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Leading edge slat/FLAP servo torque question - 2/8/2012 6:17 AM   
FenderBean



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Status: offline 		I have two new projects on coming up and I was just curious, the jet will be around 50lbs im guessing and the slats will be like the design found on F-86 sabers. Probably two servos per side just pushing the slat out and in.
I will most likely have the full out position straight on the servo to help with load like I did with my UP-6 brake valve. Its works perfect and my little HS-81 servo has no problem going full brake, not saying i would use a small servo for this but just giving it as an example.
Something around 200s would be fine? Im going for weight saving like always so if I dont need over kill like I do on elevators . Thanks up front


< Message edited by FenderBean -- 2/10/2012 3:50 PM >


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RE: Leading edge slat/FLAP servo torque question - 2/8/2012 7:28 AM   
Dave Wilshere


 

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Status: offline 		You need to give an idea of slat size to be able to answer. BVM F-100 uses two 65oz servos

Dw

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RE: Leading edge slat/FLAP servo torque question - 2/8/2012 10:05 AM   
Jgwright


 

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Status: online 		I had a Fibre classics F-86 with servo driven slats. They use 2 DS341 or 368 mini servos each side to drive the slats out. As you say they can be arranged to go near straight out so there is little load on them. They work well and had no problem deploying the slats. The torque is 3.2 Kg.Cm. Interestingly the plane uses the same size servos for the ailerons and flaps. The plane weighed 28 lbs dry and flew with a Merlin 90.

John

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RE: Leading edge slat/FLAP servo torque question - 2/8/2012 11:06 AM   
olnico



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Status: offline 		Hi Fender. This is a very interesting question but quite complex to answer.
The loads transmitted to the servos will largely depend on the slat size and extension geometry.

I see mostly two cases:

1. The slat is hinged from the bottom skin and actually acts as a leading edge flap.

In that case you can use the flight controls calc sheet to compute the servo torque requirements.
Refer to this:
http://www.rcuniverse.com/forum/m_9266994/anchors_9266994/mpage_1/key_/anchor/tm.htm#9266994

The spreadsheet is available on my ftp:
http://www.geohei.lu/olin/data/modelisme/mechanical%20advantage/Controls%20calc%20imperial.xls

2. The slat is moving on rails

In that case, the servo will mostly have to fight against the relative airspeed and rail friction.
The rail friction can be roughly measured on the plane itself, using a dynamometer while moving the slat.
The air friction force could be computed from the following formula:
F=1/2 x ro x S x v2 x Cz
Where ro is the air density, S the frontal surface of the slat, v is the plane velocity in m/s and Cz is the coeficint of drag of the slat alone.

As you can see, the big problem is to get the value Cz for a slat. I have no answer about this at the moment but would say that a conservative value of 0,5 could be a good starting base.
Then checking the servo current in flight for a given speed with telemetry systems like Weatronic would confirm the validity of Cz.


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RE: Leading edge slat/FLAP servo torque question - 2/8/2012 12:41 PM   
Jgwright


 

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You raise a very important question, the F86 had guide rails that were curved and made from paxolin. When I acquired the plane they were very stiff and incorrectly installed. I took it apart reglued them and spent a while getting each guide to slide freely in the rails. In the end they moved very easily and had very little friction. If the servo goes so the arm is almost pointing straight towards the leading edge, then it takes little effort to keep it there. On my plane we could have the slats half out without any problem.

John

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RE: Leading edge slat/FLAP servo torque question - 2/8/2012 5:47 PM   
FenderBean



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Status: offline 		hmm lots to think about and until I get the wing I guess I cannot answer anything yet. I have no idea how its done, from the looks of things the speed brake will be on rails and the servo just pushes in and out. If its direct servo I will more than likely add a carbon rod on each side for a guide.


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RE: Leading edge slat/FLAP servo torque question - 2/8/2012 5:56 PM   
grbaker



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Status: offline 		There was a real nice F104 back in the DF days that was lost when one slat retracted and one did not. Of course, we weren't using servos near as powerful as we do now.

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RE: Leading edge slat/FLAP servo torque question - 2/8/2012 6:16 PM   
airraptor


 

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Status: offline 		Why use a servo when you could use a electric retract actuator. I know TBM sells a upgrade kit for thier retracts, Lado has retro kits and other mfg's sell worm gear actuators. This might make it easier for you???

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RE: Leading edge slat/FLAP servo torque question - 2/9/2012 1:07 AM   
rctech2k7



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Status: offline 		This is a good question. I believe the only help I can give is using estimate on this one. First we need to get aerodynamic force on your slat then transfer the torque load to the linkage then to the servo. If your slats are big enough or heavy we need to get static load also. Servo requirement is based on net moment torque (dynamic and static) and product of airspeed factor. To get aerodynamic force we need to get size of your surface control, root and tip chord, span and thickness. Also the angle of max deflection. To get static load we need to get approximation from the weight and dimension of your wings but it's better to get exact weight of your slat. Lastly, to transfer load to the servo, we need to get distance of your control horn from the hinges for moment turn as well as the length of your servo arm.

