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Author Topic: Side Force Generators  (Read 2843 times)

Offline phil c

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Side Force Generators
« on: September 28, 2006, 06:09:20 AM »
do stunt planes need more line tension?  Maybe.

It's pretty obvious that each size of lines needs a certain amount of tension for the plane to fly well.  For 015 x 60 ft. lines it takes about 9 lbs, a 45 oz. model flying 52-55 mph.  for 018 x 65 ft lines it takes more, about 12-13 lb., a 60 oz. model flying 55mph.  That much tension keeps the lines fairly straight, with less than 3 deg. of sweep, and as long as you don't slow the plane down too much with wild control movements in sharp corners, the lines stay tight  in maneuvers and don't whip the plane around too much.

Flying slower makes it easier to fly precisely, at least when it isn't too windy.  At 45 mph the 45 oz. stunter above would lose about 1/3 of its line pull and it wouldn't fly well at all.  You could add 15 oz. of weight and get the pull back, but then you'd need 700+ squares and a 40 size motor wouldn't be enough to fly it.  So side force generators that could add 3 lb. of line pull at 45 mph might be good.

Wind doesn't help.  when it gets much over 30% or so of flight speed it really becomes a problem.  I can't see sideforce generators helping much with the wind, unless you can figure out how to make them self trimming.  Upwind they will cause the plane to get blown in.  Downwind they will generate more pull than you want.
phil Cartier

Offline Ron King

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Re: Side Force Generators
« Reply #1 on: September 28, 2006, 02:22:27 PM »
Phil,

I moved your post up here to the design board. Your post offers more information for all of us, not just the electric guys.

Thanks,

Ron
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Offline Dick Fowler

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Re: Side Force Generators
« Reply #2 on: September 28, 2006, 02:51:19 PM »
Interesting points  Phil. I think it's easier though to think in terms of G's in some cases.  Some rambling thoughts!

Typically when we are flying we generate about 3 G's of Centripetal force (CF) in level flight.  That seems to be about where we are comfortable with the "feel" of the handle. So regardless of model weight that’s where we end up.

If nothing else changes, at the top of a wingover the line forces drop to 2 G’s as gravity works against the CF. Still enough tension  to not cause us any trepidation… still feels good.

A certain amount of line tension is  needed to allow the control surfaces to deflect (The Netzeband Wall). Too little and the model simply moves inward and the control surfaces don’t deflect.

It’s possible to get down to  small control handle forces by simply increasing the mechanical advantage of the control system.

The increase in line rake angles and model yaw at lower tension (read lower model speed) can be overcome by adjustment of the leadout guides.

The only one that can’t be tweaked out without some aerodynamic help (enter SFG’s) is the 1 G  force acting overhead. So the model must exert at least 1 G plus control forces of tension over head or it will fall on us.

Problems with SFG’s

If we take Phil’s 45 oz. Stunter on 60 ft lines, it takes about 30 mph or 8.6 second lap times to get down to 1 G line tension. So with no aerodynamic help the line tension is about zero overhead. Not practical.

Picking some arbitrary number let say that we want line tension contribution from model speed to be 1 G so we  fly at 30 mph using Phil’s stunter and then another G from SFG’s so we still have 1 G at the top.

This means the SFG must create lift equal to the weight of the model! That translate into a very large device… little wingtip gadget aren’t going to get it. Add to that the fact we are flying much slower and lift is inversely proportional to the square of velocity, it takes a very large lifting device (SFG) to accomplish the job. Not practical.

Just for the heck of it , if I had to pick an arbitrary number where control feel and mechanics are practical, I’d say that level line tension would be somewhere in the range  of 2 to 2.5 G’s. So the real world is probably somewhere around  1.5 to 2.0 G’s from model speed and maybe .5 G from a SFG. Model speed would now be in the 37mph  to 43mph and lap times of 6 to 7 seconds. (For reference our normal operation would be lap time of 5 seconds, 51 mph and about 2.9 G’s of tension).

