|Jun 03, 2012, 03:36 PM|
Ramps for Hotliners / F5B
So i thought i would share a little secret sauce from the F5B world for those who have no idea what all this stuff is about. Save you some work, make life a whole lot more easy. Some people don't want me to post this, and it is indeed a lot of invested time and effort i am about to share with you. You can buy me a beer some time.
For starters you have to realise that whatever you do with the signal coming from the TX is going to be between the end points you have set on the TX. If those end points are a long way beyond what the ESC is looking for, then your ramps will not take the amount of time / have the effect you expect them to have. So you might want to figure out where the end points need to be for your ESC before you start dialing things in to much. You can test this on the bench with a couple of cells less and no prop. When you have found the end points you need, you want to add a little extra for reliable operation.
Forward speed vs RPM;
The angle of attack (AOA) of a prop is what makes or breaks performance. Just like a wing, get the AOA to high and it will stall. There are two factors that determine the AOA during normal operation, the RPM and the forward speed of the plane. Fortionately unlike a wing, when the prop starts to stall it will only start to stall near the root of the prop blades and not stall completely. This means you still get a lot of thrust and just a small loss in efficiency. The bigger the part of the prop stalled, the lower the efficiency.
Max start power;
The maximum power you can put into the prop at the start of the climb is determined by the pitch of the prop and the forward speed of the model. A prop with less pitch can take more power at this point. Flying slower means you can put less power into the prop before it starts to stall. Keep that in mind when you rig your ramps for duration mode when you are flying far slower then when flying legs.
Roughly speaking, you can put about 4KW into most modern F5B props at the start of the climb before the prop starts to stall. This excludes the launch and assumes you are doing the distance task with a reasonable return speed from the last set. The forward airspeed is somewhere around 120 to 150 Kph.
In duration mode, your plane is only flying about 60 to 80 Kph and so you can only put about 1.5 to 2 KW into the prop before it starts to stall.
Since these planes accelerate very fast, the power you can put into the prop also increases very rapidly. However, if you put in 500W, the plane will accelerate much slower then if you put in 4000W. So you are looking at a sort of exponential deal where the faster the plane goes, the more power you can put in and the faster it will accelerate.
With the Futaba 8FG, 12FG, 12Z and similar TX's you have a motor menu that works perfectly for what we want to achieve. It will allow you to do simple strait line power ramps with 2 stages. The first stage is how fast you get the power up to the start power of the climb, the second stage dictates how long it takes for the rest of the power to feed in.
Hope it all makes sence so far.
If you are looking at my blog page, please click ''comments'' to continue reading.
|Jun 03, 2012, 03:47 PM|
Here are some plots pulled from a very nice piece of software / programming made by Mr Magic. If you also want to be able to do the same and nicely overlap each climb of your flight then i suggest you get in contact with him.
You will notice in these graphs we are looking at the RX signal and the Power.
In this case the motor OFF signal from the RX is about 1230 us and full power is at about 1800 us. The signal is already rising a little before the ESC responds.
In this example the power is ramping up to just over 3000W before it then ramps a little slower up to 5000W when full power is reached and subsequently the power collapses further as the speed of the plane continues to increase and unloads the prop.
When looking at both sets of lines in the same graph, be sure not to get confused between the units given on the left and the right hand side. The RX signal is on the right hand side and given as A2 from the Unilog.
|Jun 03, 2012, 03:49 PM|
Effectively we have cut the RED triangle away. This is where the prop would likely have been partly stalled and the energy that you would have put in would have just gone to waste.
|Jun 03, 2012, 03:54 PM|
The Futaba 12FG motor menu gives you several options to play with. I have highlighted in the respective colour what does what.
The first part of the ramp is called SPEED 1 and the second half is called SPEED 2. The number ranges from 0 to 27 where 27 is really slow and 0 is instantanious. Sensible numbers are between 10 and 24. I always use 10 for speed 1 and something around 22 for speed 2.
Then there is the switchover point. This is called (SPEED 1 > 2 ) and can be adjust from -100 to +100. Sensible numbers are around -25 to +10 and you should adjust in steps of 5 or 10% at a time.
Note that there is an IN and OUT speed given. You want to add a delay on the ON side only and not when switching OFF the motor.
Note that in some cases things may be reversed, it may also depend on if you use a throttle stick or a switch. Try the manual for the TX you have and/or experiment a little with a servo plugged into the throttle channel.
Page 103 of the 12FG manual reads MOTOR;
|Jun 03, 2012, 04:21 PM|
The YELLOW lines is the first part of the ramp and is what keeps your prop from blowing up due to excess acceleration forces.
Be aware of WIDE BLADE RFM PROPS, these blades need a soft power up.
A slower ramp gives a smoother initial power delivery but wastes more time.
|Jun 03, 2012, 04:24 PM|
The second part of the ramp is the BLUE line and this is how long you want to take to feed the rest of the available power in.
A slower ramp means better prop efficiency and reaching max power further on in the climb.
It does however mean more work for the ESC.
|Jun 03, 2012, 04:28 PM|
The switchover point is perhaps the most critical number of them all. It will raise or lower the GREEN line as shown.
A little adjustment on the switchover time will make a lot of difference as it will change the power at the start of the climb as well as effect how long the second ramp will last for. This is perhaps not what you would expect.
Different brands of TX/RX might make a difference here, so please keep in mind that my experiance has only been with Futaba.
|Jun 03, 2012, 04:35 PM|
Here is one from a duration run.
You can see the switchover point is much lower and the result is a power curve that starts from about 1500W and goes up to 4900W over about 1.5 seconds. The power is cut shortly after reaching full power.
At no point during this motor run was the prop stalled.
The RX signal is about 1500us at the time of switchover between the ramps.
|Jun 19, 2012, 04:56 PM|
Here's one from todays results. Sergey Anashin's old plane back from 2009 with a new fuselage.
The graph also shows a free climb (ends short) and a huge duration climb, climb 1 is not shown.
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