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RE: Leading edge slat/FLAP servo torque question - 2/9/2012 1:58 AM   
FenderBean



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Status: offline 		I really like that idea since my f16 gear was converted to electric and I am planning to do that with this project. That's actually a super idea and if the are strong enough I could even use them on the spoilers and air brakes. Time to head over to TBM.
quote:

ORIGINAL: airraptor

Why use a servo when you could use a electric retract actuator. I know TBM sells a upgrade kit for thier retracts, Lado has retro kits and other mfg's sell worm gear actuators. This might make it easier for you???



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RE: Leading edge slat/FLAP servo torque question - 2/9/2012 2:42 AM   
invertmast


 

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quote:

ORIGINAL: FenderBean

I really like that idea since my f16 gear was converted to electric and I am planning to do that with this project. That's actually a super idea and if the are strong enough I could even use them on the spoilers and air brakes. Time to head over to TBM.
quote:

ORIGINAL: airraptor

Why use a servo when you could use a electric retract actuator. I know TBM sells a upgrade kit for thier retracts, Lado has retro kits and other mfg's sell worm gear actuators. This might make it easier for you???




Google Firgelli linear actuators...

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RE: Leading edge slat/FLAP servo torque question - 2/9/2012 3:40 AM   
FenderBean



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RE: Leading edge slat/FLAP servo torque question - 2/9/2012 4:46 AM   
RBardin



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Status: offline 		On my 1/5 FEJ Sabre slats, I used 2 Airpower 3/4" stroke air cylinders (Dreamworks) per wing. Mounted them in the wood servo box by adding a few blocks and used nuts/screws that they came with. The pushrods for the slats are connected to the air cylinders with BVM brass gear door connectors, but a wheel collar would do.

The air system is strong, lightweight, reliable and a third of the price of decent servos. I used restrictors to slow the actuation, but you could use wheel collars for that as well.

Since the actuation is linear, there is no binding to worry about as long as you install them straight. It also allowed me to make the factory cutouts for the pushrods a lot smaller - I thought the holes were way too big out of the box.

I paired my slats with the speed brakes using a BVM hi-flow valve I had on hand. Robart makes a red one with needles that you can adjust the speed with easily that might work well for this too.










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RE: Leading edge slat/FLAP servo torque question - 2/9/2012 4:58 AM   
SCALECRAFT



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quote:

ORIGINAL: FenderBean

I really like that idea since my f16 gear was converted to electric and I am planning to do that with this project. That's actually a super idea and if the are strong enough I could even use them on the spoilers and air brakes. Time to head over to TBM.
quote:

ORIGINAL: airraptor

Why use a servo when you could use a electric retract actuator. I know TBM sells a upgrade kit for thier retracts, Lado has retro kits and other mfg's sell worm gear actuators. This might make it easier for you???



On the screw drive system, you may want to look at Nitroplanes or Hobbyking. I figure you can get an entire electric retract and remove the screw drive for way less. Unless TBM has a specialized screw drive.

You have a BVM F-16 with electric retracts. I hate my BVM air system.

Steve

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RE: Leading edge slat/FLAP servo torque question - 2/9/2012 5:03 AM   
SCALECRAFT



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quote:

ORIGINAL: grbaker

There was a real nice F104 back in the DF days that was lost when one slat retracted and one did not. Of course, we weren't using servos near as powerful as we do now.



Just for the sake of info, the F-104 has LE flaps and not slats.

Steve

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RE: Leading edge slat/FLAP servo torque question - 2/9/2012 10:42 AM   
HarryC


 

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Status: offline 		just for background info -

Regarding the F-86, if you were to be perfectly scale, the servo would be holding them in against being sucked out, not pushing them out against the airflow pushing them closed!

The F-86 had automatic, spring loaded slats. There is no control for the pilot to operate and they are not powered. They work purely due to the airflow over them and are either fully in or fully out, there is no in-between. Normal airflow pushes them shut. Leading edge suction tries to open them but the dynamic pressure of the flow and the spring loading to the closed position is enough to overcome it and keep them shut. As the AoA increases, the stagnation point moves further underneath the wing and leading edge suction increases and moves forward until it can pull the slats open. The spring loading and the geometry of the linkages ensures that the slats either pop fully open, or pop fully shut, with some damping to avoid slamming, and suitable hysteresis to avoid rapid cycling open and shut at some critical value.

There is no take-off setting or any sort of intermediate setting. The F-86's automatic slats open or shut by themselves with no control from the pilot. So if you manage to be perfectly scale and only open them at the correct speed/AoA, they would be being sucked open anyway! The need to push them open is due to opening them at too high a speed and too low an AoA, but since we have no ASI/AoA info from the model then it is inevitable this will happen.
H.

