Leadout spacing?

[Shug Emery]

The query......in general how far apart should leadouts be from each other once set for flight?
Shug

[FLOYD CARTER]

Many builders use the commercially available line adjusters.  They typically have fixed spacing of 1" to 1 1/2".
Many Old Time planes show "fixed" leadout eyelets, but that is a crap shoot, and hard to adjust once built-in.  Closer is better, but not so close that the line clips could tangle together.

[Tim Wescott]

... and a lot of the top guys make the leadouts so that they're not only individually adjustable, but so that in a pinch you can swap which one is in front.

This can be as super-zoot as making screw-tightened leadout adjusters like the commercial ones, but with just one hole, to using brass eyelets in a slot (that's what Paul Walker usually does, or maybe always).

I suspect I'm not good enough to recognize problems small enough to be fixed with line spacing, at least not at the moment.

[Shug Emery]

Many builders use the commercially available line adjusters.  They typically have fixed spacing of 1" to 1 1/2".
Many Old Time planes show "fixed" leadout eyelets, but that is a crap shoot, and hard to adjust once built-in.  Closer is better, but not so close that the line clips could tangle together.

That sounds close to where I am. Thank you.

... and a lot of the top guys make the leadouts so that they're not only individually adjustable, but so that in a pinch you can swap which one is in front.

This can be as super-zoot as making screw-tightened leadout adjusters like the commercial ones, but with just one hole, to using brass eyelets in a slot (that's what Paul Walker usually does, or maybe always).

I suspect I'm not good enough to recognize problems small enough to be fixed with line spacing, at least not at the moment.

Yes but do you have any spacing info from those top guys? Thanks.

[L0U CRANE]

All good answers!
If you have access to one of the LINE programs, you can get a good estimate of the angle of line rake needed. Whatever leadout separation you're comfortable with should be centered on that line - from the CG to through the wingtip guides.

The angle should not change with CG fiddling to get the proper, or desired, 'feel' and response. With both leadout guides adjusted on a single 'carrier' you should be able to keep the center between them on the calculated angle from perpendicular to fuse centerline (or tangent to the CG's path if the model flies crabbed...)

It's not live-or-die critical, but should be very close, and is still 'tunable' with the adjustment.

Floyd made a very good point. Lines that far apart can make small inputs of their own if your handle points ahead of or behind the model. That's one of the reasons PAMPA's OTS rules allowed moving the location of bellcrank and leadouts. We'd learned better by then.

[Paul Walker]

It depends on your skill level and airplane.

Why spacing?
To counteract some of the precession. Bigger, and heavier props create more precession. With a normal rotating prop, the up line should be in the front. Most recognizable is the outsides in the square 8, where it wants to yaw in. Moving the down line back helps this. Insides are less noticable as it doesn't come loose, but more forward up line helps the quality of the insides.

Me, they are close together. I use an Igor hollow prop and the precession is minimal, although not the reason they were made, it sure helps.

So, it depends on where you are. More diameter, more spacing.

[Shug Emery]

All good answers!
If you have access to one of the LINE programs, you can get a good estimate of the angle of line rake needed. Whatever leadout separation you're comfortable with should be centered on that line - from the CG to through the wingtip guides.

The angle should not change with CG fiddling to get the proper, or desired, 'feel' and response. With both leadout guides adjusted on a single 'carrier' you should be able to keep the center between them on the calculated angle from perpendicular to fuse centerline (or tangent to the CG's path if the model flies crabbed...)

It's not live-or-die critical, but should be very close, and is still 'tunable' with the adjustment.

Floyd made a very good point. Lines that far apart can make small inputs of their own if your handle points ahead of or behind the model. That's one of the reasons PAMPA's OTS rules allowed moving the location of bellcrank and leadouts. We'd learned better by then.

I appreciate the intel.

Hi Shug. Keep it simple and make them one  inch apart.  Simple question,such complex diverse answers. sheesh.

I am at about 3/4 of an inch right now. May widen a wee bit. Thanks.

