Powering the Rx

[Martin Harris - Moderator]

This subject is why I was an early and enthusiastic adopter of telemetry from when it first became available commercially in 2009. It probably saved my Airsail Chipmunk when a cell in its 4.8V pack failed in flight and gives early warning of any unexpected voltage drops through battery or servo issues. 

[GaryW]

39 minutes ago, Jon - Laser Engines said:

 

In fairness, spektrum rx's are (or at least were) vulnerable on 4.8v packs even using standard servos. I worked at a model shop and the advice we gave about using 6v packs came from Horizon themselves. While it is not unheard of for model shops to flog you something more expensive than you need we were under instruction to do it in this case. There clearly was an issue as they sold that plug in capacitor thing to help guard against voltage drops. 

 

Admittedly this was all 10-15 years ago and a great many things have changed since then so it may not be true today. My only spektrum rx's have operated off of bec's in electric models so i have no first hand experience with them on a battery. All of my futaba 2.4 rx's have been fine on 4.8v so i have no worry there. A club mate had some hitec 2.4 rx's that flat out refused to work on 4.8v for some reason, but i have a sample size of only 3 so that is hardly conclusive. 

 

Ahhhh a fellow Model Shop Worker....... I worked at what was Biggin Hill Models for 3 years from 2007 - 2010 be fore medically retired... Me & my better half ( Caz ) also worked the Bring & Buy at the Hop Farm Show ( Southern Model Airshow 2004 - 2013... One trade i would happily go back into if I could ,,,, in that time I don't recall ever trying to sell anything a customer didn't need or ask for or a more expensive option .. I always worked within their intended budget........  I've been flying on Spektrum since the Original DX7 came out around , since then I've had ... DX7, DX8, DX9 2x DX7S and now the NX8

Edited February 17 by GaryW

[Jon H]

2008-2011 for my model shop service.

 

I always tried to give good advice, much to the annoyance of my boss on occasion. A customer wanted to buy an ASM Hercules (100 inch, 4x 30 4 strokes) and i got in trouble for recommending standard 3kg servos for it as it 'needed' much more powerful digital servos as its a big model. My argument was the size of the model is irrelevant. The model might be big but the control surfaces are small, its pretty slow and generally needs to just wander round the sky looking fat. As time went on i ended up with one myself and it flew fine with standard gear in it. I would also get in trouble for spending time with customers buying nitro cars. If i sold one i would tell them to charge up all the gear and come back the next day. I would show them how to start it and not break the pullstart. I was told this was a waste of time (costing money) and we would make money on selling pullstarts when they broke them. I considered this short sighted as it wont take long to turn a customer off a product if they keep breaking a part and dont know why. Especially when the repair cost was nearly 20 quid a go and an hours work. I worked on the basis that the time spent showing the kid (usually father and son bought the cars) how to do it was not only the right thing to do, but also good for the business as said kid would have friends, and those friends would want a new toy as well. Sure enough, many times friends/brothers/cousins also wanted a car so they could race and it turned out my 'waste of time' was actually time well spent. Didnt stop me getting in trouble though. 

 

At laser i have turned many a customer away or onto a cheaper engine as they one they selected was unsuitable. One in particular i recall was a chap wanting to fit our 70 into a flair se5a as his club mates told him the 40 4 stroke on the plan would never be enough. They were wrong, as i have flown the same model using a 52 and it had massive amounts of reserve power. I recommended a saito 45, but they were discontinued as were the 50's and the OS56 was crazy money. He bought a saito 56 in the end and called me back after the models first flight to report that it flew like a pitts special at full power and he was extremely glad he never bought the 70. He then told me about his next project and asked which engine i would recommend. I cant recall the details of that project, but he bought an engine and keeps coming back as, in his own words, he 'values honest advice'. 

 

Again though, to give shops some leeway margins on big ticket items like engines, radio and models are pants and that sort of forces them to flog other stuff just to break even. I know shops would be much happier if we all went back to building kits as a tube of glue and 3 sheets of balsa probably has more margin on it than a mid range radio. 

 

Anyway, we digress. 

[steve too]

On 15/02/2024 at 11:53, Erfolg said:

It is an old DB Mascot, a trainer type model, about 56" span.

 

4 cell Eneloop AA or 2 cell LiFe or 2 cell LiPo and UBEC.

 

In all cases I would advise having a cheap voltage indicator on the battery.

[Cuban8]

Modellers should go with what they feel is is best for them - I'm just pointing out an issue that does exist with nimh packs - I've seen it with my own batteries and confirmed with test equipment after investigation........... albeit found rarely. Anyway, if a little extra care saves just one model from being wrecked, it's worthwhile.

