Throttle curve
 

There was mention over on the IMAC forum of using a throttle curve in order to get a linear throttle response with gas engines. That got me to thinking about whether anyone uses a throttle curve with nitro engines for the same purpose or for any other reason. Then I thought: "What better place than the Pattern Forum" to ask about this?

Jim

RE: Throttle curve
 

Yes a throttle curve can fine tune the response. I find it better to have good starting point. This means that your throttle ATV, travel adjust, AFR or EPA however your radio calls it is symmetrical. That means that it has equal travel up and down.

If you have 115% on the high side you need 115% on the low side. This gives a balanced feel and a good starting point. Now you use the throttle curve to tweak to your liking.


I usually will increase the curve right off idle as most engine glow or gas has a sluggish response right off idle. So i increase it there...From this initial slight jump up I carry a pretty linear curve from there.

However if you don't set up a good mechanical linkage you will constantly be fighting yourself with electronically changing the curve.

Just my two cents.

Troy Newman

RE: Throttle curve
 

I agree with Troy. I also adjust the speed of the plane in the air to 1/2 throttle using the throtle curve moving up or down at 1/2 stick. I have a dent at that position so I always now when I am in exactly 1/2 throttle.

Vicente "Vince" Bortone

RE: Throttle curve
 

Thanks for the responses.
OK then let me throw this out. Would it be reasonable to use an optical tach to figure out what the idle RPM is and what the full throttle RPM is, on the ground. Then use the optical tach to set up a throttle curve so that 1/4 throttle on the stick is 1/4 of the RPM difference between idle and full throttle RPM. Then 1/2 stick is 1/2 of the RPM difference between idle and full throttle RPM. And so on. Hope this makes sense.

Jim

RE: Throttle curve
 

It depends on the motor/prop/airframe you're using,and the torque curve (not the bhp curve) of your motor/prop in particular.
What you'll get with most systems,if you use the set-up as you described,is something that has very little,and slow, response in the bottom half of the throttle stick,and then a rapid increase in power above that,with little noticeable gradation between 3/4 and full throttle.
Just as one example, what I do,with an OS 160/pipe/APC 17x12,is:
Idle---1700
Full---8300
Mid-stick----6000(i.e.1000rpm more than midway between idle and full)
Then,as Troy said, I have a fairly steep upward rise in the throttle curve just immediately above idle,as the response is slow just there.

That sort of set-up gives me what I think I need----a smooth response to the throttle stick---in the air.

With the current set-ups in use,just from my experience, I'd say that piped 2-strokes are the most fussy in throttle curve requirements, some of the electrics require much the same sort of curves, and the YS 160s are far less critical---as long as you move the stick somewhere,it'll do what you expected it to without much bother!

RE: Throttle curve
 

I saw a linear servo conversion kit here...

http://www.emsjomar.com/linear.aspx

I was thinking of getting one just to see how it would work for the throttle.

RE: Throttle curve
 

OK, using your example here's what I had in mind for a throttle curve:

Idle---1700 RPM
1/4 stick position---3350 RPM
1/2 stick position---5000 RPM
3/4 stick position---6650 RPM
Full sitck position---8300 RPM

It seems like that would give you a linear thrust curve (at least on the ground).

Jim

RE: Throttle curve
 

As I said,you'd find,when flying, that that set-up would have little power below half throttle,and the it would all come in a rush in the top half of the stick movement.
A working curve,to give a linear feel when flying, would look something like the red line on the (very rough--my computer graphics skills are minimal!) drawing below, with the black line representing your set-up:
http://img132.imageshack.us/img132/5...rocurvejl9.png

RE: Throttle curve
 

Linear rpm change is not what you want.

You want linear power output. As with all engines or electric motors...The planes response tot eh power is what you are looking for. The boys that have flown electric understand this too....Linear does not feel linear in the models pull department.

Part of this is the way the motors work and getting into their power curve...they just don't pull down low rpms, and part of it is the efficiency issue with the props and how they are interacting with the airframe.

Like I said before.....You will have to tune it to your tastes... Personally for me linear doesn't work. You can try all you want but I think linear response will leave you with not much happening before half power.

We all try to say we want linear throttle response....this does not mean that we want linear rpm change with throttle....Fly the model and tune it to your liking....

If you are doing 3D stuff you will want a fairly flat curve where your hovering power will be. This way you can make fine tuning adjustments up and down.....but not have one click barely hold the model and the next click have it climbing out....

If you are flying sequence stuff you will want a cruise throttle stick position you are comfortable with. Some guys like to be 1/2 stick...they might use a notch at this cruise point. I have tried this and I don't think it really helps you fly better. I feel it is a handicap in some cases.

What I do think is important is that you get a power output change and that would require a rpm change too....with every click of throttle. SO you don't want to get to 3/4 stick and the power or rpm of the model doesn't change.

So this is a play with and see what you like. Don't shoot for linear as it may not be the best for you and your needs. Everytime I fly someones model and they set it up like you suggest it feels absolutely dead below half power.

