Re: [COUPERS-FLYIN]

[Wayne]

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Fellow Coupers::

 

It has been a long time since I flew an Ercoupe without manual rudder control input, but as I recomember it, the reason we want to let the nose strut to fully extend upon touchdown is for the nosewheel to make terra-firma contact at the same time as the main gear.  Done right in a crosswind, the nose wheel will be aligned with the runway, and the aircrafts momentum, at the moment of touchdown the strut begins to take the forward weight of the airplane and compress, the robust main gear moves the aircraft line of thrust configuration to the line of momentum, (The cross wind vector) the controllable nose wheel slips gently though the fingers of the highly experienced airframe controller, allowing the strut to fully compress, causing a properly rigged Ercoupe tail to rise to 75 inches, and the negative angle of incidence of the tail surface to effectively cause the aircraft to quit flying, and pass on down the runway centerline bleeding off speed and forward momentum in the process.

 

Is that about right?

 

Still doesn't answer the cable restraint question.  I never have figgured that out, I think the nosewheel should be below the turbulent airstream created by the propeller.  hence, no need for the cable at all, the nutcracker does all the work of a restraint cable, that is if the bumper is on correctly, and the nutcracker cannot go "over center."

 

Wayne W.

 

 

----- Original Message -----

From: Ed Burkhead

To: [EMAIL PROTECTED] ; Cflyin

Sent: Monday, October 16, 2006 5:17 AM

Subject: RE: [COUPERS-FLYIN]

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Dr. R. Beeman wrote:

>>>>> If you land an Ercoupe on the mains in a crosswind and wait "...until the coupe itself swivels straight down the runway..." please tell me specifically what force will cause the aircraft to "swivel straight down the runway"?

The answer is there is no magical force available to an Ercoupe or a spam can for that matter. A spam can uses rudder to counteract the crab just before touchdown, but a rudderless Ercoupe must use the runway itself to counteract the crab, for if the Ercoupe pilot tries to take out the crab before touching down, he'll simply fly away from the centerline.

The only counterforce to a crosswind crab that a rudderless model of the Ercoupe possesses is the force exerted through the nose wheel touching and counteracting the cocked main gear. This is what the POH calls for and this is all there is. There is no automatic swivel straight down the runway and if you wait for it before planting your nose wheel you'll be cutting daisies.<<<<<

 

Doc,

 

Please review Chapter 17 of Stick and Rudder, which in my copy starts on page 312.

 

Yes, there IS an automatic swivel, straight down the runway.

 

The force that rotates the airplane to line up with the direction of travel is exerted through friction with the ground.

 

The friction vector comes at an angle along the direction of motion.  It can be represented by two vectors at right angles, one along the longitudinal axis of the aircraft and one directly sideways.

 

|\

| \

|  \

|   \

|    \

|     \

|      \

|       \

-------à

 

When touching down in the crab (whether the nose gear touches at the same time or not), there is a side load on the tires – they are not lined up with their motion.

 

The tires do start to rotate and almost instantly stop providing resistance along the vector of the aircraft’s longitudinal axis.  But, the frictional push is at an angle and part of the force may be vector diagrammed as being from the side of the aircraft.

 

It is that side vector which pushes sideways on the main gear – and the main gear is behind the aircraft’s center of gravity.  So, there’s a sideways push on the rear of the aircraft.

 

But, if the nose gear has already touched, any sideways vector on that is almost instantly relieved because the nose gear will turn and align itself with the direction of motion.  So, there’s no sideways push vector on the front of the aircraft.  (This is why we must not hold the yoke firmly when landing – a fingertips only grip will allow the nose gear to turn to line up with its travel across the ground.)

 

Summary:  There’s a sideways push to the rear of the aircraft but no sideways push on the front of the aircraft.  So, the aircraft rotates.

 

When the aircraft is in line with the direction of motion, all sideways vectors disappear.

 

Side notes:  the mass of the aircraft is concentrated near the center of gravity.  The structures far from the center are light weight.  So, the aircraft automatically rotates quickly.  The sideways force vector is relieved quickly and is not very violent.  An egg in a saucer in your lap will probably stay there. (Please use hard boiled eggs for your personal testing.)  J

 

If this isn’t adequate, I’ll try to hand draw, scan and send a proper vector diagram of the forces involved later.  (I’ve got to get my daughter up and get her to school.)

 

Ed Burkhead

http://edburkhead.com

ed -at- edburkhead???.com          (change -at- to @ and remove "???")

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