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Servo Stall Current?

Old 10-15-2020, 04:30 PM
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GorillaToast
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Default Servo Stall Current?

What (if any) is the importance of knowing the "stall current" of a servo in a 1/4 scale airplane?
Old 10-16-2020, 05:48 AM
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ahicks
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It's about preventing receiver "brown out" where the receiver shuts down due to low available voltage from the battery. Everything may look just dandy on the bench, but in use, after a flight or 2, the battery voltage may sag under heavy use during a maneuver- a snap or extended knife edge for instance.

It's not just about the servo draw, it's also about the battery's ability to feed the system adequate power, as well as the wiring's ability to carry enough. Not as important when just flying a lap or 2, or maybe doing bounce and go's with a Cub, but can bring a plane flying hard acrobatics or 3D down pretty easily if power delivery is not done carefully. -Al

Last edited by ahicks; 10-16-2020 at 05:50 AM.
Old 10-16-2020, 02:42 PM
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speedracerntrixie
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All very good advise above. I would like to add that you should never be in a situation where a servo gets completely stalled. If that happens the servo is way too weak for the application or you have a linkage bind. Servos tend to not live very long under those circumstances.
Old 10-16-2020, 05:39 PM
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GorillaToast
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Thanks guys for your replies. do either of you consider stall current ratings when choosing servos for non- aerobatic planes? I'm scratchbuilding a 1/4 scale Rearwin Speedster and don't plan on even doing stall turns or loops with it.
Old 10-16-2020, 06:35 PM
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Sounds like you are building a Bridi Rearwin speedster. That airplane although pretty large was designed when servos were around 50oz. By today's standards that airplane has a boatload of potential. Some things to keep in mind is that it was designed around a Quadra 35. If using a modern engine like a DLE 35 it will be lighter then the Quadra. Now if I were building it ( and I may one of these days ) I would also want to mount the elevator and rudder servos in the tail. All that being said, I would be looking at servos in the 75oz to 100oz range. In this case I would be more concerned with centering then overall power. I'm not a fan of Futaba. IMO they are overpriced and many of them are limited to 6V. Current battery tech is either 6.6V LiFe or 7.4V lipo. I think the best performance for the dollar is KST and Savox in standard size servos. Lately I have been using MKS servos and honestly they are the best performing servos I have used. They do come with the premium price tag as well.
Old 10-17-2020, 02:51 AM
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Thanks for your insights, speedracertrixie.

Yes, it is loosely based on the Bridi design. I was planning on using HiTech digital servos (D645MW) @ 6.6v using LiFe batteries. I'm being shipped a DLE 35RA. Not sure about rear-mounted servos as I'm trying to make this plane as scale-like as I can.

As an aside... have you ever heard of anyone making the horizontal stab adjustable like on the real one?
Old 10-17-2020, 04:38 AM
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ahicks
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Just as a counterpoint, if building a plane where I'm likely going to have to add nose ballast (I HATE ballast!), I would AVOID rear mounted servos, especially larger ones, as that's going to require a BUNCH more counterweight to offset! Pull-Pull works awesome on the rudder, and if the plane isn't going to be flown hard, a sturdy push rod operated elevator would help keep the tail weight down as well. My thoughts anyway, FWIW. -Al

Old 10-17-2020, 05:10 AM
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My preference for tail mounted servos is simply a solid control feel. As Al has suggested there is nothing wrong with pull pull. I have had a couple airplanes that had pull pull on both elevator and rudder. I just will not use a pushrod system on anything that large. Figuring this stuff out prior to construction really helps keep the CG where it belongs. At this point you can select appropriate wood, tailwheel assembly, hinges and control horns. You would be amazed how much weight these can add up to if you're not paying attention.

