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 Post subject: TAILWHEEL TAKEOFFS
PostPosted: Mon Sep 17, 2012 1:55 am 
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Aviation Writer and Pilot

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TAILWHEEL TAKEOFFS


The following applies to all kinds of aeroplanes with tailwheels from Cub to Mustang. As with all discussions of aerodynamic an aeronautical subjects, entire (thick and often quite dry) volumes may be and have been written. This discussion is, of course, not in any way nor is it intended to be as complete and as thorough as such as those, and ought to be considered to merely be an introduction to the subject.

A number of unpleasant but fortunately controllable things happen to tailwheel piston-powered aircraft on takeoff, some or all of which may require immediate and skilful pilot correction to keep the nose tracking straight. For the purposes of this discussion we will assume that the propeller is turning clockwise (left to right) from the point of view of the pilot. For anti-clockwise turning propellers, the direction of forces described herein are reversed.

Until the dawning of the jet age in the waning years of World War Two, most aeroplanes had tailwheels to steer them whilst on the ground; this why an aeroplane with a tailwheel is still said to have “conventional landing gear”. The reason for this design preference is obvious and discernable by observing the basic layout of these aircraft: the engine and its systems at the front of the fuselage take up whatever space might have been available for a nose wheel assembly. Fighters such as the P-38 and P-39 could have nosewheels because their engine(s) were placed elsewhere.

With regard to multi-engine bombers, while the positions of the engines are not a factor; however, crew stations in the nose must be considered when designing a nosewheel assembly. The B-17 was the last U.S. heavy bomber to have a tailwheel and contemporary bombers such as the B-25, B-26 and B-24 had nosewheels which were installed under or just behind the bombardier’s/navigator’s/front gunner’s position in the nose.

It took so long for the nosewheel to become the ubiquitous landing gear design because the tailwheel design concept is a convenient solution to a number of important matters. A tailwheel provides a relatively simple/small/light and low-drag means of providing a means of steering aircraft of all sizes on the ground. It can be designed to be retracted or not, with relatively small airspeed advantages or penalties. Tailwheels are less expensive to repair and a failure thereof will not cause propeller and engine damage; although ground loops and tip-ups which commonly occur with tailwheel equipped aircraft certainly cause more than their share of such and other damage. Propeller clearance is far greater with a tailwheel (at least until the tail comes up on takeoff and during a wheel landing). The vertical tails of tailwheel aircraft sit lower than those of nosewheel aircraft of the same size, reducing the required height of hanger/storage space.

The debit side to tailwheel aircraft is that taxiing, takeoffs and landing rollouts are far more complicated, delicate, compromised, and require far more pilot control input and care in most instances than do such operations in a nosewheel aircraft.

P-factor

When the propeller disk is at a high angle of attack (AOA), that is, positioned at an differential angle to the direction of the aircraft such as when a tailwheel aircraft is beginning its takeoff run or when any piston powered aircraft is flying relatively slowly, steeply climbing or turning, a force is created which tends to yaw the nose of the aircraft in the opposite direction of the direction in which the propeller is turning. This yawing force is called “P- effect” or “P- factor” and is not caused by torque as is commonly thought, said and written. As said, P-factor operates in the yaw axis. However, torque operates in the lateral axis and may, when the engine is powerful enough, the propeller massive enough and the throttle is opened tend to bank the aircraft in the opposite direction of the propeller. Torque may also compress the left landing gear strut and tyre and thereby cause a left turning effect due to tyre physics and landing gear geometry; however it is not torque that yaws the nose to the left or right on takeoff or in a steep climb or turn, and it is that yaw that we are concerned with herein.

P-factor yawing effect occurs when the propeller disk is at a positive AOA. As a propeller turns, one blade rises whilst another blade descends. When the propeller disk is at a positive AOA a descending propeller blade is at a greater AOA than a rising propeller blade. A propeller blade is an airfoil, much like a wing, and operates in the same manner; accordingly, a descending propeller blade creates more thrust (lift in the horizontal plane) than the rising one (greater AOA, to a point = greater thrust/lift). A propeller blade moving through the vertical produces no P-factor. Since the right (descending) side of the propeller disk produces more thrust than the left (rising) side, this asymmetrical force tends to yaw the aircraft to the left. It may be helpful to think of this as if the propeller-shaft itself has been displaced to the right of the longitudinal centreline, creating a turning force not entirely unlike when a multi-engine aircraft loses power in an engine on the port side.

