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May 18, 2018, 04:05 AM
What could possibly go wrong?
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Hi, just a place to continue the discussion started in John Morgan's build log about servo power. Stemming from the servo calculator page at
To me the real issue is what exactly is the airspeed to plug into those equations. I've seen all sorts of estimates for different types of models but really, they're guesses. Who can actually tell us a *measured* speed for, say, a warbird of JMorgan's type size and weight, or a sports 60 model, or a trainer, or what. It's not too hard to get pylon results as they're recorded to help judge the races, but I think we'll find they're little use against other types like the above. I saw a post about a 2m J3 Cub with a stall speed of 28km/h, landing speed 35 km/h and "best speed" (full throttle?) of 64. That's not much, 64 k's is just over 40 mph. All the assumptions I've seen for model speeds were much faster than that. A reliable speed is the first thing we need to use that servo calculator page.

Anything anybody would like to add about that, go for it, right here. Help yourself.
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May 18, 2018, 04:58 AM
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eagle60's Avatar
I agree with your ideas and the only way to know a model speed is to mesure it as it can vary not only from size or wings aifoil, but also the general aeridynamics of the plane is important like nose shape (p39 or corsair) karmans ecc. Radar gun is useless as you must be close to it and probably in leveled flight and you need the maximum reached speed, wich is probably at the end of a nearly vertical dive. So you need a gps either data logger (like voltron) or telemetry (like on frsky radio). And when you have a result do not forget a safety margin (your result is 10, place 15).
May 18, 2018, 06:23 AM
What could possibly go wrong?
Thread OP
There's a GPS unit with telemetry in my model but no way to log it, I need someone to watch it for me. I'm too scared to take my eyes off the model. You are right also about the error margin, I hope to make several passes and remember as many as possible. I will also have to remember that the GPS only measures speed over the ground, not true air speed, but at our club there are instruments to measure wind speed and direction. I should be able to take that into account. It would be best to fly straight up or down wind.

I was going to say we don't really need the maximum airspeed such as from a steep dive, but you are right, that is exactly when we need to know what force is on the servo. If I was able to log the GPS data I could find that speed, maybe in future. I could buy parts for the telemetry that can log all that data but I can't afford it now (Hitec telemetry with an Aurora 9X Tx and Optima 9 Rx). If someone is watching the display then if it updates fast enough maybe I can get a vertical speed, but I can't really expect it. I will just have to estimate from the fastest level speed, maybe 50 to 100% higher. There are people who should be able to help calculate that speed.

It would be good to know for calculating required servo torque, but there are other reasons too. The first piece of information you need when designing a new model is the airspeed, and for many other reasons. But I think most people are just guessing. I did try to order extra telemetry equipment including a real airspeed instrument, but there was some kind of error in the transaction and then I wasn't able to afford it later. I would like to know more about this for other types of models too, such as large scale models like John Morgan's, for trainers, old timers and more. I will have to search for it again.

I have attached a spreadsheet I found with some data for a 2M Cub model, it's surprisingly slow, but that is not a fast model. It is the only real data I have for anything except racing models. The models that most people fly are somewhere in between. Thanks for your post.
May 19, 2018, 12:16 AM
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eagle60's Avatar
Voltron is a gps data logger and you can find it here : or
and you can find a video explaining how it works.
The other option,as you mentioned, is to have telemetry with data logging, like the frsky taranis x9d plus as I suggested, and consider that the gps sensor costs only 40 $ (US).
I suggest also to have a set of servos ( only for ailerons and elevator) with highest possible power to be used in the speed test flight, after wich you can change them with servos selected according to your calculations.
Last edited by eagle60; May 19, 2018 at 12:21 AM.
May 19, 2018, 03:18 AM
What could possibly go wrong?
Thread OP
Cheers I really just need the Hitec Navi wireless adapter which will then receive the telemetry data from my Tx, and I can log it on a laptop. What I'd like to do is get a Windows mini tablet and mount it on the handle of the Tx and then I don't need anything else, I've already got the wired interface and it's only light and tiny, I could just cable it to that then to the tablet. THe standard telemetry software it comes with will already do the logging. The only trouble now is I've been off work with an injury and it's the quiet time of year, it's hard to justify the expense right now. BUt I expect some income soon and I have already got the stuff waiting in store for me to get, assuming someone else doesn't buy it first (not too likely).

I tried to go flying today and I would have got someone to watch my GPS telemetry on the Tx screen for me, there's a nice display but I don't dare take my eyes off in flight. However I had so many other problems with the model I couldn't end up flying it, I spent the day doing field repairs and in the end I wasn't confident enough in it to launch, I was afraid of faults in flight that could crash it. Oh well, still got some useful stuff done. I'll try again during the week.

As far as servos I have 2x Hitec HS-5245MG "mighty mini" servos for ailerons, they're 76 oz-in / 5.5 kg-cm on my 6V NiMH pack, one for each aileron. For elevator I have my new D-625MW which is 122 oz-in / 8.8 kg-cm, that's plenty for this model. The manufacturer recommends several digital servos in a memo regarding flutter, just a very few Venture 60's had elevator flutter so the designer lists a few affordable digitals with high stall torque. In the list is the Hitec HS-5485HB, just their cheap digital economy std servo. There's now a D-485 to replace it, same torque etc. but like all D-series it has the new processor and high resolution circuit. Torque is 89 oz-in / 6.4 kg-cm @ 6V so my D-625 is about 1/3 stronger than that. With a 5245 on each aileron I'm confident they're more than enough. They only have moderate torque but it isn't shared over both ailerons and that's one of their better mini servos. Really improved the feel of the plane over using 2x 5485, which had higher torque but less speed and resolution. The 5245 mini's aren't the latest but they are good quality. I know someone who uses these instead of std. servos all through most of his medium sized models, including elevator in another Venture 60. The Venture 60 is by BTE, Bruce Tharpe Engineering, the orig. designer of the Sig 4-star 40, 90 and 120. He considers it his evolution of the design though he did not design the 60-sized 4-Star. It's a very nice sports model, it's agile, fairly quick and lightly loaded. By no means a 3d plane so I believe these servos are perfectly adequate - the economy std servos are on the recommended list and they don't burn out in that model.
Last edited by BernardW; May 19, 2018 at 03:37 AM.
May 19, 2018, 11:48 AM
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eagle60's Avatar
good luck

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