[GaryW]

3 hours ago, Cuban8 said:

Modellers should go with what they feel is is best for them - I'm just pointing out an issue that does exist with nimh packs - I've seen it with my own batteries and confirmed with test equipment after investigation........... albeit found rarely. Anyway, if a little extra care saves just one model from being wrecked, it's worthwhile.

 

If I understand " Brownout " correctly it can happen with any battery type not just Nimh regardless whether it be 4.8 or 6v or the condition of the battery pack is in.. It can even happen with Lipos if the current f=draw is that high that the battery can't cope with an excess load draw on the battery pack being used

Edited February 17 by GaryW

[steve too]

1 hour ago, GaryW said:

IfI understand " Brownout " correctly it can happen with any battery type not just Nimh regardless whether it be 4.8 or 6v ...

 

Indeed. Brownouts happen when the supply voltage drops below the reset voltage of the receiver's processor. This was a problem with the first Spektrum receivers (e.g. the AR7000) which would do a full signal acquisition scan on power up. It was fixed back in 2008 with Spektrum's Quick Connect firmware but the story will not die.

 

The only time that I have had a cell fail in a receiver pack, it was a LiFe... 

[Frank Skilbeck]

16 hours ago, GaryW said:

 

If I understand " Brownout " correctly it can happen with any battery type not just Nimh regardless whether it be 4.8 or 6v or the condition of the battery pack is in.. It can even happen with Lipos if the current f=draw is that high that the battery can't cope with an excess load draw on the battery pack being used

Or if the current exceeds the BEC rating, I have seen this on models with electric retracts and also on one that had a faulty servo, which worked fine when the model was being set up with a 4 cell nimh but overloaded the ESC UBEC, it was only after we'd tried 3 ESCs that we tracked down the problem.

[Jon H]

15 minutes ago, Frank Skilbeck said:

electric retracts

 

all servo/electric retracts need an isolated power supply. A dual battery setup using a modified y lead is simple and effective.  

[Martin Harris - Moderator]

That might need qualifying a little. It’s a good principle to adopt but not always practical, especially in smaller models. In the days of mechanical servo operated retracts I’d agree 100% but most electric units have quite fast acting over-current protection and as long as the power source is adequate this precaution should be unnecessary.  Most EP models with retracts are powered directly from their BECs and I don’t recall ever having seen or heard of a suspected retract induced brown out. 

[Jon H]

I wouldnt take the chance personally. While they are admittedly quite big the electric retracts in my sea fury draw masses of current and when testing them they would knock my futaba rx into failsafe about 50% of the time due to current spikes. On test i saw spikes over 6amps and constant draw around 1.5. I also tested their protection features and found they did not react very quickly to being jammed. In fact, they managed to crush my hand through one of the wing skins when i was a bit slow getting it out of the way, and this did not trip their protection features. 

 

In any event i would find space even in a small model to wedge a battery in there as its not worth the risk. That or i would simply use air. 

[steve too]

3 hours ago, Jon - Laser Engines said:

all servo/electric retracts need an isolated power supply. 

 

No they don't. I did the separate battery for retracts a couple of times and then decided that I was better off fitting one big battery. No issues. If I look at receiver voltage telemetry, I can see when the retracts are operating, but the drop is not significant.

 

PS On my P47, landing flaps hit the battery harder than the retracts.

 

Edited February 18 by steve too

[Martin Harris - Moderator]

There’s a good argument for routing the current draw away from the receiver bus in cases like this, especially given the small conductor size supported by standard servo plugs but we’re straying into larger model territory.  I was really thinking of models like the Durafly Vampire where the retracts’ protection parameters are well within the capacity of the ESC’s BEC.  

[PatMc]

Somehow, I don't think Erfolg will be fitting retracts to his DB Mascot. 😉

Edited February 18 by PatMc

[Stuart Z]

Somehow it didn’t seem so complex in times gone by.  I had a Joker and a Calypso both on mechanical retracts (single bang/bang retract servo in each) running on a 500mah Nicad and got around 3 flights each.  Never thought I might be close to disaster.  All seemed fine. 

[Martin Harris - Moderator]

8 minutes ago, PatMc said:

Somehow, I don't think Erfolg will be fitting retracts to his DB Mascot. 😉

I don’t know about that.  He has an enquiring mind and isn’t afraid to experiment! 😉

[PatMc]

But he's sensible enough to always asks for advice on the choice of any gizmo he hasn't used before or for a long time. 😉

[Jon H]

2 hours ago, Stuart Z said:

Somehow it didn’t seem so complex in times gone by.  I had a Joker and a Calypso both on mechanical retracts (single bang/bang retract servo in each) running on a 500mah Nicad and got around 3 flights each.  Never thought I might be close to disaster.  All seemed fine. 

