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Old 10-12-2010, 04:28 PM   #1
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throttle expo on DX7

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I found a write up on how to program expo into the throttle channel of a dx7 in acro mode. I thought this may be of some interest to some people wishing to tame down the throttle curve of there gassers. I found the throttle on my DLE20 to be very sensitive at lower throttle settings.

I know it helped me and this is a copy and paste so i cannot take any credit for it.

Programming Throttle Curves on specific JR radios – how to do it, why do it, and some theory behind it

What radios does this apply to?

Specifically, I have done this many times with my JR 7202. I am 90% sure this also directly applies to the Spectrum DX-7, which I think is really a clone of the 7202. The screens may differ, but the theory and practice should be about the same. There is a good chance this will also apply exactly or closely to other programmable JR radios with capabilities at or above the 7202 / DX-7, and the application (at least the theory) may apply to other programmable radios of other brands. The programming may be different, but you likely will be able to find a way do it with your radio too.

What type power plants does this work with?

It is ESPECIALLY applicable to gasoline engines, and also glow engines and other internal combustion engines of any type. All that is required is that they have a carburetor. I think there is also application with various electric motors too. Glider drivers, go back to sleep!

Why would I want a throttle curve programmed for my airplane?

I expect you have noticed with your motor or engine (assuming you have the normal linear throttle response from your throttle servo) that the engine revs up a lot with the first movement away from idle of your throttle stick. Also, you will have noticed that from the mid-throttle position of the stick and the carburetor to the full throttle position of the stick and carburetor barrel or butterfly that almost no further increase in RPM or power happens. What a pain! This leaves your with (1) a real touchy throttle from idle on up to mid power setting, and (2) a throttle control that seems not to work at all from mid throttle on up to full. This is not a desirable control setup. Your throttle seems more or less an on/off affair than giving you real linear control over it.

From a practical sense, for instance, I have a plane that has a lot of drag with flaps, and I have found is lands most smoothly and under the best control if I “fly it on” the deck with some power on- not too much, just a little; maybe 3000 RPM where idle is 1800 RPM. This also allows me to control the descent on final approach the best. For this I need good throttle control. With linear throttle, the difference between idle and 3000 RPM is about three clicks on the throttle stick. This is much too touchy! With a programmed throttle curve, 3000 rpm is at about ¼ movement of the throttle stick. Now with some real movement of the stick available, I have fine control of the throttle on approach and landing. I’m sure you can think of other situations where better throttle control would be of benefit – 3D hovering for example (Don’t get me started – I am not a 3D fan – get a helicopter if you have the need to hover!!)

Built-in throttle curves

Most computer radios that have a helicopter function (including my 7202) recognize the need for fine throttle control of helicopters. To this end, they have a throttle curve function available in the programming in the helicopter mode. I believe mine has a 5-point setup, with adjustments at 0%, 25%, 50%, 75%, and Full throttle. These same programmers, at least up until now, have not seen fit to give us airplane guys the throttle control that we need and want. I have seen in other forums that some airplane drivers (probably the 3D guys) wanting good throttle response have set up their airplanes in the helicopter mode just to get this feature. I am not sure what all is involved with this, but I expect that it is doable and is another option for you should you want to pursue it. I have not tried and don’t know exactly what is involved at this time. Since it offers 5-point curves, it offers more fine control than my method, which only offers a 3-point curve. The 3-point is good enough for most applications, I think.

Some practical (and hopefully interesting and educational) theory - power vs. speed

First, it is helpful to understand some basics about power and speed. For an airplane, the power required to reach a given speed is a function of the cube of speed. For example, flying at 60 mph takes “X” engine power. To make the same plane fly at 120 mph (assuming same altitude, density, temperature, level flight , etc. etc.) takes 8 times “X” power! Not double but 8 times. Wow! The calculation here is 120/60=2 or double the speed. Now cube this, or 2x2x2=8 times the power. These calculations for an aircraft are almost exact. Small variations will enter, most specifically at the higher speed the wing will fly at a lower angle of attack, and will therefore fly at a slightly lower drag coefficient. This will lower the drag at a given speed, but all in all, the calculation is accurate within a percent or two.