It depends on your skill level and airplane.

Why spacing?
To counteract some of the precession. Bigger, and heavier props create more precession. With a normal rotating prop, the up line should be in the front. Most recognizable is the outsides in the square 8, where it wants to yaw in. Moving the down line back helps this. Insides are less noticable as it doesn't come loose, but more forward up line helps the quality of the insides.

Me, they are close together. I use an Igor hollow prop and the precession is minimal, although not the reason they were made, it sure helps.

So, it depends on where you are. More diameter, more spacing.

Thank Paul. Had to look up what precession means. Makes good sense. Mainly talking about my Strega with a .61 and a 13x4 prop. Will try some changes at the field in the morrow. Appreciate you taking the time to give your advice. Means a lot.

[Istvan Travnik]

Dear Friends,
some years ago we analyzed this complex problem, very thoroughly. See:

https://stunthanger.com/smf/building-techniques/bellcrank-normal-or-reverse/msg386941/#msg386941

Istvan

[Shug Emery]

Dear Friends,
some years ago we analyzed this complex problem, very thoroughly. See:

https://stunthanger.com/smf/building-techniques/bellcrank-normal-or-reverse/msg386941/#msg386941

Istvan

Wow...lots of info there. Thank you.

Hi Shug. You  didn't say if your lead outs were individually adjustable, as in two  separate sliders or in a two hole slider. I made a wrong assumption. Sorry.

They are. Mainly tweaking the ones on my Strega.

[Ted Fancher]

It depends on your skill level and airplane.

Why spacing?
To counteract some of the precession. Bigger, and heavier props create more precession. With a normal rotating prop, the up line should be in the front. Most recognizable is the outsides in the square 8, where it wants to yaw in. Moving the down line back helps this. Insides are less noticable as it doesn't come loose, but more forward up line helps the quality of the insides.

Me, they are close together. I use an Igor hollow prop and the precession is minimal, although not the reason they were made, it sure helps.

So, it depends on where you are. More diameter, more spacing.

Hmmmm.  Never asked Paul about this and Al never debated it when I brought it up a few decades back, but....

How about accounting for "P-factor", Paul, which more or less does the opposite of what precession is accused of.  Given the high thrust to weight ratio of our little airplanes with (comparatively) large props (and conventional rotation) I've always felt it logical that the yaw caused by P-factor throughout the high g corner would have a far greater impact on the yaw and in the opposite direction of (conventional rotation) precession with light carbon props and small diameter spinners.

As "semi" evidence I've flown out of a lot more first inside loops of clovers (a number of times) than the third (outside) loop...both performed at the same "exposed to yaw problems" 45 degree and above line angles (I know, I know, the rule book probably calls for something other than 45 degrees but i defy anyone to perceive the difference!).

As I recall it, precession was predominately an item of concern with high powered piston fighter planes when "rotating" up on the main gear on the takeoff roll: very high RPM status and a fixed point (the interface between the ground and the bottom of the tires) about which the mass of the rotating mass is pitched.

My feeble memory banks recall tales of an event at the Renton airport when I was very young (most likely plus or minus a couple years around 1950) when a war surplus P-40 was taking off and rotated too rapidly, rolled and crashed for which the purported cause among the hangar crowd was precession.  (Remember...we're talking old-timers disease and memory challenges here!  FWIW I couldn't find reference to the event in a short "desktop search".)

My more basic reason for suggesting this argument counter to precession is the hours I spent in Cessnas, etc. of all shapes and sizes holding right rudder to counteract  P-factor during climbs with a "high" angle of attack and takeoff and/or climb power settings with conventional rotation propellers.  (to be clear, none of the climbs were accomplished while inverted!)

Not being argumentative so much as respecting the input from guys such as yourself (and Howie) with serious backgrounds in aerodynamics.

[Paul Walker]

I simply find that the precession yaw is greater than the P factor yaw. I is obvious when changing from a solid core prop to a hollow core prop of the same diameter. The mass difference IS noticable. Then try a larger diameter prop. Plenty of yaw to adjust to!