P-factor increases as engine RPM increases. For this reason it is not good practice to open the throttle too quickly when beginning the takeoff run in an aeroplane with a powerful engine. When beginning the takeoff run in some aircraft, the as yet ineffective rudder combined with high power initial settings may create a P-factor yawing force which is greater than the application of full rudder correction and even differential braking can correct. Take it easy on yourself; slowly open the throttle and let the aircraft accelerate with full up elevator control (stick back) to keep the tailwheel firmly on the ground until the aeroplane is moving fast enough for the rudder to take hold and to control the yaw axis. When flying the Mustang, Corsair and other powerful tailwheel aeroplanes, many pilots initially open the throttle very slowly to around halfway (30” MP or so) and let the aeroplane accelerate at that power for a while before opening it further. As you may know, in the Mustang when the control stick is held back beyond neutral the tailwheel is limited so that it can only steer 6 degrees to either side and will not castor. This is a great aid to directional stability in the early takeoff run.

As the tail rises and the aircraft’s pitch rotates downward, P-factor diminishes until the propeller disk is at zero relative AOA at which point it disappears. The faster the propeller disk changes its AOA, the stronger the P-factor yaw. This is one reason why it is not good practice to hurry the tail up with forward stick. Another reason is the danger of a prop strike should the nose come down too fast and far before it can be halted. On a tail-low takeoff the propeller disk is at a positive AOA during the entire takeoff run and P-factor yaw will continue right into and during the climb-out. However, a new force is introduced as the tail comes up and the propeller disk begins to rotate downwards.


Precession

We have all heard about gyroscopic forces. The two greatest of these forces is Nutation and Precession. Nutation is the force that tends to sway or nod a spinning axially symmetric object about its axis of rotation; however, we are only concerned with precession here (and aren’t you glad of that).

Precession, simply put, is the gyroscopic force that displaces a symmetrically spinning object (such as a propeller) at a right angle (90 degrees) to its rotation when the pitch of the spinning object’s axis of rotation is changed, such as when the propeller disk rotates downward as the tail rises. (You call that simple?) When this happens, precession creates a force which tries to yaw the propeller disk to the left, at a right angle (90 degrees) to its downward pitch rotation. Conversely, if the propeller disk’s pitch was rotated upwards (tail brought down) precession would try to yaw the propeller disk to the right.

So, we see that as the tail is rising rotating the propeller disk downward and propeller disk is approaching and achieving zero relative AOA, just as P-factor is diminishing, precession is commencing and is trying to yaw the nose even harder to the left. Once the propeller disk stops rotating downward and the aeroplane is rolling along in a level or near level attitude, precession also stops. This is one reason, along with increased rudder effectiveness at higher speed, why when you are in the last phase of the takeoff and are rolling straight and level, far less if any rudder input is usually required to maintain a straight takeoff track.

Depending upon the power of the engine, the mass of the propeller and the general design of the particular aeroplane, you also might need to hold some right aileron during the takeoff run to counter torque. Lest we be unconfused, aileron displacement into a cross wind is also required during the takeoff run. It may occur that a strong enough left crosswind vector might tend to counter some of the torque and permit neural aileron displacement, but don’t hold your breath for it; nature is rarely that kind. Remember that all of the differentially generated forces mentioned herein overlap and mix during the takeoff. It is unlikely that you will be able to accurately divine which force is doing what at any given time. In any event, you will be busy enough keeping things going straight to much worry yourself sussing this; and in the end, a straight takeoff run is what really matters, eh?

Rudder Trim

Military Technical Orders for virtually all powerful tailwheel aeroplanes require some right (or left as the case may be) rudder trim during takeoff and climb to help to offset P-factor and precession yaw. The amount of rudder trim required for each type of aircraft is based upon accumulated flight test data and is determined by a number of factors:

One factor is the amount of aerodynamic aid that an average pilot may require to physically hold a straight track during a full-power takeoff. In the RW, rudder pedal pressure required to hold a straight track on takeoff in powerful piston single-engine aeroplanes without the aid of trim may be too great. Of course, we do not have equivalent RW rudder pedal pressures in most flight-sim rudder pedals, so we have more latitude with regard to setting rudder trim on takeoff.