This means that a SFG designed to produce lift equal to .5 X  45oz. = 22.5 oz at 39 mph would have an area of about 30 to 40 sq. in. using a typical lifting airfoil. ( if  I’ve done the numbers correctly).

With a device or devices whose cumulative lift is this large, a whole bunch of problems crop up.

Lots of drag – It’s a rather large device.

Appearance – can’t really hide that in current designs.

Location is critical –  Just about any place it is mounted creates some sort of moment that affects roll, pitch, yaw or all. If it were possible to have it located so its CP was right on the CG, Center of lift it might have a chance… problem is that in most cases these points on a model don’t coincide to start with.

At first blush it seems to be a big hassle requiring more time than I would like to spend.

What is interesting is that we already have about 30 to 40 sq in. of vertical surface working for us already in the form of the fuselage. That’s why I made my comment about using rudder to get some AOA and from that some lift.

Sorry I rambled. Disclaimer – As a Physicist I tend to look at these things as rocks on strings… in my world models never reach level flight! So the aerodynamics could be flawed and comments or corrections are welcome.
Dick Fowler AMA 144077
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Offline Ron King

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Re: Side Force Generators
« Reply #3 on: September 29, 2006, 07:56:30 AM »
Dick,

At least you have generated some numbers for us to play with.

But since the average stunt ship has far more area aft of the CG, am I correct to consider this area you mention to be what is needed right around the CG? Say within 6 inches fore and aft?  ???

If so, then we could consider the side area of the fuselage above and below the wing, then subtract that and come up with a closer estimate of our SFG area.

More food for thought,

Ron
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Offline Dick Fowler

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Re: Side Force Generators
« Reply #4 on: September 29, 2006, 08:34:31 AM »
"But since the average stunt ship has far more area aft of the CG, am I correct to consider this area you mention to be what is needed right around the CG? Say within 6 inches fore and aft? "

Maybe not... not sure. Stability is better if the CP is aft of the CG.

 Just like real estate... I think these kinds of gadgets are all a matter of location, location, location!. ;D Biggest problem I see is that all of the "centers' are not in the same place on our models. CG is at a different location than CP both lateral and spanwise. The apparent center of drag is at an other location so where do you put it?

From my uneducated point of view, I can't think of a location that doesn't present other issues so why bother? To me it's another complication that we could end up chasing our tails.

Dick Fowler AMA 144077
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Offline Bill Little

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Re: Side Force Generators
« Reply #5 on: September 29, 2006, 11:01:46 AM »
Hi Phil,

I don't think it's a matter of "needing more tension", just a matter of needing the proper tension.

I have found that a lot of stunt planes are dialed in with so much line tension that flying precisely is a problem.  All that is needed is to have control of the plane.

Bill <><
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Offline Ron King

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Re: Side Force Generators
« Reply #6 on: September 29, 2006, 12:27:54 PM »

From my uneducated point of view, I can't think of a location that doesn't present other issues so why bother? To me it's another complication that we could end up chasing our tails.


Upon further review... I Agree.  <=

If, as they say, you learn more from your failures than from your successes, I must be getting smarter.  n~

Ron
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Offline the original Steve Smith

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Re: Side Force Generators
« Reply #7 on: September 29, 2006, 09:14:29 PM »
Has anyone actually tried to use SFG devices on a real stunt plane?

It seems that the RC guys - particularly the 3D flyers - use these to help with knife edge flight which is what we are doing in the overhead 8.

I have been tempted to try an RC electric design and modify it for C/L but haven't had the time or money to do so yet.

Might be worth experimenting with.   y1


Thanks,
the original Steve Smith
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Offline phil c

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Re: Side Force Generators
« Reply #8 on: September 30, 2006, 07:08:35 AM »
One of the reasons I don't see a lot of reason to experiment with sideforce generators is that they only really help in two maneuvers, the Reverse Wingover and the Square Eights.  Since the judges can't even see the square eights it's unlikely that they will even notice big improvements, so why bother.

