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Author Topic: Engine Response Speed  (Read 10972 times)

Offline Tim Wescott

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Engine Response Speed
« on: August 26, 2014, 11:10:03 AM »
This is pulled in from another thread, where we were wandering off the topic

I have followed Dave's thread about the regulated IC setup. I find it very interesting, but I don't think it will work. Not because of anything you did, but because of the way IC engines behave. I don't think they can change rpms fast enough to get the timing right, unlike electrics which can change rpms immediately.

Nothing can change RPM immediately.  The electrics may be able to change RPM faster than engines set up the way we're used to, but they're still taking a finite amount of time to change RPM.  The question is whether engines do, or can be made to, change speed quickly enough.

I don't have a lot of good data on engine response, but here's what I do have.  If you squint at the plots, you can see that the engine acceleration and deceleration starts with a sharp, quick speed change, followed by a more lazy increase or decrease in speed.  I suspect that the slow engine response you perceive when you goose the throttle on a plane is that latter part, while the initial burst may be all you need.

I also suspect that there's room for improvement on throttle response, either purely with how the throttle is treated by the control system, or with changes to the engine and/or carburetor.  It's pretty well known in full-scale automobile performance circles that when you go for all-out horsepower in an engine you pretty much throw quick throttle response at anything but a narrow band of engine speeds into the toilet.  Engines built for racing classes that involve going around corners a lot often get built with smaller passages in the manifolds, smaller carburetor bores, and specific cam timings for exactly this reason.  I'm not nearly as up on 2-stroke engines, but I'm sure the same principles apply there.

So I haven't written off the notion of controlling an engine with this stuff, but I'm not going to bet money that it'll pan out, either.
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Online Howard Rush

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Re: Engine Response Speed
« Reply #1 on: August 26, 2014, 11:37:14 AM »
Of interest for an autothrottle is how fast it will change RPM a thousand or so around the cruise speed. 
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Offline Tim Wescott

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Re: Engine Response Speed
« Reply #2 on: August 26, 2014, 11:54:54 AM »
Of interest for an autothrottle is how fast it will change RPM a thousand or so around the cruise speed. 

Actually, I think 1000RPM or so around nominal is all that I'd need for TUTting a model engine effectively in stunt.
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Offline Jason Greer

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Re: Engine Response Speed
« Reply #3 on: August 27, 2014, 11:10:01 AM »
Would a variable pitch prop be a viable alternative to varying the engine RPM?  The TUT could be connected to a servo to control the pitch.  I would think the response time could be much less than waiting for the engine or motor to change RPM.
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Offline Dave_Trible

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Re: Engine Response Speed
« Reply #4 on: August 27, 2014, 12:45:22 PM »
The project for me is sitting on the back burner right now but I found while testing I got a reliable instant jump of around 500 rpm up or down.  I think that's plenty for what I see the need to be.  In fact bigger power bursts probably wouldn't be wanted.  As I think about it I'd be more interested in the power-down phase than the power-up.  That would help with wind wind up.  As I left it I was having fits trying to fit the servo in a workable position while avoiding the header and coupler.  I will likely need to cut a hole in the wing sheeting under the pipe to inset the servo.  I'm working on a new take-apart Desperado and have found some space there since there is no wing structure between the sides.  Time marches on.

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« Last Edit: August 27, 2014, 01:03:04 PM by Dave_Trible »
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Offline Tim Wescott

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Re: Engine Response Speed
« Reply #5 on: August 27, 2014, 02:39:38 PM »
Would a variable pitch prop be a viable alternative to varying the engine RPM?  The TUT could be connected to a servo to control the pitch.  I would think the response time could be much less than waiting for the engine or motor to change RPM.

That's been suggested.  A variable-pitch prop at the least, and possibly that plus a regulator keeping the engine RPM constant.

I found it interesting to look at engines designed for nitro-powered RC helicopters -- they use not two, but three needle valves, with adjustments for idle, full power, and mid-range.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline RandySmith

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Re: Engine Response Speed
« Reply #6 on: October 05, 2014, 01:59:05 PM »
One of the  quickest ways to do this with an IC engine is an exhaust throttle

Randy

Offline Steve Fitton

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Re: Engine Response Speed
« Reply #7 on: October 08, 2014, 07:15:54 AM »
One of the  quickest ways to do this with an IC engine is an exhaust throttle

Randy

Isn't that what the pipe (more or less) is already doing?
Steve

Offline Brett Buck

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Re: Engine Response Speed
« Reply #8 on: October 08, 2014, 09:25:01 AM »
Isn't that what the pipe (more or less) is already doing?

