Max Headspeed question

[ryguy76]

Hey,

I'm using a HW60A ESC but will be governing via the Spirit. Something that I've never really understood is the "Motor Efficiency" parameter when doing Headspeed calculations.

What determines this setting? It makes a fair bit if difference when calculating the optimal headspeed so not sure what my actual max head speed is to input into Spirit.

I know I don't have to enter the maximum achievable headspeed, just my preferred max head speed which also makes setting throttle curves afterwards easier too. For now, I have chosen 3200 and my IU3 is 100% flat so in theory I should get my 3200 provided my calculations are correct. From changing some fine tuning values around in Mr. Mel's calculator, the max RPM range is anywhere from 2900 to 3600.

Just wondering how one accurately figures out the absolute max headspeed their chosen pinion will allow for and it seems knowing the ins and outs of the motor efficiency aspect is a must and it has a large effect on the calculations.

[treys1]

Motor efficiency is something you would have to find from the motor manufacture or a similar source. Most people use 90-95 to account for some loss.

As for the spirit portion, you can do it either way. You can put the max theoretical HS in and adjust your throttle curves off of that (e.g 3600max but flying in your highest IU2 @ 3200 would be a 89% throttle curve), or you could put the max you want to fly at in (again 3200) and run a 100% for that and base your lower IU1 and Normal off of that (2800 Normal would be 88% in this case). It is completely up to you.

Now to find max using MrMel I would use 90% efficiency, take the 100% at beginning of flight and multiply it by .85 or 85%. That is what i would target as a sustainable max HS for a specific motor / pinion.

[ryguy76]

Originally Posted by treys1

Motor efficiency is something you would have to find from the motor manufacture or a similar source. Most people use 90-95 to account for some loss.

As for the spirit portion, you can do it either way. You can put the max theoretical HS in and adjust your throttle curves off of that (e.g 3600max but flying in your highest IU2 @ 3200 would be a 89% throttle curve), or you could put the max you want to fly at in (again 3200) and run a 100% for that and base your lower IU1 and Normal off of that (2800 Normal would be 88% in this case). It is completely up to you.

Now to find max using MrMel I would use 90% efficiency, take the 100% at beginning of flight and multiply it by .85 or 85%. That is what i would target as a sustainable max HS for a specific motor / pinion.

Makes good sense, thanks. In a round about sort of way, I figured 3100 is my optimal rpm for my chosen pinion based on reading different ways of calculating governed HS last night in several threads.

Your method confirms I'm in the right optimal HS too (3677 x 0.85 = 3125). Where does that 85% value come from? Is that a typical governor sweetspot value?

I had 3200 entered in the Spirit as Max, but since have changed it to 3100 and am running 93.5%, 97% and 100% TC values for a ~2900, ~3000 & 3100HS. Will get a iphone Tach on it today and see what I'm actually getting but am now more confident with obtaining the optimal HS for a motor, pinion and battery combo. Thanks.

[Xrayted]

The 85% is a round about value used by most because you must leave some headroom for the governor to be able to maintain the set RPM under hard loads or as the pack drains.

For example if the HS calc says that the max RPM at the beginning of the pack for a given setup will be 3000 RPM, then that RPM is only going to drop as the pack drains during flight. The governor isnt going to be able to keep that same 3000 RPM the whole time. There must be some headroom below that 100% throttle value to account for the voltage drop.

Another way to do it with Mr Mels is to look at the value that shows as the "lowest RPM due to voltage and efficiency" and the hover RPM at end of flight as rough guide of the range of the max value you will be able to effectively govern. The 85% will be somewhere in the middle of those two values.

If the RPM starts at the beginning of the flight at 3000 but ends at 2400 due to voltage and efficiency, then logic dictates that you wont be able to maintain more than 2400 RPM effectively throughout the entire flight due to voltage drop at the end of the flight

[ryguy76]

Originally Posted by Xrayted

The 85% is a round about value used by most because you must leave some headroom for the governor to be able to maintain the set RPM under hard loads or as the pack drains.

For example if the HS calc says that the max RPM at the beginning of the pack for a given setup will be 3000 RPM, then that RPM is only going to drop as the pack drains during flight. The governor isnt going to be able to keep that same 3000 RPM the whole time. There must be some headroom below that 100% throttle value to account for the voltage drop.

Another way to do it with Mr Mels is to look at the value that shows as the "lowest RPM due to voltage and efficiency" and the hover RPM at end of flight as rough guide of the range of the max value you will be able to effectively govern. The 85% will be somewhere in the middle of those two values.

If the RPM starts at the beginning of the flight at 3000 but ends at 2400 due to voltage and efficiency, then logic dictates that you wont be able to maintain more than 2400 RPM effectively throughout the entire flight due to voltage drop at the end of the flight

It's all coming together now. I see that playing with the C rating of the battery and the packs age doesn't affect the max rpms at the beginning or end of the flight but it does affect the bogging rpm, which makes good sense based on the voltage sag an older pack would endure. Subtracting half the value of the difference between the bogging RPM and the ungoverned hover rpm at the end of the flight gets you pretty close to that 85% mark of max non governed rpm. I like it.