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RE: Leading edge slat/FLAP servo torque question - 2/9/2012 6:25 PM   
airraptor


 

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quote:

ORIGINAL: HarryC

just for background info -

Regarding the F-86, if you were to be perfectly scale, the servo would be holding them in against being sucked out, not pushing them out against the airflow pushing them closed!

The F-86 had automatic, spring loaded slats. There is no control for the pilot to operate and they are not powered. They work purely due to the airflow over them and are either fully in or fully out, there is no in-between. Normal airflow pushes them shut. Leading edge suction tries to open them but the dynamic pressure of the flow and the spring loading to the closed position is enough to overcome it and keep them shut. As the AoA increases, the stagnation point moves further underneath the wing and leading edge suction increases and moves forward until it can pull the slats open. The spring loading and the geometry of the linkages ensures that the slats either pop fully open, or pop fully shut, with some damping to avoid slamming, and suitable hysteresis to avoid rapid cycling open and shut at some critical value.

There is no take-off setting or any sort of intermediate setting. The F-86's automatic slats open or shut by themselves with no control from the pilot. So if you manage to be perfectly scale and only open them at the correct speed/AoA, they would be being sucked open anyway! The need to push them open is due to opening them at too high a speed and too low an AoA, but since we have no ASI/AoA info from the model then it is inevitable this will happen.
H.


+1 this would be very scale indeed. Could just use springs for actuators.

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RE: Leading edge slat/FLAP servo torque question - 2/9/2012 10:46 PM   
rbxbear44


 

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Status: offline 		FB,
I just finished the install and set up on the 1/6 FEJ F-18 for a friend. The whole slat design was changed so I don;t know if you got the newer version but there is no longer live hinging. It all works on pivots the full length of the slat and wing internally and a single servo set into the wing fairing to drive it. There is not a whole bunch of movement allowed for the surface but being a leading edge slat, it will be very effective on flight characteristics.

Rex

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RE: Leading edge slat/FLAP servo torque question - 2/10/2012 12:56 AM   
FenderBean



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Status: offline 		Hey Rex long time, I like the design u described. My two projects were just ordered so im guessing it will be the new design. I agree its more for scale. Im guessing it will make an already gentle flyer even nicer on landing. Are you going to Florida jets or Perry?


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RE: Leading edge slat/FLAP servo torque question - 2/10/2012 5:28 AM   
rctech2k7



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Status: offline 		Absolutely, aside from more scale and gentle landing it increase the effectiveness of the wings at higher angle so you would have plenty of added lift on lower speed and be able to land at higher angle of attack. On takeoff it directs airflow producing higher pressure under the wings and more suction on the top. This will also reduce your takeoff speed and shorten your distance for heavy and high wing loading jet.

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RE: Leading edge slat/FLAP servo torque question - 2/10/2012 9:05 AM   
HarryC


 

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quote:

ORIGINAL: rbxbear44

FB,
I just finished the install and set up on the 1/6 FEJ F-18 for a friend. The whole slat design was changed


Rex and Fender, those are not slats, they are leading edge flaps. They work in different ways, do different things and place different loads on the servos. If you are dealing with leading edge flaps you need to start the thread all over again and name them properly!
H.


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RE: Leading edge slat/FLAP servo torque question - 2/10/2012 12:09 PM   
rbxbear44


 

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quote:

ORIGINAL: HarryC


quote:

ORIGINAL: rbxbear44

FB,
I just finished the install and set up on the 1/6 FEJ F-18 for a friend. The whole slat design was changed


Rex and Fender, those are not slats, they are leading edge flaps. They work in different ways, do different things and place different loads on the servos. If you are dealing with leading edge flaps you need to start the thread all over again and name them properly!
H.



LOL...I love these types of posts....



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RE: Leading edge slat/FLAP servo torque question - 2/10/2012 12:48 PM   
HarryC


 

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quote:

ORIGINAL: rbxbear44
LOL...I love these types of posts....

Well, what you said was the equivalent of saying the F-18's tailwheel design had changed! It doesn't have a tailwheel and it doesn't have slats. It's important to name things correctly otherwise you get the answer for something different.

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RE: Leading edge slat/FLAP servo torque question - 2/10/2012 3:48 PM   
FenderBean



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Status: offline 		Here we go again splitting hairs and killing a good thread no wonder people stop coming to this forum.


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RE: Leading edge slat servo torque question - 2/10/2012 3:57 PM   
HarryC


 

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Status: offline 		That's right, expecting people to say slat not flap, say aileron not rudder, gear not door, its just splitting hairs!
If you use a word that means something completely different, readers take a totally different meaning to what you think you told them.

It is not splitting hairs to expect people to use the correct word. If you ask what power of servo you need to move the rudder, when actually you are looking at the tailplanes, don't get arsey when you are told about it and learn something new.
H

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