  Yes.

    Brett

Offline Tim Wescott

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Re: Engine Response Speed
« Reply #9 on: October 08, 2014, 09:57:02 AM »
One of the  quickest ways to do this with an IC engine is an exhaust throttle

If you were going to try to apply an Igor Burger style inertial speed regulator to an IC-engined plane, then the engine would have to spend long periods of time at a high power yet still part throttle setting.

If this were done with an exhaust throttle, wouldn't it heat up the engine to an objectionable degree?
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline RandySmith

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Re: Engine Response Speed
« Reply #10 on: October 08, 2014, 03:51:25 PM »
If you were going to try to apply an Igor Burger style inertial speed regulator to an IC-engined plane, then the engine would have to spend long periods of time at a high power yet still part throttle setting.

If this were done with an exhaust throttle, wouldn't it heat up the engine to an objectionable degree?

No ,l really no reason for it to heat the motor, and it has been done several times before, the setup runs normal, but when the bellcrank is moved it opens the exhaust diameter larger, the venturie then "acts" larger, the exhaust flows instantly better, then it is immediately closed back to normal size, It would be interesting to 1-- do this with a pipe, and  2 do this with electrics opening the exhaust instead of a rod off the bellcrank, you would need a sensor to see the crank move, and how much, then convey that to a servo

Randy

Offline RandySmith

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Re: Engine Response Speed
« Reply #11 on: October 08, 2014, 03:53:17 PM »
Isn't that what the pipe (more or less) is already doing?

NO, not  exactly, what I am referring to is the exhaust physically gets larger..ie  the hole  gets larger, then back to normal instantly

Randy

Offline Tim Wescott

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Re: Engine Response Speed
« Reply #12 on: October 08, 2014, 04:08:57 PM »
No ,l really no reason for it to heat the motor, and it has been done several times before, the setup runs normal, but when the bellcrank is moved it opens the exhaust diameter larger, the venturie then "acts" larger, the exhaust flows instantly better, then it is immediately closed back to normal size, It would be interesting to 1-- do this with a pipe, and  2 do this with electrics opening the exhaust instead of a rod off the bellcrank, you would need a sensor to see the crank move, and how much, then convey that to a servo

Got any pictures of what the actual throttle mechanism looks like?
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Phil Krankowski

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Re: Engine Response Speed
« Reply #13 on: October 08, 2014, 08:37:38 PM »
Got any pictures of what the actual throttle mechanism looks like?

Cox engines use an exhaust throttle.  Quite a few other engines from 60 years ago did too (Fox and everybody else).  Exhaust throttling is nothing new.  There are examples of butterflies in the exhaust of larger sized engines as well as in the side of mufflers.  

http://rctruth.com/index.php?topic=1750.0
http://www.rcgroups.com/forums/showthread.php?t=1159239&page=2
http://www.rcgroups.com/forums/showpost.php?p=23436511&postcount=68
http://www.coxengineforum.com/t1048-throttle-for-cox-049-medallion
http://www.modelaviation.com/enginehistory
http://www.rcgroups.com/forums/showthread.php?t=1159239

I think the real reasons exhaust throttling went away, mostly, cost of manufacturing.  Setup complexity with the extra moving parts probably is the other main reason.  

I have not run anything other than an exhaust throttled cox on a bench.

With modern computer radios endpoint setup and servo throw is used to adjust the positions easily.  With older radios selecting the correct arm length would do the job too.  For CL use with a computer governor I am at a loss, but I am sure something exists.  A retract sequencer or similar device might turn on/off into +5 degrees, -5 degrees...

Phil

There are other throttles too like the "maples" EGR throttle once popular on rc cars.

Offline Tim Wescott

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Re: Engine Response Speed
« Reply #14 on: October 08, 2014, 09:46:20 PM »
I was thinking of how one might arrange an exhaust throttle on a new engine, with a muffler.

I assume that you want to locate the throttle at the exhaust stack, not at the end of the muffler if you want speed.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline RandySmith

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Re: Engine Response Speed
« Reply #15 on: October 08, 2014, 10:31:58 PM »
Got any pictures of what the actual throttle mechanism looks like?