Another factor for determining the amount of rudder trim required at takeoff is what is required to maintain a straight track at the initial climb power setting (in the Mustang - 46”, 2,700 RPM). Some pilots prefer to reduce or to return rudder trim to neutral when at lower cruise power settings. Some well-muscled civilian pilots may choose not to use any rudder trim at all during takeoff and rely solely upon physical rudder control input to keep the nose tracking straight. This is not, however, an option in military aviation where Technical Orders must be obeyed and precisely executed at all times upon pain of washout and/or reassignment to non-piloting duties. However, to replicate the most accurate flight experience and therefore increase immersion, we recommend that the RW trim settings as stated in the Dash-1 or POH be used.

There are some aeroplanes such as the early Mark (I-V) Spitfires, and others whose fin/rudders are designed and configured so that they naturally counter P-factor and precession yaw on takeoff and climb with little pilot rudder input required. Accordingly, in these aeroplanes, little or no right rudder trim is required. Later Spitfire Marks (VIII- F.24) with more and more powerful engines did, certainly, require rudder trim for most flight conditions.

Tailwheel aeroplanes whose "sit" whilst on the ground is at a relatively high angle due to long landing gear legs and/or a short fuselage (early, short fuselage P-40s for instance) will experience a greater degree of both P-factor and precession yaw during takeoff since the propeller disk's AOA when commencing the takeoff as well as the amount of pitch rotation required to achieve relative zero AOA will necessarily be greater. To avoid excessive precession yaw, these kinds of aeroplanes are often not rotated to level and the tail is kept fairly low during the takeoff run.

So, here is the gavotte that the tailwheel pilot must, more-or-less, perform during the takeoff:

1. Before opening throttle, stick fully back and towards cross wind - roll forward a few feet to determine that the tailwheel is straight, rudder amidships;

2. Throttle slowly opened –anticipate and correct any change of tracking from straight ahead, then gradually anticipate and correct the increasingly powerful left turning tendency with rudder input as the throttle is opened (P-factor);

3. Allow aircraft to accelerate to the point that the rudder becomes effective before permitting the tail to rise, reduce rudder input as necessary as the rudder begins to take hold;

4. Stick to neutral or very slightly forward - as tail rises P-factor yaw diminishes but precession yaw commences. Be prepared to increase rudder input to counter precession yaw and then to decrease rudder input as increasing speed causes the rudder become more effective;

5. Be prepared to reduce or neutralize rudder input when rolling at a steady level attitude and near liftoff speed (rudder now fully effective and both P-factor and precession yaw substantially diminished or gone);

6. After lift off, rudder trim and/or pilot rudder input as required to track straight (centre ball).

In some light aeroplanes such as the Cub, the elevator and rudder become effective in the propeller back-wash as soon as the throttle is opened. Accordingly, on takeoff the tail may be permitted to rise immediately. The relatively low power of the Cub’s engine and small size of the Cub’s propeller do not produce large amounts of P-factor or precession yaw, unlike the Mustang’s powerful engine and enormous four-blade propeller. Some rudder input on takeoff is necessary in the Cub; but it is minimal and subtle compared to that which is required for an aeroplane with a powerful engine and a large propeller.

Aircraft with nosewheels generally do not encounter P-factor or precession yaw to the same degree as do tailwheel aircraft since they are already in a more-or-less level attitude before beginning the takeoff run. However, that does not meant that they will not require some right rudder correction when the throttle is opened on takeoff. To the extent that the propeller disk is at a positive AOA during the takeoff run relative to the direction of the aeroplane, which is often the case, especially if the aeroplane is heavily aft-loaded, a nosewheel aeroplane will encounter some P-factor and precession yaw as the nose lowers during the takeoff run, requiring rudder input to maintain a straight track.

I hope that the foregoing is of some help regarding taking off in the Mustang and other high-performance taildraggers. We do not make arcade- game aeroplanes; however, a bit of knowledge, some practice and the acquisition of skill and experience will increase your enjoyment and satisfaction when flying these aeroplanes, just as in the RW. Please do not hesitate to ask questions on this forum if you are curious about any aspect of flying and/or operating our aeroplanes, or are encountering any difficulties of any kind.

Mitchell


Last edited by Mitchell - A2A on Sun Feb 17, 2013 3:05 pm, edited 3 times in total.

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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Mon Sep 17, 2012 11:32 am 
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Technical Sergeant

Joined: Sun Feb 08, 2009 5:03 pm
Posts: 680
With both Mitchell and Dudley posting about tailwheel takeoffs lately, I'm smelling an Accu-Sim Core Update coming very soon.

