junkjet

My Feedback: (55)

I need to know if I can mix analog servos with digital servos as in analog flaps and digital everything else.

Desertlakesflying

My Feedback: (28)

I've done it on Spektrum

Len Todd

I have done that also. Just can't gang analog and digitals on the same channel output.

If you are using HV Digitals on a > 6 volt supply, watch out for analog servos that are low (<6 Volt) power. LV servos don't last to long on HV when you really load them like we do. You MIGHT get away with one on the nose gear or a gear door and that is about it.

wfield0455

My Feedback: (7)

The real answer is it depends on the frame rate your receiver is sending. If it ~20ms / 50Hz then you can use analog servos. If it's faster than that in general digital servos are REQUIRED. It has nothing to do with mixing analog and digital, analog servos generally can't operate at as high a frame rate.

If you're using Spektrum, you need to make sure it binds using a 22ms frame rate, not 11ms. Some have been complaining that some of the new Spektrum receivers don't seem to be able to be foreced to operate using 22ms frame rate and simply don't operate properly with analog servos.

The bottom line is if analog servos work properly with your receiver than digital should too and mixing the 2 makes no difference as the receiver has no idea what type of servo is connected.

junkjet

My Feedback: (55)

Just a shockjet on 6.6v with 6v Hitec servos. Wanting to put a set of Hitec 646 hv capable analogs on the flaps. Using jr dmss.

Dave Wilshere

Why on earth would you want to put servos with the least holding power on Flaps...its the one place they are 100% needed!

c/f

My Feedback: (1)

Don't flaps lock in place with air pressure? For this reason I prefer analogs on flaps cause air pressure and full holding torque at any position specifically against a hard stop such as flaps uses up some MAH.

Perhaps blow back on flaps in deployed position may be a good thing as it would be proportional to speed, just sayin....

The real point to consider is the signal speed as mentioned earlier. This subject will become more prevalent with the use of smart flight controllers and performance with the faster frame rates. With my Graupner receivers and using the built in heading hold 3Xg axis I can "FEEL" the difference between a 20ms frame rate versus 10ms frame rate in a 3D model, not so much in a jet application. That's why slow precision JR servos are the bomb............

alasdair

Like Dave Wilshere says, flaps need held down using powerful servos.
I recently calculated (again) the servo torque needed for my JayTee sport jet/trainer.
I used Craig Tenney's Excel spreadsheet and also Carl Risteen's published formulae.

I calculate the servo torque required for 15 degrees aileron, 20 degrees elevator, 30 degrees rudder and 60 degrees (plain) flap - for the drag. Craig's sheet does not cover flaps, but calculates 20, 18 and 60 oz.in on ail, elev & rudder respectively at 130 mph
Carl's calcs give answers 50% higher. I generally use Carls, and double the answer when choosing servos.
For flaps Carl's calc gives 42 oz.in at 50 mph, for full 60 flap.

So flaps (and rudder) are the torque hungry controls, but I am quite happy using analogue servos, whereas for aileron and elevator I use digitals.
If there is a little bit of blowback, due to inadvertant high speed, they are less likely to pull lots of amps. Unlike digitals, or so I am led to believe.

rhklenke

My Feedback: (24)

Now Dave, don't lambast the guy for asking the question!

The last time I build a BVM Kingcat and a BVM F-100F, the instructions called for analog servos (yes, but high torque ones) on the flaps and digital everywhere else. The thought is that digital servos reach their maximum stall torque very quickly when off-position and thus, if the "flaps up" position is not adjusted properly and the flaps hit their mechanical stops before the servo reaches its commanded position, a digital servo is going to stall and draw a high current. An analog servo is a bit more forgiving in that sense and generally won't ramp up to its maximum stall current in the same situation...

I know of two instances where guys burned out 8611's on Kingcat flaps because of that very reason...

Bob

luge_racer

I always thought a linear actuator would be a better option for flaps, similar to what we use for electric retracts now. This way they could mechanically lock themselves into position reducing the strain on the electronic system. They can provide a lot of force with relatively little power. They don't need to be fast either (I always slow down my flap servos a lot).

junkjet

My Feedback: (55)

This is the reason for the switch. These new servos have as much power as my digital on the other functions. Had a servo on the flaps burn out one time when I turned on the radio before they went back to center. Thanks for all the replies all are educational.

Dave Wilshere

I always thought BV's request for non digitals was odd. I know his reason was people not being able to set travel and not burn out, particularly on his stuff where the flap comes up against a hard edge...but you can still fry non digitals...
Its holding power, not torque we need. Yes, geometry has a big influence, I have seen some terrible mechanical set ups, having the servo go over center is all well and good if the rod passes exactly through the servo center...I see so many models where the atv (travel) and sub trim is wildly different, that's why flaps go up and down at a different speed. Rotary into linear movement will always rely on matched set ups...when many wings don't match in distances!!

Digital servos are the way to go, they have great accuracy and holding power...everything a flap needs

junkjet

My Feedback: (55)

I currently have digital servos in it now but the problem I had only happened once when I turned on the radio the flaps went to full deflect for a couple seconds and one burned one time. I haven't got my new jr system in yet. Have a XG14 and 1131 receiver.

Len Todd

When setting up a digital servo on flaps, I always put a current meter in line and then set the travel adjust so that the flap is full up with minimal current applied. It usually winds up being only a couple milliamps to hold a flap up firm against the stop. When starting and/or just rolling around, I set the flaps at half flap where almost zero milliamps are used.

