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Author Topic: Reducing control forces using spades  (Read 20864 times)

Offline Mark wood

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Re: Reducing control forces using spades
« Reply #50 on: November 29, 2021, 09:28:50 PM »
Lasted longer than most.  It was getting somewhere too.

Ken

We're not done yet.
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Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
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Offline BillLee

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Re: Reducing control forces using spades
« Reply #51 on: November 30, 2021, 07:54:23 AM »
Maybe to get back on track a little .....

Mark, from my perspective, a "spade" as you are describing is an aerodynamic feature intended to balance the torque created by the flaps.

How does this differ from a balanced surface where the balance is achieved by having a portion of the surface in front of the hinge line? Note: not a mass balance, but an aerodynamic balance.

Example in attachment, taken from Wikipedia:
https://en.m.wikipedia.org/wiki/Balanced_rudder
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Offline Mark wood

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Re: Reducing control forces using spades
« Reply #52 on: November 30, 2021, 08:36:25 AM »
Maybe to get back on track a little .....

Mark, from my perspective, a "spade" as you are describing is an aerodynamic feature intended to balance the torque created by the flaps.

How does this differ from a balanced surface where the balance is achieved by having a portion of the surface in front of the hinge line? Note: not a mass balance, but an aerodynamic balance.

Example in attachment, taken from Wikipedia:
https://en.m.wikipedia.org/wiki/Balanced_rudder

Great move Bill, thanks. There is actually little difference in terms of the moment generation between a balance horn and a spade. I think there is a little in this discussion which actually says specifically that, a spade is more or less an aerodynamic balance tab or horn. There are a bunch of NACA technical briefs on the horns and are a lot of the source material we have used in the development of the spades during efforts I have been involved with. Basically a spade is a bit of an afterthought in sorts. The history can be traced to one guy, Leo Laudenslager who was an American legend and world champion the  full size aerobatics arena. He began adding spades in the late 70's or early 80's.

The Stephens Akro he was flying had long ailerons, see my photo earlier here, which have a lot of torque necessary to drive. They are driven by a steel tube which winds up quite a bit and limits the max deflection and therefore the roll rate of the airplane not to mention the stick force which is quite high without the spades. During my testing of the Laser, I dove to 240 MPH and did full deflection tests. On the ground my non "fixed" aileron would deflect 27 degrees and during the high speed test the twisting of the torque tube made it so the aileron was only deflected 12-15 degrees. That and the stick force was very high which I was barely able to physically push hard enough to get to the stick to the stop. The roll rate in this test was actually slower than the roll rate at 150 MPH.

The photo I posted of the Laser aileron is the "fix", actually I did two I inserted a heavy wall tube in the existing tube and I added the spade. The result when I went back to the dive test was an improvement in the deflection where I was getting more like 20 degrees deflection and I was physically able to push the stick to the stop. During normal aerobatics the laser flies between 60 and 180 MPH. It is desirable to have one handed capability of moving the stick which the spades achieved. Without not so much. This knowledge is what I bring in to this discussion and this is a bit of history where it comes from.

So Leo had the idea of mounting a strut and putting a flying surface on in front of the hinge line, in essence a balance horn. Why he did it this way rather than the end of the wing, I don't know. Suffice it to say it worked. The Pitts and other airplanes were doing this a little different at the time and they were using what is know as a Frise aileron which has its hinge offset and the nose is allowed to dip down in to the airstream. This is beneficial for an airplane that always flies upright but not so much for an airplane which is intended to operate inverted. The current methodology is is an offset of the hinge in to the aileron (flap) to about 25% of the chord. Some are more and some are less. This is also common on jets which has high control forces as well.

Honestly, I wasn't planning on messing with spades at all on the models as my direction is towards the more recent approach but I decided it was worth giving it a try. I'm glad I did as it really worked out well. Well enough that I abandoned a wing design I had  that was going to use a 22% flap hinged at the 20% flap chord line. Actually that design was a constant hinge offset with a tapered surface which will also be the case on the new wing design. I was actually preparing the laser templates to cut parts when I stopped. So the new design will use a 30% average chord flap and I haven't yet decided how I am going to do the hinge but no less than 20% flap chord. That decision makes an impact on the airfoil selection so I have spent some time doing some analysis on airfoils specific to this geometry. I'm not sure where that is headed.