[Xrayted]

Originally Posted by ryguy76

It's all coming together now. I see that playing with the C rating of the battery and the packs age doesn't affect the max rpms at the beginning or end of the flight but it does affect the bogging rpm, which makes good sense based on the voltage sag an older pack would endure. Subtracting half the value of the difference between the bogging RPM and the ungoverned hover rpm at the end of the flight gets you pretty close to that 85% mark of max non governed rpm. I like it.

And the way you fly will play a lot into it as well. Someone who is just a big air sport flyer may be able to push the envelope with a higher governed rpm a little outside of the suggested range and never really notice it, however a 3-D pilot could see significant bogging if doing the same thing and need to use different gearing if he wants to maintain the same rpm

[ryguy76]

Originally Posted by Xrayted

And the way you fly will play a lot into it as well. Someone who is just a big air sport flyer may be able to push the envelope with a higher governed rpm a little outside of the suggested range and never really notice it, however a 3-D pilot could see significant bogging if doing the same thing and need to use different gearing if he wants to maintain the same rpm

Ok, so for example, if 3100 rpm was the 85% of max ungoverned rpm at the start of the flight, and it bogged to 2900, on a given pinion let's say... We could tackle that by setting the max rpm in the Spirit at 3100 and then 100% TC in the radio. BUT, if the 2950 bogging voltage was bothersome for 3D flying, we'd have no other option than to change the pinion to provide a bogging rpm that matched the 3100 rpm we want to fly at so there would be no actual bogging during flight, right?. Could we then keep the 3100rpm max setting in the Spirit and the 100% TC without any real ill-effect to the ESC that is actually having to throttle back it's output to the 3100rpm which would be significantly less than the 85% of it's ungoverned max rpm at the start of the flight? Or would that just not be something to worry about?

[Shonkonray]

Originally Posted by ryguy76

Ok, so for example, if 3100 rpm was the 85% of max ungoverned rpm at the start of the flight, and it bogged to 2900, on a given pinion let's say... We could tackle that by setting the max rpm in the Spirit at 3100 and then 100% TC in the radio. BUT, if the 2950 bogging voltage was bothersome for 3D flying, we'd have no other option than to change the pinion to provide a bogging rpm that matched the 3100 rpm we want to fly at so there would be no actual bogging during flight, right?. Could we then keep the 3100rpm max setting in the Spirit and the 100% TC without any real ill-effect to the ESC that is actually having to throttle back it's output to the 3100rpm which would be significantly less than the 85% of it's ungoverned max rpm at the start of the flight? Or would that just not be something to worry about?

As far as my understanding goes i would say if your esc has active free wheeling it is no problem. If afw is deactivated, it depends on how far off you are with the gearing respectively at which % the esc is running and is then a matter of esc temperature (gets too hot or not).

[Xrayted]

Originally Posted by ryguy76

Ok, so for example, if 3100 rpm was the 85% of max ungoverned rpm at the start of the flight, and it bogged to 2900, on a given pinion let's say... We could tackle that by setting the max rpm in the Spirit at 3100 and then 100% TC in the radio. BUT, if the 2950 bogging voltage was bothersome for 3D flying, we'd have no other option than to change the pinion to provide a bogging rpm that matched the 3100 rpm we want to fly at so there would be no actual bogging during flight, right?. Could we then keep the 3100rpm max setting in the Spirit and the 100% TC without any real ill-effect to the ESC that is actually having to throttle back it's output to the 3100rpm which would be significantly less than the 85% of it's ungoverned max rpm at the start of the flight? Or would that just not be something to worry about?

Yes, a 3-D pilot may need more overhead than a sport flyer which might be able to get away with less for the same value.

ESC issues typically won't occur unless you are way overgeard for the rpm you are running. Being geared for 3100 and running at 1700 for example would be a very inefficient setup both mechanically from a gearing and mechanical leverage standpoint and electronically as the ESC runs most efficient near 100% output.

Keep in mind these rules really only come into play when improperly set up combined with trying to do demanding moves that create hard loads. Simply dropping the throttle curve down to a much lower rpm for basic low head speed flying should not cause any electronics issues. In this case the much lower RPM also causes the current draw to be much lower, so everything will work just fine even if the gearing isn't ideal, although the bogging will still be an issue.

It’s the combination of low ESC output due to over gearing combined with high demand loads which spike the current to high levels that are the deadly combination for the electronics. For example being over geared for the RPM you are running and trying to lift a lot of extra weight with the model such as in a heavy scale fuselage setup, or trying to do smack 3-D moves. This can be a recipe for an ESC fire depending on which ESC brand you are using.