 

The issue comes if they bind and continue to draw current. Then you can drain your flight battery before its time to land. 

 

As for the park fly micro retracts i guess you can run them without a separate power supply, but i wouldnt be doing it on anything big. 

[Paul De Tourtoulon]

6 hours ago, steve too said:

No they don't. I did the separate battery for retracts a couple of times and then decided that I was better off fitting one big battery. No issues.

No Issues "yet", I also ( sadly) use a few planes with big batteries, but some retracts are delivered with a timer that one day will activate

the 'magic smoke', that will change your way of setting up electric retracts.

[Stuart Z]

1 hour ago, Jon - Laser Engines said:

 

The issue comes if they bind and continue to draw current. Then you can drain your flight battery before its time to land. 

 

As for the park fly micro retracts i guess you can run them without a separate power supply, but i wouldnt be doing it on anything big. 

Thankfully- never had a problem but always checked pre-flight.  Fortunately they were both easy landing aircraft and the strip was maintained well so no “hard” landings.  Still have one set of retracts and still have the servos, who knows, one day!

 

PS Still getting the pop up and equally annoying I’m getting logged out almost every time I try to read:respond to a page.

[leccyflyer]

8 hours ago, Martin Harris - Moderator said:

That might need qualifying a little. It’s a good principle to adopt but not always practical, especially in smaller models. In the days of mechanical servo operated retracts I’d agree 100% but most electric units have quite fast acting over-current protection and as long as the power source is adequate this precaution should be unnecessary.  Most EP models with retracts are powered directly from their BECs and I don’t recall ever having seen or heard of a suspected retract induced brown out. 

That is consistent with my experience of multiple electric powered models with the electric retracts supplied via the receiver and using ESCs with good quality switch mode BECs onboard. Some of the minority of models with mechanical retracts do have provision for a separate battery for the retracts.

[Nigel R]

On 17/02/2024 at 17:37, steve too said:

 

Indeed. Brownouts happen when the supply voltage drops below the reset voltage of the receiver's processor. This was a problem with the first Spektrum receivers (e.g. the AR7000) which would do a full signal acquisition scan on power up. It was fixed back in 2008 with Spektrum's Quick Connect firmware but the story will not die.

 

The only time that I have had a cell fail in a receiver pack, it was a LiFe... 

 

It was also fixed by improving the tolerance of supply voltage in the next generation of receivers, if I remember rightly what I have read about the subject.

 

If I'm honest, there was no way a healthy 4 cell nimh pack (and a decent switch with good wiring) would have dropped below the original operating voltage range - providing the pack was not overstressed.

 

Of course, one could take a knackered old 4 cell pack, a dodgy old switch with dirty plugs or an icky solder joint, and combine them with some fancy new high power digital servos - trouble is being requested and would no doubt be delivered.

[Dickw]

Many modern servos will operate at quite low voltages with torque data now being quoted at 3.8v (even for some HV servos), and in the world of Discus launch glider (DLG) flying it is quite common to use only a single Lipo cell to power the receiver and servos. The receivers don't usually have a problem on 1 cell Lipo either.

Of course Lipos generally have a low IR so the volt drop on load would not be as great as for a similarly sized Ni based cell.

 

Dick

[FlyinFlynn]

22 hours ago, steve too said:

 

No they don't. I did the separate battery for retracts a couple of times and then decided that I was better off fitting one big battery. No issues. If I look at receiver voltage telemetry, I can see when the retracts are operating, but the drop is not significant.

 

PS On my P47, landing flaps hit the battery harder than the retracts.

 

There is no way any telemetry is going to pick up all transient spikes, the uP however will and it *could* only take one for the uP to enter reset. A larger capacity battery or even a largish capacitor on the power feed will help reduce the chances of a brownout but a separate battery removes it entirely.

 

As others have mentioned, the problems occur when an adverse condition materialises, not during normal operations and I dare say you have not conducted sufficient testing to confidently make your assertation. I suggest you re-try your retracts operating while watching the telemetry but this time jam both the legs and subject the airframe to some serious vibrations in a noisy RF environment.

 

Everything you do to reduce liability increases the reliability.

[Nigel R]

22 hours ago, Martin Harris - Moderator said:

There’s a good argument for routing the current draw away from the receiver bus in cases like this, especially given the small conductor size supported by standard servo plugs but we’re straying into larger model territory.

 

I agree. Servo connectors are the very opposite of heavy duty.

 

A Y lead and separate retract battery achieves the necessary separation perfectly well, and isolates the likely mechanical failure away from the main flight controls.