and differential power applied to start the turn momentum, and also to assist long straight taxing in crosswind. I don't remember MAP readings, but generally it was well less than than 50% to initiate roll and only slightly above idle -- or at idle -- once the aircraft started to roll.
Props were at full rpm, mixtures, usually in auto-rich as they were normally set for start and take-off. This prop and mixture setting was used NOT specifically for taxi, but rather to preclude any abnormal configuration change that could be overlooked (through established habit patterns) in the pre-takeoff checklist prior to the initiation of take-off roll.
We always taxied at full rich -- as memory serves me -- unless we were doing prolonged taxiing at either high elevations or very hot ambient temps, which was very infrequent. And I am not sure that it really made any significant difference on plug fouling before run-up check. In a year in Vietnam, I don't recall ever using anything but full rich for taxi. Chuck Miller.
As to taxiing the "3" I've done my share of it around MDW and ORD in the 50's and 60's. I suppose it would add up to the hundreds of miles! At MDW we had a gate in the "crotch" of the finger we were assigned. It was to say the least, a tight fit. There was a strategically placed steel plate that was the mark for placing the right tire (or tyre) precisely, and applying right brake to rotate the craft around that point. If you were to deviate an inch or two, your wing tip would connect with the light pole or the restaurant! It was definitely a two man job. While the left seat was aiming for the plate, the right seat,(usually a radio operator), would give directions and rap your hand on the throttles when centered on the plate. Left power was added as the right brake was held and left brake at the ready in case a lamp was in danger of removal. There was a way to "jink" the craft back and forth to extricate yourself from damage using left and right brake but I never had the opportunity to do that. I was a tip watch when someone else did it. I still have not figured out the physics or geometry of it but it did work. I was involved in a close call or two at ORD. The tower had directed me via the (then existing) "outer circle" taxiway, and one of two routes to the hangar on the west side of the field. As I entered the way between the inner and outer circle taxiway, I came face to face with a UAL DC-8 coming in! (United "owned" ORD so we put up with this kind of thing plus double and triple parking to accommodate them in those days at the first terminal which no longer exists. It was about where UAL check in is now). Well, I called "ground" to see if I'd misunderstood my directions. They indicated I had not but could we turn around and let the "8" go by. I called back." We may be old, but we're still more agile the they are". Approaching the same entry to our gate with a CV-440, I'd found the braking action poor to nil. Between the outer and inner was down hill. As I entered, I was to hold for a plane on the "inner" The brakes had no effect! I hit the prop reverse button and throttles at the same moment and brought the CV to a halt to keep clear of the taxiway. Reverse was NOT to be used while taxiing so this does not appear "in the books"! A friend was caught when he tried to adjust his parking faux pas by backing into a space. When in reverse, the nose of the CV rises to the maximum extension of the nose strut. In this configuration he back into place, right over a frozen snow drift. When the throttles were retarded' the nose settled and caused a very noisy wake-up call to my friend. He couldn't deny the scared prop! John Wells
The C-47 was my first "tail dragger" experience. I was an Air Force Senior Pilot and instructor pilot in the KC-135 when I was sent to England AFB in 1968 to check out in the C-47 enroute to Vietnam. I finally got it on the runway for takeoff, but then the fun started. My first takeoff roll was a long series of S turns till I finally got rudder control. My next couple of takeoffs weren't much better. I finally tried a "toe dance" on the rudder pedals, just lightly tapping left and right until the rudder became effective, and from then on I had no problem. I found out early that if the correction was held until the plane responded, that was too long and it was going to go in the opposite direction! My next 8-900 flights were a thorough pleasure, even considering the often hostile conditions. I consider it a great honor to have been given the opportunity to fly the C-47! A wonderful airplane!! Don Croston
WARBIRD NOTES # 4 3 Apr 95 (26) MIXTURE MISCUES & MIX-UPS
This concerns the very prevalent habit, origin unknown, of placing the mixture(s) in AUTO LEAN while taxiing (one more form of mental masturbation) to "keep it from loading up". I've seen this habit (I really don't know what else to call it) in pilots with every possible level of experience and type of background. Manuals published by the manufacturers, military services and the airlines (and I've looked way back to just before WW II) make no mention of this procedure. Every manual that I can find - without exception - simply calls for RICH after start, then recheck it prior to the takeoff. You'll notice that I've used the word "manual" here, in some cases checklists may be a completely different story! Everyone and his brother seem to have gotten into this act. Contrary to popular belief, a general aviation checklist isn't really examined or looked at for its accuracy by any responsible authority prior to being placed in use. It very easily may be just a list of items by anyone, easily containing one - or more - "old wives' tales". Such is the case of a number of "checklists" I've looked at on some vintage airplanes, this article addresses just one of the more glaring mistakes seen in them. If my aversion to old wives' tales were the only important thing, then I probably couldn't care less. However, this really needs to be addressed as the FLIGHT SAFETY problem that it really is. Sometimes people forget them for run-up but the following is the real problem! I can distinctly remember three times watching pilots get busy, skip them somehow on the before takeoff checklist and then start a takeoff in AUTO LEAN, causing them some amount of embarrassment when I mentioned it as they were advancing the throttles.