Adjustable stab, I don't see the need on this type of airplane. It is used on full scale to cope with a shifting CG due to cargo and passengers. Once we dial in the CG on our models it pretty much stays there. I have adjustable stabs on my pattern airplanes but again they get adjusted once and forgotten. It's more about having removable stabs at that point. I would mount the stab at zero or one degree positive depending on your flying style. Setting to one degree positive would help reduce climbing under power and allow a CG a bit closer to nuetral.
Old 10-17-2020, 01:31 PM
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Originally Posted by speedracerntrixie View Post
My preference for tail mounted servos is simply a solid control feel. As Al has suggested there is nothing wrong with pull pull. I have had a couple airplanes that had pull pull on both elevator and rudder. I just will not use a pushrod system on anything that large. Figuring this stuff out prior to construction really helps keep the CG where it belongs. At this point you can select appropriate wood, tailwheel assembly, hinges and control horns. You would be amazed how much weight these can add up to if you're not paying attention.

Adjustable stab, I don't see the need on this type of airplane. It is used on full scale to cope with a shifting CG due to cargo and passengers. Once we dial in the CG on our models it pretty much stays there. I have adjustable stabs on my pattern airplanes but again they get adjusted once and forgotten. It's more about having removable stabs at that point. I would mount the stab at zero or one degree positive depending on your flying style. Setting to one degree positive would help reduce climbing under power and allow a CG a bit closer to nuetral.
In trying to get as close to the full-scale as possible, I saw that there is a very noticeable opening in the fuse sides near the front of the h-stabs. Upon examination I learned that the stab is adjustable via a crank in the cabin. I understand that adjustability would not be necessary from day -to-day on an r/c plane. I do appreciate the insight about 1 deg. up, because I do not like a plane that climbs significantly under power. the wing incidence will be adjustable like on pattern planes, set to 0 to start and trimmed as necessary after initial flights. If i cut the openings in the fuse sides, do you think it would cause any problems?
Old 10-17-2020, 02:11 PM
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The opening shouldn't be an issue at all. We actually channel air through the fuselage of electric powered models to cool components. Just to clarify, I suggested positive stab incidence of 1 degree, that would be the same as down elevator not up.
Old 10-17-2020, 04:07 PM
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Originally Posted by GorillaToast View Post
In trying to get as close to the full-scale as possible, I saw that there is a very noticeable opening in the fuse sides near the front of the h-stabs. Upon examination I learned that the stab is adjustable via a crank in the cabin. I understand that adjustability would not be necessary from day -to-day on an r/c plane. I do appreciate the insight about 1 deg. up, because I do not like a plane that climbs significantly under power. the wing incidence will be adjustable like on pattern planes, set to 0 to start and trimmed as necessary after initial flights. If i cut the openings in the fuse sides, do you think it would cause any problems?
As a pilot, I can provide some insight as to how/when that crank is used. It's about trimming the aircraft for hands off flying - to take the for/aft load off of the yoke or stick - to make for more comfortable flying. Not unique to this plane at all. Rag wing Pipers use a similar system. Every plane I've flown has some sort of trim. It's something you are constantly messing with (to the point you don't even think about reaching for it to make an adjustment) as your speed changes during take off and landing sequences. As you mention, the airfoil provides different amount of lift with speed changes. To complicate that, there's a range of CG that the plane is flown in, which also changes trim requirements, so it's not possible to associate a single speed with a desired amount of trim. Without the ability to trim the angle of the horizontal stab, you'd have some pretty substantial flight loads on that stick/yoke at opposite ends of the plane's flight envelope (and CG range). Trying to keep the plane from dropping like a rock on final approach, and climbing like a homesick angel when under power means trim frequent trim changes. Sounds like a lot to do, but in real life, it's just another small piece of everything else going on. You make constant corrections seeking a "neutral" stick/yoke - that's not pulling or pushing in flight.