A correlation to this can be seen with a normal car vs. a race car. You may be able to drive your family sedan at 100 mph using 100 HP (watch out for the cops!) Man, you think, if I had 200 HP I could go 200 mph. Wrong! Here again, the speed has doubled, and the power required goes up by the cube factor, or 8 times, and you realize then that you need 800 HP. Ask the NASCAR boys. For the 200 mph speedway speeds, they use engines that dyno out in the range of 700 – 800 HP. Of course, with a race car, there are the aerodynamic influences in addition to the friction components (tires mostly) but the cube function, it turns out, pretty much applies to these forces too.

Now think of your airplane engine. It turns a propeller, which of course is just a rotating wing. The same cubic function applies to the engine power vs. RPM calculation. The RPM is of course in direct relation to the forward rotational speed of any point on the propeller, and is very proportional to the airspeed of the plane in flight. When thinking of relative power output between engines turning a given prop, remember the cube function. If one engine will turn a given prop 10,000 rpm and another will turn that same prop 12,000 rpm, the second engine is about 73% more powerful.

Your carburetor, it turns out, pretty much meters the POWER output of the engine (at least for the first half or so of its potential full throttle opening), with the rpm following as a function of this power available. Even this power vs. rpm curve is not linear with carburetor opening (remember, there is a cubic function to this parabolic curve), but is not as far off the straight line relationship as is the carburetor opening vs. rpm curve. The effect of this is that opening the carburetor barrel or butterfly angle for instance 10% of the available movement from the idle position will increase the power to the prop by about 800% of the idle power, or to about 13% of the MAXIMUM engine power it will have at wide open throttle (WOT.) Keeping in mind the cube function, this will move the rpm of the prop from idle (let’s assume this is 2000 rpm) to 4000 rpm. So – an increase of 10% throttle opening doubles the rpm, with a similar doubling of airspeed!

Please note – these throttle angle calculations are based on engineering formulas – not actual test data for model engines, BUT they give values that are “in the right direction” – please don’t chime in with statements how your measured rpm’s vary some from these projected values. I KNOW they probably will. My real aim is to demonstrate the hair trigger nature of an aircraft engine throttle in the idle to half open range, and the lack of sensitivity after the throttle is at least half open.

As a bit of a visual aid, I have constructed the following chart that should be fairly accurate for a typical gasser with a Walbro type carburetor:

RPM Throttle angle, degrees from closed % Power output comments

2000 <2 1.6 idle
3000 4 5.3
4000 8 12.5
6000 15 42.2
8000 70 100 WOT

Note the sensitivity up to 4000 rpm (only 6 degrees of throttle movement) and the lack of sensitivity from 6000 to 8000 rpm (only 2000 rpm increase with 55 additional degrees of throttle movement)

Here again, your engine may be more or less sensitive than this hypothetical situation, but the high sensitivity at low and low sensitivity at high will prevail in all cases.
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Old 10-12-2010, 04:28 PM   #2
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Re: throttle expo on DX7

Programming the JR 7202 (and hopefully the DX-7):

A couple of notes are in order at this point.

(1) In the Airplane Mode, the throttle curve is generated using the Programmable Mixing 1-6 feature.
(2) In the JR 7202 manual this is in Section 6.2, pages 55 -57.
(3) The throttle curve is created by mixing Throttle with itself, or Thro-Thro.
(4) The throttle curve generated is really a three point curve, or more precisely two linear segments joined at a mid point. The slope and end points of each line segment is programmable, and the position of the mid point is adjustable with “Offset” value.
(5) From the background information, hopefully you will see that the object is to have very little throttle servo movement with the first half (or so) of the throttle stick movement, and an exaggerated amount of movement during the stick movement from the “half throttle” stick position on your Tx to the “full throttle” position of your stick.
(6) The other functions that effect the throttle channel still function and can be superimposed upon the throttle curve – end point adjustments, servo reversing, travel adjustments, trim settings, etc.
(7) I find it helpful to use the travel adjustments and trim setting in the end to actually set the full throttle and idle positions.