I recently acquired an USAF Manual 52-12 (Powerplant Maintenance for Reciprocating Engines) published in 1953. (It's surprising what you sometimes find at estate sales.) In looking through it for a description of how to accomplish an idle mixture check I found something that directly applies to the subject I mentioned in the first paragraph. Quoting from page 182: "NOTE - On all carburetors, except the Holley Pressure-Type carburetor, effective leaning of the idle mixture will not occur until the mixture control approaches the IDLE CUT-OFF position. On Holley Pressure-Type carburetors, effective leaning of the idle mixture is accomplished between the RICH and LEAN positions as well as between the CRUISING LEAN position and the CUT OFF position." Everything that I've flown for many, many years has had Bendix-Strombergs. The only Holley around that I'm aware of at this time is the EAA's B-25, although a few others may become known as a result of this article. It would appear that the only person who might offer some sort of an excuse for this might be an old time B-25 pilot from back in the days when (mid-fifties) we had "J" models with Holleys in the USAF. And even then, only if he had a mis-adjusted carburetor. I don't, however, remember ever seeing anyone do it back then - when we still had a lot of those WW II pilots around.
A Bendix carb functions like this. During the first 10° of throttle travel the mixture control plates aren't really in the picture since the airflow at this low power is not enough to provide a stable idling speed. Instead - back in this idling range - the idle spring contacts the diaphragm poppet valve and holds it partly open, providing a fuel supply deliberately in excess of that required for idle power. This rich flow is then reduced by the idle mixture control valve (manually adjusted with the idle mixture setting screw, more on this later) and enters the engine's induction system. Properly adjusted, this flow will provide a stable idle at a proper fuel / air ratio designed to avoid "loading up". So you can move the mixture control back and forth between RICH and LEAN till the cows come home and - - absolutely nothing happens. I agonized over how to paint a verbal picture of this operation and finally tried to draw a couple of diagrams figuring a picture is worth a thousand words. Bill Harrison thought about this for awhile and reduced it to a simple analogy. If you've got a river bed (flow of fuel) it doesn't make any difference how many dams (mixture plates and jets) you place downstream if everything has to first pass through a little culvert (idle mixture control valve).
While writing this a friend said "why don't you say exactly how you'd perform this idle mixture check". I think I'd be more comfortable telling how most mechanics I've known and talked to over the years seem to want it done since they're the ones who'll have to adjust the idle screw based on the information you provide them from this check.
First, don't just read this and decide to go out and do a check after pulling the airplane out of the hangar and running it for a few minutes. The engine and carb need to be at operating temperature, cleared out and stabilized decently. So, do it after flying it if possible. Find an area on the ramp where you won't have to look outside for a couple of minutes. Run the RPM up to around 1500 for thirty seconds or so to clear it out and then gently close the throttle (remember it's an idle mixture check). During the next step you can save yourself a little time by placing the mixture maybe half way between AUTO LEAN and IDLE CUT OFF. (You could start way up in RICH but it wouldn't accomplish anything, the rise in RPM is going to occur when you get the mixture to a position very near IDLE CUT OFF.) Note the idling RPM. If the tachometer needle jiggles a little bit "average" it in your mind to a specific RPM. Then, slowly start moving the mixture towards IDLE CUT OFF and keep watching the idling RPM. This is kind of like s-q-e-e-z-i-n-g a trigger during target shooting, it should surprise you when something happens. I should mention here that in talking to Steve Hinton he's had good luck with moving the control pretty normally but most manuals describe the slower method. All of a sudden, the RPM will start a "perceptible" rise to reach a peak (best power) and then start dropping towards zero. In the rare event that the RPM doesn't rise at all, you're idling at best power and the idle mixture needs to be enriched slightly. Another thing that I definitely want to include here is an indication favored by many professional mechanics I've talked to. They keep a close eye on the manifold pressure, you should see a drop of about 1/4" while the RPM rises, any more is too rich. At this point you'll probably want to place the mixture back to a running position and catch the engine to repeat the check, most people need to do so in order to really get a valid or good RPM reading. Or, you can just let it quit if you've seen what you need and are finished.