Even with the offset hinges many of the full size airplanes use a small spade to adjust the control force feel. The size of the spade in these cases is actually very small and is mostly a trim device and helpful at reducing the weight required to balance the surface. So, the spade is kind of like the application of Gurney flaps. They work really well at making desired changes without having to resort to complete redesign.
 
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Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
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Online Dan McEntee

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Re: Reducing control forces using spades
« Reply #53 on: November 30, 2021, 08:54:15 AM »
Maybe to get back on track a little .....

Mark, from my perspective, a "spade" as you are describing is an aerodynamic feature intended to balance the torque created by the flaps.

How does this differ from a balanced surface where the balance is achieved by having a portion of the surface in front of the hinge line? Note: not a mass balance, but an aerodynamic balance.

Example in attachment, taken from Wikipedia:
https://en.m.wikipedia.org/wiki/Balanced_rudder

   I have been trying to get around to asking this myself. There have been many designs over the years that have these types of "balanced " control surfaces, with the SIG Chipmunk being one of them. Back when I built one during my intermediate class days, it was suggested that they be eliminated and just go with a straight hinge line. I don't know if I have the "feel" that some of you have for subtle control differences. I think I would need several models all lined up in a row with different set ups and fly them ion sequence and hope I could tell the difference!!

   On some models, we experience a difference in how a model turns inside and outside and usually address this with line offset from center on the handle. Could the flaps be contributing to this effect?  I built two Jetco Shark 45 models about 10 years apart. Both were just copies of the Jetco kit. Both were as close to being an exact copy of each other as I could get. The second model was just a bit lighter and both models powered by the same engine, the ST. G-51. But both models, to me, flew almost exactly the same. The second model had it's maiden flights at Tucson at VSC in 2005. Bob McDonald was at the grass circles with me, and eyeballed up right and inverted flight for me for a few laps and gave me a thumbs up, so I went tight into the pattern with complete confidence and had no problems. I think I was even using the same lines and handle as the first model. The main issue I had with both airplanes was that they turned outside much easier than inside. I tried all the usual trimming options on both. Both models use Dave Brown tube type push rods and I have never had any problems with these flexing in use before. The Jetco Shark does seem to have quite a bit of spacing on the thrust line, wing centerline, and elevator centerline. I just eventually thought that this might be the factor on this flying trait, and the turn rate is the nature of the beast. Would a spade located on one side or the other of the flap have any effect on how a model turns? Could the flaps be loaded more or less in direction than the other? I couldn't think of any other way to ask the question.
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Offline Mark wood

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Re: Reducing control forces using spades
« Reply #54 on: November 30, 2021, 09:38:44 AM »
   I have been trying to get around to asking this myself. There have been many designs over the years that have these types of "balanced " control surfaces, with the SIG Chipmunk being one of them. Back when I built one during my intermediate class days, it was suggested that they be eliminated and just go with a straight hinge line. I don't know if I have the "feel" that some of you have for subtle control differences. I think I would need several models all lined up in a row with different set ups and fly them ion sequence and hope I could tell the difference!!

   On some models, we experience a difference in how a model turns inside and outside and usually address this with line offset from center on the handle. Could the flaps be contributing to this effect?  I built two Jetco Shark 45 models about 10 years apart. Both were just copies of the Jetco kit. Both were as close to being an exact copy of each other as I could get. The second model was just a bit lighter and both models powered by the same engine, the ST. G-51. But both models, to me, flew almost exactly the same. The second model had it's maiden flights at Tucson at VSC in 2005. Bob McDonald was at the grass circles with me, and eyeballed up right and inverted flight for me for a few laps and gave me a thumbs up, so I went tight into the pattern with complete confidence and had no problems. I think I was even using the same lines and handle as the first model. The main issue I had with both airplanes was that they turned outside much easier than inside. I tried all the usual trimming options on both. Both models use Dave Brown tube type push rods and I have never had any problems with these flexing in use before. The Jetco Shark does seem to have quite a bit of spacing on the thrust line, wing centerline, and elevator centerline. I just eventually thought that this might be the factor on this flying trait, and the turn rate is the nature of the beast. Would a spade located on one side or the other of the flap have any effect on how a model turns? Could the flaps be loaded more or less in direction than the other? I couldn't think of any other way to ask the question.
  Type at you later,
   Dan McEntee