Trim system is something that requires some attention during your annual inspection, where that whole system is gone over pretty thoroughly. Frayed cables, frozen pulleys that guide those cables from front to rear, and slop in the horizontal stab due to worn linkage are all issues that will ground a plane until repaired. -Al

Last edited by ahicks; 10-17-2020 at 04:10 PM.
Old 10-17-2020, 06:42 PM
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GorillaToast
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ahicks,

Not that I'm going to try to make a working trim for the horizontal stab, but I'm still curious about the mechanics of the trim mechanism. Are you able to describe how it works?
Old 10-17-2020, 06:46 PM
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Originally Posted by speedracerntrixie View Post
The opening shouldn't be an issue at all. We actually channel air through the fuselage of electric powered models to cool components. Just to clarify, I suggested positive stab incidence of 1 degree, that would be the same as down elevator not up.
Came across this...
Incidence.pdf

it seems to suggest that the stab be set at 0deg and the wing set to +.5deg relative to the stab. Am I understanding that correctly?
Old 10-18-2020, 05:08 AM
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Originally Posted by GorillaToast View Post
ahicks,

Not that I'm going to try to make a working trim for the horizontal stab, but I'm still curious about the mechanics of the trim mechanism. Are you able to describe how it works?
Sure! There's a pulley, maybe 3"- 4" in diameter front and rear, and small cable that runs around them, with small pulleys from front to rear to guide the cables. Front pulley has a small hand crank mounted to it. Cables are wound around both pulleys several turns so it doesn't slip. In back, there's a jack screw arrangement. It's a vertically mounted threaded rod with the rear pulley attached to the top, that turns the threaded rod. The front of the horizontal stab is held by a fancy casting (shaped like a T) with internal threads that rides up and down on that threaded rod as it's turned. It's kinda like a trailer tongue jack where the crank handle is replaced by a pulley, and instead of the trailer frame moving up or down when the crank is turned, the front of the stab. is moved up and down - within that slot you're seeing in the sides of the fuse.

The -0- incidence thing is something practiced only in RC. Ability to sense the load on the elevator servo, and changing the elevator trim to cancel that out as required would REALLY complicate things in RC. The need for the trim system in full scale DOES give you an idea of the flight loads imposed on the RC elevator servo though.... -Al

Last edited by ahicks; 10-18-2020 at 05:19 AM.
Old 10-18-2020, 06:18 AM
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Originally Posted by GorillaToast View Post
Came across this...
Attachment 2268415

it seems to suggest that the stab be set at 0deg and the wing set to +.5deg relative to the stab. Am I understanding that correctly?


Yes, that would be the correct setup for an IMAC or pattern aerobatic model and to a certain degree of what I use on my pattern models. The difference is that the zero stab works well with a symmetrical airfoil. Your Rearwin Speedster has a semi symmetrical airfoil that will produce more lift at the same given AOA.
Old 10-18-2020, 07:45 AM
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Originally Posted by speedracerntrixie View Post
Yes, that would be the correct setup for an IMAC or pattern aerobatic model and to a certain degree of what I use on my pattern models. The difference is that the zero stab works well with a symmetrical airfoil. Your Rearwin Speedster has a semi symmetrical airfoil that will produce more lift at the same given AOA.
Ok, thanks for the info. I plan to use Gator incidence adjusters on the plug-in wings so, I'll start with .5 deg positive and adjust back to 0 deg if necessary.
Old 10-18-2020, 02:01 PM
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Originally Posted by ahicks View Post
Sure! There's a pulley, maybe 3"- 4" in diameter front and rear, and small cable that runs around them, with small pulleys from front to rear to guide the cables. Front pulley has a small hand crank mounted to it. Cables are wound around both pulleys several turns so it doesn't slip. In back, there's a jack screw arrangement. It's a vertically mounted threaded rod with the rear pulley attached to the top, that turns the threaded rod. The front of the horizontal stab is held by a fancy casting (shaped like a T) with internal threads that rides up and down on that threaded rod as it's turned. It's kinda like a trailer tongue jack where the crank handle is replaced by a pulley, and instead of the trailer frame moving up or down when the crank is turned, the front of the stab. is moved up and down - within that slot you're seeing in the sides of the fuse.

The -0- incidence thing is something practiced only in RC. Ability to sense the load on the elevator servo, and changing the elevator trim to cancel that out as required would REALLY complicate things in RC. The need for the trim system in full scale DOES give you an idea of the flight loads imposed on the RC elevator servo though.... -Al
Wow! yeah, I don't think I'd like to attempt that in a 1/4 scale model. Thanks for the description.

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