Also, high quality programmable servos typically have travel limits and stall failsafes that you can set in the servo. Lately I haven't really needed that though because I pay a lot of attention to the mechanical setups and use the current meter to find the hard endpoints and then set the transmitter's servo Travel Adjust back a bit. Usually these hard stops are way past the recommended throws anyway. Then, I use Dual Rates set on the Flight Mode switch to set the recommended throws for triple rates. KISS

On a couple planes with very long servo arms I reprogrammed the servos' center and endpoints to avoid using offset. But lately, I just use the transmitter's offset. It appears to me that using the transmitter's offset does not significantly affect the linearity of travel, unless you are using some very long servo arms, which on jets we do not normally do (e.g. covers get in the way, etc.)

One other point: Spektrum has a menu option to manually set Frame Rates. I have several dozen Spektrum Rxers, including several currently offered ones, and I have never had a problem setting back the frame rates. Guess I would like to hear some specific examples about this inability to set Frame Rates on currently offered Rxers.

David Gladwin

Digital servos on flaps, don't leave the ground without them!

BVs advice to use analogue servos on flaps is something I totally disagree with.


Use their precision and holding torque to accurately position the flap,EXACTLY where you want them and that means NOT hard up against a mechanical restriction!

Len Toods advice on the use of an ammeter to check current draws is spot on!

The new JR NX 8931 servo is superb for this application, dead accurate , powerful , programmable and NO buzzing !

junkjet

My Feedback: (55)

I am going to leave the digital servos in and finish the setup. Maybe the jr XG14 won't deflect to full down flap like the old 9103 with a Spektrum module in it.

David Gladwin

Interesting stuff, Alasdair.

I always use use digital servos on every flight control inc flaps but using Weatronics radio, with its extensive data recording, I can see exactly the current draw every every second of the flight thanks to the recording of Giga Control.

The highest draw is when flaps are travelling to full down, usually about 40 degrees on most of my jets. As the servo arm at full flap is aligned , or nearly aligned giving the servo a good mechanical advantage, with the output shaft the current, an almost direct function of torque, falls significantly and is really quiite small, certainly nothing which is going to bother a strong digital servo, hence my flap servo failure rate, zero !

i3dm

My Feedback: (51)

Short answer: YES.

alasdair

David, great idea.
Does that mean you could calibrate the power consumed to the torque produced? You could thus see what torque is needed for any/every manoeuvre?
If you also know the airspeed you can relate torque and airspeed.
At the least, you can estimate what servo torque we REALLY need on our models instead of guessing as we now do.
I use the Tenney and Risteen formulae, but have no idea how accurate they may be. I believe they were done from a purely theoretical basis with no practical backup tests. If you could produce accurate answers that would be a great step forward.

The best I have done is put an ammeter on the Rx input, and it retains in memory the highest value. So I discovered my maximum current draw to be 5.2A.

Dave Wilshere

This would have been because you did not set the radio before binding-or re bind after setting the flaps up.

Dave

karolh

Very interesting topic.

wfield0455

My Feedback: (7)

I've never had a digital servo jump wildly when powering up. I've only ever had analog servos do that. The problem was because some receivers apply power to the servo ports immediately but there isn't a valid signal because the receiver hasn't connected to the transmitter yet. Some analog servos when they have power but no signal would jump to their extreme travels. Every digital servo I've ever used does nothing until it has a valid signal. If your travels are set properly you should never have a problem with a digital servo on any control surface.

With Spektrum, if you do a normal bind then all controls but Throttle simply hold but if you do a Smart Bind to set failsafe positions on all controls, make sure you have all your reversing and travels set correctly and all controls set to the desired failsafe positions and THEN do the Smart Bind ( or what ever the proper term is)..

junkjet

My Feedback: (55)

Dave I believe you just nailed it.

David Gladwin

Alisdair,
I may be able to give you some specific data when I get home ( I am in the middle of the Atlantic on a ship right now) so no access to my data which is all recorded by the Weatronics system on sd cards. After using Weatronics since its inception I cannot speak too highly of this system, it's extensive data recording facility gives REAL date to work with. When Powerbox reintroduces the new transmitter it wil be awesome.

However a a few points:

some years ago I made some simple tests on servos, applying varying loads, basically different weights, and I could see in broad terms that current drain was directly related to torque generated in an almost linear progression. No real surprise there.

I found that that modern digital servos use less current do for a specific load than analogue ( more efficient amplifiers and motors? )


I have a mix of analogue servos and digital servos on most of my jets, some old but reliable analogues being used on non critical controls such as wheel brake valves. I use Weatronics and JR. DMSS, 28x

The only problem I have ecountered was with the electronic Horizon air brakes on my Blanik, they just jittered. This was fixed by feeding the air brake signal through a Match box, after which they worked perfectly.

As you will understand, I extend flaps on my jets when airspeed is appropriate to prevent over stress so speeds are relatively low so that keeps currents down, the largest value is just as the flap servos start to move reducing as they reach a stable position. I am sure that with increased ias current would rise exponentially, v squared etc.,

I will try and give you some numbers later.

Finally, I probably use much stronger servos than needed to meet flight loads alone. I do do this to give servos an easy life and so extend longevity, so I have never had a servo fail in normal operation. I also believe that the strongest digital servos are the best defence against flutter !

David.