Programming steps:

(1) Press “Down” and “Select” at same time to access Function Mode (when Tx is on and transmitting.)
(2) Use “Up” and “Down” keys to select the Prog. Mix screen of your choice. It will by Prog.Mix1 if this is not being used. If it is, select the next available unused one from 2 through 6.
(3) Use Select button to highlight “master channel” on second line.
(4) Push “Increase” or “Decrease” button until Thro appears on line two.
(5) Press Select button to highlight “slave channel” on second line. This is the second entry after the previously selected master channel.
(6) Press “Increase” or “Decrease” button until Thro appears as the slave channel. At this point the second line should read Thro-Thro. The hyphen shown here is actually an arrow on the screen. To the right on line 2 it will say “ON” I think this is by default when a channel is mixed with itself, and cannot be switched off.
(7) Press the Select button until line 3, or “Rate” is highlighted.
( Push your throttle stick to full throttle. A small arrow on the top number, just to the right of the word “Rate” will illuminate. This means you are ready to set the shape of the mid to high throttle curve segment.
(9) Use the “Increase” and “Decrease” buttons to set the value of the curve segment. It can be varied between + 125% and – 125%. Here your want “more sensitive than normal” setting for the insensitive part of your throttle response. In this example (for a Saito FA-50 four stroke engine) I have set this to +18%
(10) Move your throttle stick to low throttle. The arrow moves to the low throttle to half throttle part of the curve. As in step 9 use the “Increase” and “Decrease” keys to set your value for the low curve. Remember, here you want to program in lower sensitivity for this area of the engine throttle that is hair trigger sensitive. In my case, I have entered (-) 47%
(11) Now it is time to set the “mid point” where the switch over from the low to high part of the curve happens.
(12) Use select to highlight SW on line 5.
(13) Press increase until it says SW:ON. This makes the switch over (mid point) of the curve active all the time.
(14) Press select to highlight “Offset.”
(15) Use the “Increase” and “Decrease” keys to set an offset value. I have mine set at (-) 20%. This value sets the switch over point 20% below the mid point of the throttle. This seemed to work well with this engine.
(16) This completes the programming. Shut off Tx and it will save the values you have entered.

The actual values you use and the switch over point you use will be a somewhat trial and error process. The important thing is that the Rates must be + for the high and (-) for the low throttle. Play with the values and gauge your engines response. Make adjustments as necessary. You will find that the Rate values effect the distance traveled by the servo arc. This can be a blessing or a curse. Rate therefore effects the end point adjustments. You can actually use the Rate settings to set an end point, or probably better is to get the rates the way you like and THEN go to the End Point Adjustment screen and use this to set the actual maximum travel of the servo. Be careful to not go past the travel range of the servo with either the end point or rate adjustments.
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Old 10-12-2010, 04:33 PM   #3
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Re: throttle expo on DX7

Thank you very much for all that effort

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Old 10-12-2010, 04:42 PM   #4
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Re: throttle expo on DX7

Like I said its only a cut and paste, I'm not smart enough to figure that out on my own.
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Old 10-13-2010, 11:09 AM   #5
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Re: throttle expo on DX7

Originally Posted by rooky View Post
like i said its only a cut and paste, i'm not smart enough to figure that out on my own.

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Old 10-17-2010, 06:23 PM   #6
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Thumbs up Re: throttle expo on DX7

You are correct it is alot easier to manage a throttle with some expo.I have set a few dle 30s at my club that way but on a 9303 or 9503 its easy.if you want it to work really good you can build expo into the servo setup right off the bat, just have the servo arm at 90deg at full throttle or a little more that way at low throt the arm rotates more before it pulls the linkage and is less responsive.Also smooths out the idle trim.
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Old 10-17-2010, 06:59 PM   #7
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Re: throttle expo on DX7

This is also a critical point of set up for gasser helicopters (almost the reverse of nitro). Usually mechanical set up is how it's done but, with the good resolution of today's radios, it can be set up electronically as well as above.

Good post.
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Old 10-17-2010, 07:07 PM   #8
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Re: throttle expo on DX7

Interestingly this same topic came up in the DX8 discussion:

Some were wondering why the DX8 has programmable 5 point throttle expo available in Acro mode. I was also curious myself as a new DX8 owner. I have never felt the need for it as I fly only electrics.

Well now, I know!

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