[Chris Cox]

Hi Shug

Like Paul and several others out here on the left course, I use the plug-in inserts for my lead-out adjustments.  The spacing is 1/4”, and the inserts are nylon which I make on my hobby lathe.  You will notice the lead-outs are right beside each other (1/4”!).   And yes, they are a long way back!  Electrics seem to like this, please don’t ask me why....

[Chris Cox]

Hi Ted

When I was teaching ground school at Air Cadets, to demonstrate precision, I would bring one of those small gyros that was in a wire cage.  You would spin the centre gyro using a string and then amaze your friends by placing the contraption on the rim of you coffee cup and if would happily sit there until the gyro finally slowed down enough that it would no long stay in place.  Anyhow, to demonstrate precision, I would spin the gyro up and hand it to a student and ask them to try twisting it.  Of course, it would resist, but the tendency to turn 90 degrees to the direction they twisted it was quite an eye opener!   I have to believe the precision induced by a large heavy prop must be significantly high, as observed by Paul. 

[Dan McEntee]

Hmmmm.  Never asked Paul about this and Al never debated it when I brought it up a few decades back, but....

How about accounting for "P-factor", Paul, which more or less does the opposite of what precession is accused of.  Given the high thrust to weight ratio of our little airplanes with (comparatively) large props (and conventional rotation) I've always felt it logical that the yaw caused by P-factor throughout the high g corner would have a far greater impact on the yaw and in the opposite direction of (conventional rotation) precession with light carbon props and small diameter spinners.

As "semi" evidence I've flown out of a lot more first inside loops of clovers (a number of times) than the third (outside) loop...both performed at the same "exposed to yaw problems" 45 degree and above line angles (I know, I know, the rule book probably calls for something other than 45 degrees but i defy anyone to perceive the difference!).

As I recall it, precession was predominately an item of concern with high powered piston fighter planes when "rotating" up on the main gear on the takeoff roll: very high RPM status and a fixed point (the interface between the ground and the bottom of the tires) about which the mass of the rotating mass is pitched.

My feeble memory banks recall tales of an event at the Renton airport when I was very young (most likely plus or minus a couple years around 1950) when a war surplus P-40 was taking off and rotated too rapidly, rolled and crashed for which the purported cause among the hangar crowd was precession.  (Remember...we're talking old-timers disease and memory challenges here!  FWIW I couldn't find reference to the event in a short "desktop search".)

My more basic reason for suggesting this argument counter to precession is the hours I spent in Cessnas, etc. of all shapes and sizes holding right rudder to counteract  P-factor during climbs with a "high" angle of attack and takeoff and/or climb power settings with conventional rotation propellers.  (to be clear, none of the climbs were accomplished while inverted!)

Not being argumentative so much as respecting the input from guys such as yourself (and Howie) with serious backgrounds in aerodynamics.

     Hi Ted;
     The phenomenon and circumstances of which you speak, was covered very well in an old issue of Air Progress Magazine back in the 70's I think. Chris McMillin could probably remember what issue it was, because he asked me to see if I had it and make copies for him, which I did, when he was living here in St. Louis. It was a lengthy article on the subject of ground handling, take offs and landings in high performance air plane(warbirds.) I read it at the time also, but have slept a few times since then, so that is all I can remember about it. If Chris sees this, or if anyone else can remember at least the year, I can dig the magazine out again pretty easy and scan it.
  Type at you later,
   Dan McEntee
   

[Gary Dowler]

I simply find that the precession yaw is greater than the P factor yaw. I is obvious when changing from a solid core prop to a hollow core prop of the same diameter. The mass difference IS noticable. Then try a larger diameter prop. Plenty of yaw to adjust to!

Paul, where do you get your props from?

Gary

[Paul Walker]

Paul, where do you get your props from?

Gary

Igor.

[Air Ministry .]