Let's discuss "perceptible" for just a moment. To me this means that it is a very small amount - but - you are able to perceive it. The maintenance manual on your specific engine should tell you how much is allowed. For instance, reading the USAF B-25 -2 for the R-2600 says a 10 RPM rise is allowable. I've always thought that in general if you see anything beyond a 25 rise it's excessively rich and needs to be adjusted. Before starting to write this I don't remember ever seeing one that would rise much more than 50-75 or so but recently I experienced something that gave me a case of the round eyes. I knew it was excessively rich since it was torching at idle - big time! I casually mentioned that we'd do an idle mixture check before shutdown and we might see something like a 100 rise. In the last little bit (3/4' at the most) of mixture travel towards IDLE CUT OFF I was astounded to see a rise of 250 RPM or more. Absolutely nothing was happening - until that last little bit of travel.
Now, for those of you who've stuck with me this far, let's go back to the first paragraph. I've debated whether to mention this or not. I think I'll have to briefly touch on this since some of the people reading this possess a level of knowledge acquired from working on these carbs or otherwise being an long time observer of aeronautical trivia and will have caught it. Maybe I'll expand on it in some future Warbird Note when there's more space. Whatever, I'm sure that someone will observe the last sentence of the first paragraph and say "even if you did forget and leave the mixtures in AUTO LEAN before takeoff the fuel flow would be enriched by the power enrichment valve and the fuel flow would be the essentially the same as if the mixtures were in AUTO RICH". This is true, the charts show at high power the flows are the same regardless of whether the mixture control is in AUTO LEAN or AUTO RICH. But (and this is a really big but) you're relying on a fifty year old mechanical device within the carb to perform this function. I'm reminded of the pungent observation of a highly experienced old naval aviator whose friendship "Connie" Edwards and I've enjoyed for many years. Art Ward sez, "well, I'm old and sometimes I forget!" Maybe some parts of your carb might also forget, I sure wouldn't want to rely on them in this case. Furthermore, after takeoff just as soon as you retard the throttles to a point where the power enrichment valve closes, you'll be in a detonation range.
I guess that's pretty much wraps up what I wanted to say about this habit. If something's wrong, then fix it! But, if everything's working correctly, why wear out the mechanical components of the fuel system for a paradigm that turned out to be untrue? It sort of makes me think of that old Johnny Cash song, "bad news travels like wildfire, good news travels slow". I've discussed it with some really experienced aviators and they also can't say where it really came from, it just started to happen somehow over the years. Merrill Wien has a plethora of experiences to share, he agrees that we never saw it in years past but somewhere along the line it started to creep into a lot of procedures. It's amazing how many people cling, almost fanatically and with highly detailed justifications if asked, to this perception in spite of manuals, illustrations and whatever. And this isn't just the inexperienced, it includes people one would expect to know better. Might be enlightening the next time you see someone automatically moving it to AUTO LEAN after start or landing to ask "would you mind sharing with me just exactly why you're doing that?" If the answer is "to keep it from loading up", that'll tell you something about their level of understanding. Randy Sohn.
Our "Classic Air" operating manual says and that is how we operate and teach new pilots: Condition, no or light cross wind: RPM 1000, tailwheel locked, whenever possible rudder inputs rather than use of brakes to keep it straight. To initiate a turn, unlock tailwheel, deflect the rudder and if necessary give here a gentle brake input to the corresponding brake. Condition crosswind: tailwheel locked, use of full rudder deflection and or assymetric power, like 900/1200 RPM whenever possible do not use brakes and never drag the brakes by all means. This procedure workes fine, is smoth for the passengers and increases the brake life. B.Kugler, Captain DC-3, Classic air, Switzerland