Hi Tim

I can dig them out and take photos, they are many types you can make, a simple one is a barrell, sort of like a RC carb, hook a ball link and rod coming from the bellcrank, it is closed when level, but opens when you move the crank either up or down
Next you can have a second exhaust outlet that opens when the bellcrank move, and closes when it goes back to neutral

Randy

Offline Brett Buck

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Re: Engine Response Speed
« Reply #16 on: October 08, 2014, 10:54:26 PM »
Hi Tim

I can dig them out and take photos, they are many types you can make, a simple one is a barrell, sort of like a RC carb, hook a ball link and rod coming from the bellcrank, it is closed when level, but opens when you move the crank either up or down
Next you can have a second exhaust outlet that opens when the bellcrank move, and closes when it goes back to neutral

   As I recall from 35ish years ago, Scott Bair's was a drum valve.

     Brett

Offline Tim Wescott

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Re: Engine Response Speed
« Reply #17 on: October 08, 2014, 11:30:36 PM »
I can dig them out and take photos, they are many types you can make, a simple one is a barrell, sort of like a RC carb, hook a ball link and rod coming from the bellcrank, it is closed when level, but opens when you move the crank either up or down
Next you can have a second exhaust outlet that opens when the bellcrank move, and closes when it goes back to neutral

I'd appreciate it.  Don't knock yourself out to get it done quickly -- this project is moving slowly, and may get slower yet since I'm flying in Expert with a Fancherized Twister.  I'm really feeling the need to build a better airplane.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Chris Wilson

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Re: Engine Response Speed
« Reply #18 on: October 09, 2014, 03:51:35 AM »
Would it be possible to change engine speed by using the far less aggressive environment of crankcase volume?

Thinking something along the lines of a bellows inside of the backplate recess and just might force more fuel air mix (like the Dellorto pumper carbys of old) at the right time when compressing and accelerating.
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Offline Steve Fitton

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Re: Engine Response Speed
« Reply #19 on: October 09, 2014, 06:04:00 AM »
I'd appreciate it.  Don't knock yourself out to get it done quickly -- this project is moving slowly, and may get slower yet since I'm flying in Expert with a Fancherized Twister.  I'm really feeling the need to build a better airplane.

You live out west, so I suggest getting some Impact plans and start building Impacts.
Steve

Offline RandySmith

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Re: Engine Response Speed
« Reply #20 on: October 09, 2014, 12:04:41 PM »
  As I recall from 35ish years ago, Scott Bair's was a drum valve.

     Brett

Yep Scott  made several that had a drum valves at the rear of the muffler, setup two way, 1 was in the exhaust outlet, the number 2 way was an additional exhaust vavle just below the exhaust outlet that opened only when the bellcrank moves.
I have one on one of my ships, the only thing I do not like about it, is the direct hookup to the bellcrank, I want to keep the controls as free as possible, and not add load to it

Scott  put one of the exhaust throttle drawings on one of his plans, I am not sure but it maybe the StuntFire plane from  M.A.
I will try to find which one.
Many people made exhaust throttles  going way back decades, most were operated by the carb linkage, this will not work for us, it is simply slow and requires perfect timing with the pilot control inputs.

Randy
« Last Edit: October 09, 2014, 06:01:51 PM by RandySmith »

Offline Brett Buck

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Re: Engine Response Speed
« Reply #21 on: October 09, 2014, 06:12:38 PM »
I have one on one of my ships, the only thing I do not like about it, is the direct hookup to the bellcrank, I want to keep the controls as free as possible, and not add load to it

    I expect Tim is looking to enlist some sort of demons, or evil spirits, encased in silicon, to do the actuation for him.

    Brett

Offline RandySmith

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Re: Engine Response Speed
« Reply #22 on: October 09, 2014, 07:15:23 PM »
    I expect Tim is looking to enlist some sort of demons, or evil spirits, encased in silicon, to do the actuation for him.

    Brett

LOL  as do I, but really  , but electrickery is perfect for a small servo and sensor to move the thing from  looking at how the bellcrank moves . That would  nix my biggest objection to it hooked to the  crank.

Randy

PS   evil spirits are  good too !!

Offline Tim Wescott

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Re: Engine Response Speed
« Reply #23 on: October 12, 2014, 11:57:18 PM »
LOL  as do I, but really  , but electrickery is perfect for a small servo and sensor to move the thing from  looking at how the bellcrank moves . That would  nix my biggest objection to it hooked to the  crank.

Electrickery #1: Gut a servo, leaving only the case, output shaft, and potentiometer.  Replace the servo electronics with something that emits pulses proportional to position, instead of the other way around.  Connect a battery, position sensor, and servo together, and the servo would follow the position sensor, giving your throttle "power steering".  You'd still need to make a mechanical linkage that drives the position sensor the way that you want the servo driven.

I posted a schematic for a suitable circuit in Howard's "antiservo" thread.