Hooking up the ' wiggly rudder ' gave imediately ' it just flows round through there , square eights .
instead of the previously not heeded pul pull pull slacken slaken etc on the corners , made ya jump .
Eye opener.
If youarnt running that , yell. I will post a picture of the Mechanism on mine . retro fit .

[Ted Fancher]

Hi Ted

When I was teaching ground school at Air Cadets, to demonstrate precision, I would bring one of those small gyros that was in a wire cage.  You would spin the centre gyro using a string and then amaze your friends by placing the contraption on the rim of you coffee cup and if would happily sit there until the gyro finally slowed down enough that it would no long stay in place.  Anyhow, to demonstrate precision, I would spin the gyro up and hand it to a student and ask them to try twisting it.  Of course, it would resist, but the tendency to turn 90 degrees to the direction they twisted it was quite an eye opener!   I have to believe the precision induced by a large heavy prop must be significantly high, as observed by Paul.

Thanx Chris.  I'm curious what percentage of the overall weight of the gyro was spinning?

How did you demonstrate P-factor to them in the classroom?  Or just in the actual airplane?

My argument sort of falls back to the basic airmanship instruction for straight ahead climbs in your Cessna or Piper: 1. Set climb power (usually firewall it on light planes); raise the nose to increase the angle of attack and achieve/maintain climb airspeed and....something else...oh, yeah, center the ball with right rudder to prevent turning to the left while in the climb due to P-factor.

IOW, if you pull the nose up to climb the airplane will want to turn to the left due to P-factor and if you push the nose down (with power left on or increased) it will turn right for the same reason albeit reversed due to the pitch attitude relative to the the direction of thrust: in both cases the turn tendency will remain unless the pilot keeps the ball centered by applying opposing rudder to center the ball and, thus, continue straight ahead flight while climbing or descending.  (Obviously, most descent are conducted with power reduced or at idle, thus, the demands for countering rudder input due to P-factor is often minimal to non-existent .)

Again, given the high thrust to weight ratio of a competitive stunt ship I continue to be amazed that there is little or no discussion of the P-factor developed especially with respect to yaw/line tension issues.

Ted

[Ted Fancher]

     Hi Ted;
     The phenomenon and circumstances of which you speak, was covered very well in an old issue of Air Progress Magazine back in the 70's I think. Chris McMillin could probably remember what issue it was, because he asked me to see if I had it and make copies for him, which I did, when he was living here in St. Louis. It was a lengthy article on the subject of ground handling, take offs and landings in high performance air plane(warbirds.) I read it at the time also, but have slept a few times since then, so that is all I can remember about it. If Chris sees this, or if anyone else can remember at least the year, I can dig the magazine out again pretty easy and scan it.
  Type at you later,
   Dan McEntee

Thanx Dan.  Those are exactly the sort of events that were experienced in the "now" phantom P-40 crash I mentioned.  I was a little surprised I was unable to find any reference to the Renton P-40 event on line.  Searching for crashes at that airport didn't even bring up the B-747 landing there that tore the aft body gear out of the airplane on the dike at the approach end of the runway which fronts on Lake Washington.  It's only a 5400 or so foot runway so they were obviously trying to get the plane on the ground early and misjudged!
 

[Air Ministry .]

"  Had to look up what precession means. Makes good sense. Mainly talking about my Strega with a .61 and a 13x4 prop ":

Vindiesays , anything over 12 ' and fit the Rabe rudder , Id say WITH 12 and over. Which might be what windy says .
Anyways as this influences the leadout Adj.  if you ever do , Theres three Du Bro hinges in the rudder , a 1/4 inch of alumn. tube at the elevator bottom front,
a 1/32 ply horn on the rudder . Light wire as its in tension under load , and can ' spring ' if you rest it on its end. So you dont , well you do , carefully .

Winds Rec. is Lead Outs 1 in aft. of C. G. , starting point .

[Shug Emery]

Thanks all. As expected...info overload.
Tweaked at the field today and some improvements in clover. A bit more to tweak but all helped.