The 2 stroke pattern engines on a pipe will be worse at this dead below half power...as no matter what the setup used it will have some pipe affects....This makes it tough...Also throttling up through the range will give one rpm at a certain point and throttling down and coming off the pipe the rpms at the same point will be different...This is one reason I don't run 2 strokes. However you can play games with a throttle curve to help this situation...and make it feel like there is not the step...

For me this is one reason I like the 4 stroke engines. Each engine will have its own curve too. I am flying and have been flying the 170DZ for a couple years now. Drop a 160 in the same spot and I tweak the throttle curve a little to get it to feel the same as the 170 did.

Another trick is on the 4 stroke is using a velocity stack or restrictor in the carb. Yes it restricts top end power by a couple hundred rpm but ti makes the bottom of the power curve come alive and smooths out the power delivery. So a 170DZ with or with a velocity stack installed will have different response.
There are lots of things that go into it.....You want the model to respond linearly not have the rpms change linearly....So is it really linear....Nope it is just what feels right to you.

Reality is 1/2 stick for me is about 60% power. This is what feels best for me...It may not for you.


Troy Newman

RE: Throttle curve
 

OK, I've been making the assumption that pull is directly proportional to RPM (example: that 4000 RPM will give you half the pull that 8000 RPM gives you). It's obvious there's a lot more at play here. I really appreciate all the responses. I'll be printing this thread out to add to my trimming an airplane file. It's obvious that coming up with a good throttle curve (or curves depending on the flight mode) is just as critical as the other trimming processes.

Jim

RE: Throttle curve
 

Jim.
The relationship between engine rpm and
1. static thrust and
2. power
are not proportional, nor are they the same. (Power=rpm^3, static thrust = rpm^2, while pitch speed is proportional with rpm.) This means that increasing the rpm from one value to an other value gives you a certain increse in power. It will also increase the thrust, but not by the same percentage value.

Taking the above example as a case, (post #5) OS 160 with APC 17x12.

Based on Pe Reivers power calculator: (Pe is the Dutch supplier of MVVS engines.)

Idle, 1700 rpm, 0,03 Hp, 0,39 kg thrust, 31 km/h pitch speed.
Mid range, 6000 rpm, 1,34 Hp, 4,91 kg thrust, 109 km/h pitch speed.
Full, 8300 rpm, 3,56 Hp, 9,4 kg thrust, 152 km/h pitch speed.

To have "half power", (i.e. 3,56 Hp/2= 1,78 Hp) would require 6600 rpm, where the static thrust would be 5,93 kg, and the pitch speed is 120 km/h.
To have "half static thrust", i.e. 4,7 kg, would require 5900 rpm, which is 1,28 Hp and give a pitch speed of 108 km/h.

If you set up the throttle linearly, you will only gain a few hundred rpm's from mid-stick to full stick, but those few hundred rpm's make a lot of power, not so big increase in static thrust, and almost no increase in pitch speed.

Magne

RE: Throttle curve
 

[quote]To have "half static thrust", i.e. 4,7 kg, would require 5900 rpm, which is 1,28 Hp and give a pitch speed of 108 km/h.
[/quote

Well that's nice to know---as I said(in post 5) 6000 is where I have mid-stick,and,for me, that gives a linear "feel" to the throttle when flying.

Now for a slightly different case--I've just started flying a little electric thing for ,hopefully, Winter practice (Vanquish f3A).
I'm a more or less complete novice with electricity.
I've set the ESC thingy to be at stop on a switch for the bottom stick position, and then at the lowest possible idle (by the switch) for flying(with no brake)
This seems to work well.
The only slight hitch, is that everything happens a bit higher up the throttle stick position,compared with my IC set-up.
Can I adjust this with a throttle curve, or would that confuse the ESC?

RE: Throttle curve
 

ORIGINAL: f3a05

You can use a throttle curve like you do with a glow engine or, if your ESC has the option, program the ESC for Logarithmical throttle curve.

RE: Throttle curve
 

Excellent information!!!

RE: Throttle curve
 

Don't forget that the throttle barrel has an arm on it just like the servo. To get the type of "linear" response that you're talking about (one degree of servo movement = one degree of throttle movement), you just need to set up your existing linkage correctly. If the arms are parallel you will get very nearly "linear" response. The helicopter guys do this all the time. Using a linear output servo (no arm) actually gives you an exponential control situation: you will get more "response" at the extremes (idle and full-throttle). Linear servos were used quite extensively before we had expo adjustments in the transmitter. :)

John Pavlick
Team Black Magic

RE: Throttle curve
 

You are quite right Troy. I looked into this some years ago and arrived at the graph below that shows throttle area as a function of throttle position assuming that the linkage is set up so that 1 degree of servo rotation equals 1 degree of throttle barrel rotation. In a zeroth order approximation one can assume that power output is proportional to throttle area. If this is the case, the graph shows exactly what you describe, i.e., right off idle the response is rather sluggish, gradually becoming more linear as the throttle is opened.