Electrickery #2: This sort of thing is TUTish enough that I'd be willing to look at making the TUT do it, but I'm not promising anything in the near term.  If I did do it, you wouldn't need any mechanical trickery -- just put a position sensor on the bellcrank, and the TUT would goose the throttle at (hopefully) the right times.

Electrickery #3: Try to sweet-talk Kim Mortimer into sharing his technology.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Steve Helmick

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Re: Engine Response Speed
« Reply #24 on: November 12, 2014, 07:09:14 PM »
Electrickery #3: Try to sweet-talk Kim Mortimer into sharing his technology.


Uh...that's not going to happen in this lifetime. Why didn't I catch this earlier? Oh, that's right, it's the Engineering Forum. What was the late Kim Mortimer's occupation, anyway? I met him at GSSC, but he was busy running a calculator.  D>K Steve
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Online Dennis Toth

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Re: Engine Response Speed
« Reply #25 on: November 20, 2014, 05:26:12 PM »
If the exhaust throttle is following the bellcrank what controls the speed up on the down side of the maneuvers? Seems it looses the brakes cause the power is always being added unless your going to put flat spots in the rounds.

Best,    DennisT

Offline Brett Buck

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Re: Engine Response Speed
« Reply #26 on: November 25, 2014, 02:59:10 PM »
If the exhaust throttle is following the bellcrank what controls the speed up on the down side of the maneuvers? Seems it looses the brakes cause the power is always being added unless your going to put flat spots in the rounds.

Best,    DennisT

  It doesn't know up and down, or the absolute speed, so it doesn't do anything for that. It applies an open loop correction for the drag of maneuvering. The other effects (load and thermal, plus the fuel pressure variation) still happen on top of the correction, so if it is working correctly it still might back off as the load is reduced.

   It's not terribly great as a solution compared to a full feedback control system, but it is adequately simple to implement, reasonably light, and relieves you of the need to continually adjust the compression/plug/venturi/muffler screws/prop pitch/prop diameter/prop airfoil/prop pitch distribution/fuel nitro content/fuel oil content/oil type to get it to blip just right in the corner. Bear in mind this was first implemented in the *late 70's" when your smart phone might have been the size of the moon to get the same capability.

    Brett

Offline Brett Buck

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Re: Engine Response Speed
« Reply #27 on: November 25, 2014, 03:40:00 PM »
Would a variable pitch prop be a viable alternative to varying the engine RPM?  The TUT could be connected to a servo to control the pitch.  I would think the response time could be much less than waiting for the engine or motor to change RPM.

   The prop is a much more likey-to-work solution in the hypothetical. The problem is how to build it so it
A) doesn't fly apart
B) is actuated fast enough to do the job but not so fast or far it bogs the RPM down and negates the effect, or make it worse. You might consider a flywheel, but then you have:
C) not be so heavy that it offsets the small advantage you might gain over a more conventional system

  The solution to the problem of how to control the engine speed in the corners without prohibitive weight penalty was found in 1986-7, and it's called a tuned pipe. It reacts in as little as 1 revolution, or 1/240th of a second or so. And can be made to "overshoot" if desired. It may or may not be better than an actively controlled system. The only downside is that it's still more challenging to set correctly and more prone to external influences than a hypothetically perfect feedback control system, but it's vastly easier and more consistent from day to day than a 4-2 break engine.

   Brett

Offline Chris Wilson

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Re: Engine Response Speed
« Reply #28 on: September 25, 2015, 12:58:03 AM »
Nothing can change RPM immediately. 

Impact with the ground maybe? ;D
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Offline Tim Wescott

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Re: Engine Response Speed
« Reply #29 on: September 25, 2015, 09:03:57 AM »
Impact with the ground maybe? ;D

Then you get to have an argument over the meaning of the word "immediately".  Usually when I pull an engine out of the dirt there's some evidence that the LE of the prop bit in more than the TE.  So if by "immediately" you mean "less than one revolution" -- yes, probably.  If I was trying to win a bet I'd probably try for "immediately" to mean "less than the Planck time", in which case the answer is "no".

https://en.wikipedia.org/wiki/Planck_time
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Phil Krankowski

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Re: Engine Response Speed
« Reply #30 on: September 25, 2015, 11:46:03 AM »
These are small combustion engines.  Attoseconds really are not that interesting.  (10^-18 seconds)  1 revolution is as close to "instant" as possible on a 1 cylinder 2 stroke engine.  If a 4 stroke I would say 2 revolutions.  

Multi-cylinder engines could have a fractions of a rotation response, but I will stick with 1 revolution being effectively instant

Phil


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