[FLOYD CARTER]

For my future builds, I am going to adopt the 1/4" line spacing.  This means I must stagger the leadout lengths to prevent line clip interference.  So this also means I have to shorten one of my existing lines, or start with a new set..  I think the effort will be worth it.

I still like to use commercial leadout adjusters.  I can drill new holes in them to get 1/4".

[Tim Just]

While I am a beginner at stunt, I have been fortunate to fly a number of high performance tailwheel aircraft.  Experience has taught me that it is possible, even easy to raise the tail before the rudder is able to do its job.  Easy, yes. Advisable, not so much. Like touching wet paint or a hot stove I of course had to find out for myself how much margin existed. When I was a novice in such matters I inadvertently self demonstrated.  It went something like this. Add power to start takeoff.  Immediately lift tail to improve visibility, reduce angle off attack associated induced drag. Airplane starts to make an uncommanded left turn. As my right leg begins to run out of travel the now improved visibility does not include the runway!  Just enough time to re avaluate all of my life choices.  Happily there was enough control power to rein it back in.  Lesson learned, don’t touch wet paint!

I’m pretty sure there was a question in the private pilot written about torque, gyroscopic precession, P-factor and spiraling slip stream.  For this discussion I’m leaving spiraling slip stream out because it’s the hardest for me to quantify.  On humid days I have seen vapor trails coming off the propeller tips that show the effect and look really cool.

Separating precession from P-factor is actually pretty straightforward in a manned aircraft.   My experience, any hard pull will require left rudder (gyroscopic procession) through the corner. Once the G is stopped, right rudder is required (P-factor).  On a 45 deg upline if held to stall the plane will run out of rudder (P-factor) but not aileron.  In a vertical climb, the roll from torque overpowers the ailerons first.  No P factor as the propellers is at zero AOA.   A ton of right rudder is however required. I wasn’t going to mention it but probably spiraling slipstream?

One figure that really shows the influence of gyroscopic procession is a push humpty bump.  No idea who named it. Starting from a vertical up line, push 180 degrees to a vertical down line with minimum speed over the top.  During the push the G is close to zero and airspeed is almost off the dial.  The propeller can easily have its way with the airplane.  If standing on the right rudder does not keep the plane on heading power must be reduced.  This was counter intuitive at first as I wanted more power and more airflow over the tail.

An inverted 45 deg push corner held to stall requires right rudder in the corner again from gyroscopic procession. Immediately followed by increasing left rudder (P-factor). If held post stall the airplane will spin against full left rudder.  This is mildly disconcerting.  Recovering only when power is reduced due to gyroscopic precession. 

Another interesting  P-factor demo is in light twins with non counter rotating propellers.  The descending blade of the right motor typically defined as the critical engine is the farthest from center line and consequently has greater moment to overpower yaw in an engine out situation. 

My takeaway from all this, gyroscopic procession is most troublesome when pitch or yaw rate is introduced and P-factor is more noticeable steady state at positive or negative Angle of attack.

As my journey with stunt continues I’m currently having significant difficulties understanding the aerodynamics of overspray and a new word just added to my vocabulary, fisheye.  Armed with a rapidly shrinking stack of 1000 grit, I will prevail!

[Chris Cox]

Ted  - What Tim says above.      

[Chris Cox]

Hi again Ted - My response above was a little curt, but Tim’s explanation of when Gyroscopic Precession plays a role during a pitching moment vs. P-Factor is very good.  Like Tim, when flying my somewhat aerobatic tail dragger, I find what I believe to be precession, the stronger phenomena taking place. 

With respect to the P-40 crash, and this is purely speculation on my part.  If it rolled in on take-off, the issue may have been more a torque roll issue than either precession or P-factor.  We have lost several P-51’s and other way birds over the years due to the pilot increasing power too quickly, especially on a go around, and rolling in due to uncontrollable torque forces.

Here is one of many examples:   https://aviation-safety.net/wikibase/wiki.php?id=167777

Interesting side issue to the above accident with respect to cannabis use and THC that remains in body tissue for extended  lengths of time.  Not good.