There's a lot to digest in this question Dan. Here's some of the problem in your experience as I see it. Two different airplanes are exactly that two different airplanes totally independent of the design. When they are of one design they are still different. I built lots of F1C's in a from that made them super consistent but the resulting models did perform better or worse. They all would pretty much fly correctly and only took a few trim flights to make them ready for competition.

Removing the horn from the elevator on the Chipmunk I can't truly comment on specifically as I don't have experience with them. However, we're discussing the total hinge moment when we're talking about the CLPA. The flaps are two, three times the area of the elevator and much larger chord. This means they are the primary contributor to control moments. Removing a couple square inches from the elevator isn't likely going to make much difference in total control moments. This is why I specifically focused on the flaps. The current spades are about 7 square inches each. Not small by any means and definitely make a difference. The only question I have is how much is too much? Stay tuned for that one.

So, in terms of A-B testing removing all of the variables as much as can be done is desirable. This makes this spade approach I have a very good candidate for answering questions like this one. Fly with and without and see what happens. I do allot of that. Way more than I should prolly.

To address your specific question I don't have one. But lets review.

he Jetco Shark does seem to have quite a bit of spacing on the thrust line, wing centerline, and elevator centerline. I just eventually thought that this might be the factor on this flying trait, and the turn rate is the nature of the beast. Would a spade located on one side or the other of the flap have any effect on how a model turns?

Done correctly no, they shouldn't. This is a place to point out that they can negatively impact the trim if not properly aligned. This is why I made my first test flights with 60 second flight times. I was fortunate and got it right first effort. They can be shimmed to get the alignment better. When they don't align properly the result is to cause a static deflection until the forces are in balance. That's to say that if the spade isn't align it will cause the flap to want to fly at some non neutral position. This won't change the ultimate maximum deflection and won't change the turning radius. The only thing they do really do is change how much force it takes to get to max deflection.

Could the flaps be loaded more or less in direction than the other? I couldn't think of any other way to ask the question.

I think the above answer covers this.


As for why the airplane turns better one way than the other I think you already have a handle on this. It's a matter of understanding the moments around the CG. The high thrust line has to be compensated by down ward trim from the elevator. If the elevator has uniform deflection, it will have less power to drive the wing AOA in the positive direction and more power to drive in the negative direction. The vertical displacement of the thrust line is a "desirable" feature to counter the precession of the propeller.  A little is good. A lot, not so much. How much, I don't know, buy some Paul Walker Impact plans and copy what he did.

I don't know what my basic opinion is on this one and my experience is that I lean more on symmetry than on balance by asymmetry. The later is always different the former is always the same. The vertical offset, when done properly removes some of the trim force to fly upright and balances this when inverted. The trouble is that it is power and propeller inertia specific. Change one the balance is no longer balanced For me, I prefer x lbs upright stick force balanced with x lbs inverted. That is generally always the case 0 offset engine wise.

The horizontal offset is another issue and may impact the turning radius as well. This one is a necessary evil. The wing changes the departing airflow, wing wake, which changes the AOA of the tail plane. Without enough separation the wing wake will change the AOA of the tail continuously resulting in hunting. Some can help this. but the trade off is that in some maneuvers the wing impacts is greater than others so insides will suffer while outsides benefit form the wing wake. I think Igor did a lot of analysis on this and a good resource is Frank Zaics book, Circular Airflow. Frank did a good job of discussing the impact of the change in flight path on the surface AOA and included some of the wing wake calculations.

Bottom line is I don't think spades will "fix" these. If'n it were me and I thought it was cool airplane, I'd just fly it and be happy. But then again, I like riding scooters too.




Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman


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