The other issue is line tension and how it contributes to better flights.  Take a really close look at how your plane flies at various lap times.  I guarantee you it will fly smoother and wobble less when it is flying faster.  Every plane I have ever flown flies better faster.  You may not like the control feel or the excessive line pull, but the plane flies better when it pulls harder and the lines are straighter.

Yes, you can trim the plane to fly square to the edge of the circle with the leadout rake.  But a light plane flying with 5 deg. of rake will have a lot more problems with the lines whipping around and upsetting the plane than a heavier plane flying with 2.5 deg. of rake.  It's also another reason for the quest for steady flight speeds.  The line rake varies with airspeed, and one setting is really only good over a very narrow range.  When the airspeed changes, the line rake changes and the plane starts to yaw.  This really becomes obvious when you take a lightly loaded ship and fly it through some hard square eights.  After a few corners you can see the lines whipping around several feet, yawing the plane in and out as much as 20 deg. or more.

It would be an advantage if you could trim the plane to fly at 45 mph or even a bit less.  At that speed most pilots would be getting into the range where their reflexes are faster than the five foot pullout alititude.  You can see and react down in the 3 ft range which lets you actually fly the plane down to the right height instead of trying to guess at 15 ft where it is going to end up.  But at 45 mph your good ol' Nobler is flying 5.8 sec laps and feels like it is going to fall out of the sky.  Side force generators to get the line pull back up to eight pounds or so(2 lb. of side force) would be great.

How about putting 10 little vertical winglets along the spar on each panel.  10 top and bottom, say 1 in. x 3 in. tall.  That would be 120 square inches of winglets.  I'd make them with a Clark Y flat bottom section to make them easy to line up with the centerline and still generate lots of lift.

Better yet, how about a Mighty Slobbery?  Biplane with 6 in. x 40 in. wings.  Space them 7 in. apart and add 4 vertical "struts" between the wings on each side, eight in all.  The struts would have a flat bottom section, positioned to lift outboard.  Each strut an 1.25 in. wide. for a total of 70 square inches.
phil Cartier

Offline Tom Niebuhr

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Re: Side Force Generators
« Reply #9 on: October 01, 2006, 12:39:43 PM »
Phil,

"Mighty Slobrery!!??

The "BiSlob" can be made to almost stop overhead now.. Imagine with the side force generator/airfoiled struts, in a good wind it will be able to hover straight over head!
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Offline Dean Pappas

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Re: Side Force Generators
« Reply #10 on: October 20, 2006, 01:02:06 PM »
Hi Gang,
One of the points of such a basic investigation is to question our cherished assumptions.
Yes, angling the plane outboard does work, but with an ugly price. The wing now flies inboard tip forward, and that amount changes with speed and line tension. We already know about the difference in leadout versus rudder offset with heavy and extremely light airplanes, and those yaw behaviors, combined with the tip banging that comes from flying the wing crooked  make life difficult. Dick is onto something, here. How do we keep the wing spar in line with the handle over a range of speeds, and how do we generate side force with a surface(s) that don't create negative line tension at the upwind end of the circle. A quick thumbnail calculation shows that the side-slip angle at the upwind end of the circle is 15 degrees when doing 5.5 seconds on 70' lines. That's pretty severe. It certrainly complicates the problem. I need to build a profile foamie to test some ideas. This day job requirement gets in the way!

Let's keep thinking, though. Phil did a good job of explaining what the potential prize is, if we figure it out.

later!
Dean Pappas
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Offline phil c

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Re: Side Force Generators
« Reply #11 on: October 23, 2006, 10:00:54 AM »
Phil,

"Mighty Slobrery!!??

The "BiSlob" can be made to almost stop overhead now.. Imagine with the side force generator/airfoiled struts, in a good wind it will be able to hover straight over head!