:)

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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Mon Sep 17, 2012 10:54 pm 
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A2A Spitfire Crew Chief
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Joined: Tue Jul 29, 2008 11:41 pm
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Location: The South West of the large island off the north coast of Tasmania
Nice run down Mitchell!

(I owe you an email when I return from Kwa-Zulu Natal)

D

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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Tue Sep 18, 2012 2:19 am 
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Aviation Writer and Pilot

Joined: Tue Feb 22, 2005 1:29 pm
Posts: 320
Killratio wrote:
Nice run down Mitchell!

(I owe you an email when I return from Kwa-Zulu Natal)

D


Thank you, Sir.

By Zeus, you do get to some exotic places! Looking forward to your missive with anxious anticipation. Don't lose your head whilst in KZN.

Coincidentally, I have a friend, a fellow musician, Bakithi Kumalo, who is originally from the Soweto township near Johannesburg but who dwelled for a time in Umzinyathi (Dundee) not more than 20-25 miles northwest of Rorke's Drift where there is, I am told, a lovely little Inn and restaurant on the site of the famous battle. Chard, Bromhead and Prince Dabulamanzi kaMpande would be so pleased, I'm sure.

M


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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Tue Sep 18, 2012 8:22 am 
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A2A Spitfire Crew Chief
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Joined: Tue Jul 29, 2008 11:41 pm
Posts: 5017
Location: The South West of the large island off the north coast of Tasmania
M

I will surely fill you in...yes, exotic places... and I have some exotic afflictions to evidence it!
Soweto is not on the plan. My travel companion has spent time in Nairobi and I in Port Moresby...neither of us has any wish for those particular experiences this time. Rourkes Drift and Isandlwana most definitely are. Alas there was no room at the Inn (no, no Messianic pretentions intended :wink: ) so we can not stay there, nor visit Fugitive's Drift (which is on that particular private property). We are, I believe, based out of Dundee.

One of these days I will find a nice civilised resort and spend a few days there sipping Pina Coladas and harrassing waitresses....but for now hard ground and mosquitos will have to continue to suffice....

Ave Amicus


D

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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Tue Sep 18, 2012 9:52 am 
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Airman
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Joined: Mon Jul 09, 2012 1:03 pm
Posts: 17
Location: Esslingen, Germany
Thanks Mitchell, my head is still spinning... :lol:


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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Tue Sep 18, 2012 12:39 pm 
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Senior Airman

Joined: Sat Jun 02, 2012 1:04 am
Posts: 156
Mitchell wrote:
TAILWHEEL TAKEOFFS
Everything Mitchell said....


What he said is spot on fellas..

Joseph


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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Tue Sep 18, 2012 8:24 pm 
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Location: Minneapolis, Minnesota USA
Mitchell,

Thanks for a nice review of these aerodynamics. Having received a taildragger endorsement several months ago (Citabria Champ) after 20 years of tricycle gear flying, your summary only reinforces those effects which typical tricycle gear aircraft tend to muffle or 'forgive'. No question the taildragger is a completely different beast, but mastery (or the constant striving) of take-offs and particularly landings is without question a worthy goal, and true artform. Lots going on simultaneously, paraticularly with crosswind influences. But because the taildragger needs to be flown, rather than flying itself, it makes the pilot technically much better. I can only imagine the true challenge of the P51, but thanks to great programs like this we can get a little closer to reality.

Greg in Minneapolis

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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Thu Sep 20, 2012 7:59 am 
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I moved this to our Flight Academy forum and made it a sticky so others can benefit from it too.

Thanks Mitchell for taking the time to write and post this,
Scott.

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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Thu Sep 20, 2012 8:08 pm 
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Joined: Mon Jan 02, 2006 8:42 pm
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Thanks Mitchell for this.


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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Fri Sep 28, 2012 12:45 am 
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Airman Basic

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Thanks by all to Mitchel

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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Wed Feb 13, 2013 1:42 pm 
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Airman First Class

Joined: Mon Feb 04, 2013 10:21 pm
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I owe you a beer, OP!!! Please share some more info like this, particularly with the Mustang!!

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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Thu Feb 14, 2013 12:49 am 
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Well thanks Wild Bill. Make that an Arrogant Bastard (ale).

M


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 Post subject: Re: TAILWHEEL TAKEOFFS
PostPosted: Fri Feb 15, 2013 10:06 am 
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Airman

Joined: Fri Mar 02, 2012 8:05 am
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Very nice reading Mitchell,
I'm glad I could read this well written poem about taildraggers. It is always nice to do some memory refreshment in a well written material.


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