[Air Ministry .]

You get this ' P ' effect in flight , when its mushing . ( Horizontal or is that Vertical ? YAW )seeing lateral yaw is yaw , it must be vertical yaw .

If you pull out in a hole in the air & it bonces , attempting to accelerate , nose up , engine on, HARD . The effect can be quite noticeable .
Requireing back stepping, smartly . As Appropriate . In some cases running helps .

[Brett Buck]

Thanx Chris.  I'm curious what percentage of the overall weight of the gyro was spinning?

How did you demonstrate P-factor to them in the classroom?  Or just in the actual airplane?

My argument sort of falls back to the basic airmanship instruction for straight ahead climbs in your Cessna or Piper: 1. Set climb power (usually firewall it on light planes); raise the nose to increase the angle of attack and achieve/maintain climb airspeed and....something else...oh, yeah, center the ball with right rudder to prevent turning to the left while in the climb due to P-factor.

IOW, if you pull the nose up to climb the airplane will want to turn to the left due to P-factor and if you push the nose down (with power left on or increased) it will turn right for the same reason albeit reversed due to the pitch attitude relative to the the direction of thrust: in both cases the turn tendency will remain unless the pilot keeps the ball centered by applying opposing rudder to center the ball and, thus, continue straight ahead flight while climbing or descending.  (Obviously, most descent are conducted with power reduced or at idle, thus, the demands for countering rudder input due to P-factor is often minimal to non-existent .)

Again, given the high thrust to weight ratio of a competitive stunt ship I continue to be amazed that there is little or no discussion of the P-factor developed especially with respect to yaw/line tension issues.

     The difference is that in your example case, the precession is essentially zero in all the cases. In stunt planes, the P-factor is happening at the same time as an exceptionally high pitch rate. Precession is proportional to the pitch rate, in the Cessna it's essentially zero, in the stunt plane in a corner, it's huge.

     Precession torque is relatively easy to calculate. I, at one point, also (with some assumptions) wrote a simulation of how much P-factor you were liable to get for a typical case at the time, an ST46 with a 12-6, assuming a relatively large body angle of attack (probably larger than you would get, peak). At the very highest AoA  and what I estimated as the thrust - in-flight, not the ground, and assuming something like 10 FPS reduction in speed, the P-factor was about 10-15% of the precession. So the net torque was strongly in the direction of precession.

   Note that the "high thrust to weight" is not exactly what you might expect, if anything, suppressing the speed drop reduces the required AoA, which reduces the P-Factor, rather than increasing it. As I recall, for the example case I did, the P-factor was about like shifting the CP laterally about 1/2", that is, take the thrust and treat it like it was 1/2" off the centerline of the crankshaft. With the much higher RPM, it will be shifted less, because the vector component of the forward motion will reduce the difference in AoA from one blade to the other. Again, my simulation was far from ideal, but it doesn't have to be when the difference is a factor of 5x to 10x. It also predicted the static thrust pretty accurately.

      Brett

[Chris McMillin]

Neat post, Brett. I've read it three times and am almost up to speed.
Chris...

[Chris McMillin]

Dan McEntee,
Mike Dillon authored "Fly Your Own Fighter Plane; or Death in a Beautiful Package", fall of 1968 Air Progress. I cant remember the month.
It was a great article giving one the basics of P-Factor of asymmetric thrust of propeller blades from at high AOA (with photos of blade angle on each side with 2x2 stick aligned with blade for effect even a 4th grader could understand), slipstream effect, torque as in engine/airframe reaction in prop fighters, and some of their individual handling and general physics (like how a Corsair stalled at 1,000 feet develops a high rate of sink because it weighs 11,000 pounds and one won't have room to unstall it and arrest the rate of descent until after ground impact.)
I think I found it in your collection and made copies for students and colleagues for years.
Chris...

[Gary Dowler]

Brett, interesting post. Just beginning to understand some of this stuff.  Keep typing, I'll keep learning.

Gary