That's the whole idea Tom.  I haven't seen anyone fly even a deccent pattern with a Bi Slob.  Too unstable and the speed varies wildly making precision very hard.  Overhead is tricky too.  Get more than a bit to the upwind side and the plane sometimes slows to a stop, losing almost all its line tension.
phil Cartier

Offline Doug Moon

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Re: Side Force Generators
« Reply #12 on: October 27, 2006, 10:41:31 PM »
I have tried SFGs on my Forerunner with horrid results.  I made them look very similar to what you see on larger RC pattern planes of the day.  I went out about 2/3 of the wing panels and cut a slot in the wing and inserted what looked like the verticale stabalizer both top and bottom of the plane.  I read several articles on area of the SFGs and placement of according to high point and wing design, and went accordingly on my plane.  I expected Line tension to go up BUT they caused so much drag and yaw it ruined line tension on the first flight.  Moved the LOs as faw forward as possible and tried again.  The tension was back but more like normal instead of increased.  EVERY trim probelm with that plane was maginfied 10 fold.  In the maneuvers the roll was so bad some manuevers were not able to be completed.  Such as verticles AND get this OVERHEAD EIGHTS!  Yes they were so poorly effected I could not complete them.  I trimmed on the plane for about 7-8 flights and decided it wasnt going anywhere productive.  Removed them went back to prior trim setup and the plane was happy and stable again with excellent flying abilities.  CLPA planes have what is referred to by my RC friend as VOO-DOO aerodynamics.  I agree.  Typical logical 3 deminsional thoughts about aerodynamic additions to CLPA planes just dont fly sometimes....pun intended.  Since we operate in a 2-D world we have to stay in that environment.  Introduce 3-D aerodynamics and the delicate balancing act of a stunt plane is gone.

VOO DOO Aerodynamics.  For instance, for years we have heard "tape the hinge lines."  that is a logical thought and I have done it many times myself.  Frank Williams, a very smart man I might add, has flaps on his newest plane with hinges that deflect the flap down/up and away from the TE.  When deflected it looks disconnected from the plane.  The gap is HUGE.  He says he gets an increase in line tension overhead with such modifications.  Should it be?

As RO once said.  "When "they" say go right I go left just to see what happens."
Doug Moon
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Offline Dick Fowler

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Re: Side Force Generators
« Reply #13 on: October 29, 2006, 04:36:21 AM »
Hey Doug... I just consulted the "chicken bones" and wonder what would have happened if when you did your experiment, you would have had the tabs mounted with the  front slighty inboard. Again though... center of pressure of these gadgets with respect to yaw will dictate behaviour.

Although I think that we truely are flying in 3D... unlike RC airplanes ours never reach equilibrium because we are flying in a circle.
Dick Fowler AMA 144077
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Offline Howard Rush

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Re: Side Force Generators
« Reply #14 on: October 31, 2006, 09:24:50 PM »
I've experimented with a couple of different side force generators.  They worked fine and had more L/D (sideforce/drag, actually) than I expected. 

One phenomenon I've noticed with a really good stunt plane is that when the wind increases, line tension increases uniformly all around the circle. 
The Jive Combat Team
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Offline Will Stewart

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Re: Side Force Generators
« Reply #15 on: November 09, 2006, 06:38:48 PM »
Phil and All,

Never flew Stunt, but have held a few CL Speed records (Recip, Jet and Electric) over the years.  And pay the bills as an aero and thermo engineering consultant.

Some comments from an outsider:  Aerodynamic side force generators can be mounted to pivot around their neutral axes, spring loaded to develop sideforce with leading counterweights to reduce angle as centrfugal force builds.

If only a few pounds of outforce is necessary, a small ducted fan installed perpendicular to the line of flight, powered by an 09 could do this in 3 inches or so diameter.

Will Stewart
« Last Edit: November 10, 2006, 10:44:40 AM by Will Stewart »


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