Tower Pro MG90D - RC Groups
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Feb 09, 2018, 08:27 AM
Phil or TazziE
farmerjack61's Avatar
Mini-Review

Tower Pro MG90D


I was very excited to receive the six MG90D servos from the Hobbyking mother store. I purchased them on the 7th of November 2017

When they arrived I plugged them into a rx and gave them a whirl. They travelled quickly back and forth corresponding to the stick movement on my tx. They also centred consistently well. I thought all was well, and packed them away for an upcoming project.

I dragged them out today to do some tests and thought they were all duds. But I got it all wrong, then I posted my disappointment here. But I am editing all the mistakes I made earlier! - hence springers post below. Sorry mate, I messed up.

I made up an amateur test rig and compared two servos, a Hitec HS85MG+ and the MG90D. The latter were much quicker and centred just as well - very good and consistent centering. At this stage everything went wrong.

I did a load test on both units (HS85 first and then MG90) and the servos jittered and then failed to respond. I made a misguided assumption that the load test (only 200g applied with a digital scale at a radius of 1cm), had burned out my servo tester and both servos. I was furious and lashed out on the MG90D in a thousand bad words from satans dictionary.

Turns out that I got it all wrong! Oops

The home made battery pack that I used for the test was shorting in the area where I crimped the servo plug years ago. This caused the servo malfunction.

I have learned at least two things from this traversty. Firstly I need to replace that plug on my battery - I'm lucky it wasn't in a model at the time. Secondly after a restart, The humble MG90D came out looking very good indeed.

Finally I can say confidently; By comparison the $50Au Hitec is much stronger (torque) and has equivalent centering accuracy but the $6Au Tower-Pro was much faster, just as accurate in the centre and still strong enough for most medium sized control surfaces on an aircraft.

The tower pro now looks like very good value indeed. But will it last as long as the Hitec? Only time will tell.

Phil
Last edited by farmerjack61; Feb 09, 2018 at 11:07 AM. Reason: to admit I made a mistake & correct it.
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Feb 09, 2018, 09:59 AM
treefinder
springer's Avatar
On the TowerPro site the mg90d torque rating is 1.8kg/cm and 2.1kg/cm. Same numbers as HK, but with the added /cm, which makes sense for torque (Not sure what the vn means in your comments). I've not used the digital version of the TP mg 90 servos, but started getting the analog version (mg90s) when I saw them 6 for $13.89 with free shipping on banggood. My previous fave was HXT 900 analogs from HK. I probably have purchased a hundred of them over the last 15 years and never a DOA, or early failure. After years of use and abuse the tall spur gear breaks up. If the MG90s is as good I will be happy. The 90s is rated a bit lower torque than the d, but has been fine on the planes i have used them on. Personally I see no advantage in digital for the foamies I fly, so stay with the lower tech analogs.
Feb 09, 2018, 10:41 AM
S.A.D. member
ivanc's Avatar
"kg/cm" or any "force/distance" unit is meaningless and NOT used for torque. The units used for torque are "force-distance" or "force.distance". In the servo case it is measured in kg-cm or oz-in.

At specific torque value, you can get higher force at closer distance to the center of rotation and less force farther from the center of rotation. The normal force multiplied by the distance at which it is applied equals the torque - that is why using the division sign "/" in the torque unit is incorrect.

Ivan
Feb 09, 2018, 11:17 AM
Phil or TazziE
farmerjack61's Avatar

Mg90d


Quote:
Originally Posted by springer
On the TowerPro site the mg90d torque rating is 1.8kg/cm and 2.1kg/cm. Same numbers as HK, but with the added /cm, which makes sense for torque (Not sure what the vn means in your comments). I've not used the digital version of the TP mg 90 servos, but started getting the analog version (mg90s) when I saw them 6 for $13.89 with free shipping on banggood. My previous fave was HXT 900 analogs from HK. I probably have purchased a hundred of them over the last 15 years and never a DOA, or early failure. After years of use and abuse the tall spur gear breaks up. If the MG90s is as good I will be happy. The 90s is rated a bit lower torque than the d, but has been fine on the planes i have used them on. Personally I see no advantage in digital for the foamies I fly, so stay with the lower tech analogs.
The abbreviation Vn is the number 'n' of volts 'V' - I made that up earlier today!

I adore the little blue HXT900 also - what a fantastic product.

Cheers, Phil
Feb 09, 2018, 11:21 AM
Phil or TazziE
farmerjack61's Avatar
Quote:
Originally Posted by ivanc
"kg/cm" or any "force/distance" unit is meaningless and NOT used for torque. The units used for torque are "force-distance" or "force.distance". In the servo case it is measured in kg-cm or oz-in.

At specific torque value, you can get higher force at closer distance to the center of rotation and less force farther from the center of rotation. The normal force multiplied by the distance at which it is applied equals the torque - that is why using the division sign "/" in the torque unit is incorrect.

Ivan
Thankyou Ivan


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