Electronic solutions to modifying glow engines of all sizes to gasoline
10-10-2022, 01:17 PM
#1026
Senior Member
Thread Starter
I probably should have said the minimum throttle slowdown should be the servo speed rating.
I want to account for the separate components that all add up to the total instantaneous fuel requirement. The obvious first component is the fuel required at steady state (constant rpm). I want my base mixture curve to corrspond to this and I want the throttle position to exactly track with this base mixture curve.
Next, I want to add the fuel required for acceleration to that base mixture only when accelerating. I do not want the base mixture to include the acceleration fuel via having the mixture lead the throttle because I won't be able to model that in software. It also negates the requirement of a mixture curve with a positive slope.
By the way, with a broken airbleed screw, there's not much that can be done to change the mixture curve.
I want to account for the separate components that all add up to the total instantaneous fuel requirement. The obvious first component is the fuel required at steady state (constant rpm). I want my base mixture curve to corrspond to this and I want the throttle position to exactly track with this base mixture curve.
Next, I want to add the fuel required for acceleration to that base mixture only when accelerating. I do not want the base mixture to include the acceleration fuel via having the mixture lead the throttle because I won't be able to model that in software. It also negates the requirement of a mixture curve with a positive slope.
By the way, with a broken airbleed screw, there's not much that can be done to change the mixture curve.
FWIW, I think that the only reason cars and such have "exact" amounts of acceleration fuel, is because excess fuel disturbs the functioning of a catalythic converter and that messes up emissions I don't think that is a consideration.
The other thought I have is that the rates of acceleration that we are working with, are of a completely different nature than that of automotive engines.
For example, my old Ducati still has a carb with an acceleration pump. That is a buffered device, actuated by the thottle movement. Whenever the throttle is moved, the pump actuates, and squirts in a bit of fuel, with a rather fixed quantity and duration, but the actual RATE of change of RPM the engine undergoes, as wel as the absolute change (in RPM nmbers) is different depending on the gear in use, the road conditions (level, uphill, downhill, headwind, tailwind, etc) and the amount of throttle given.
Our engines ONLY need that extra fuel under the harshest of acceleration (from low to WOT) and the rate and duration of that acceleration is always roughly the same.
Therefore I would expect the exact amount of fuel needed to be not too precisely defined.
Let's assume that with the system as we have it now, we are running a 0,7 sec slowdown on throttle. Let's assume also, that we could reduce that slow down to 0,5 seconds, but the engine becomes a bit unpredictable at that point and 0,4 or less it simply stumbles and dies.
Let's assume that our human thumb needs 0,1~0,2 sec to move the throttle forward.
So our not slowed down solenoid signal needs 0,2 sec to reach its endpoint, giving that extra fuel and that would allow for a safe throttle movement of 0,7 sec under all circumstances.
Now I would expect that a driver that would de-energize the solenoid as soon as the stick is moved quicker than say 0,5 sec/full stroke (so also for 0,25 sec for half stroke) would give so much additional fuel, that the throttle slow down maybe could be safely reduced to 0,5 sec, possibly even less.
I am just "thought-experimenting here", but de-energizing the solenoid, WILL dump fuel in the carb, more so than a rapid moving fuel curve would. Seems reasonable that the throttle possibly could move faster too. Whether the engine will actually accelerate faster, I have no idea,
The thing is, will this result in better stability of operation or not? I don't see gains in acceleration as even remotely interesting. Planes don't need that. We do not do dragracing.
If it increases stability and reliability, then I am all for it.
Right now, I have a level of reliability that absolutely is ok to any standard. But better than OK is always welcome.
10-10-2022, 02:27 PM
#1027
A question derived from more playing around.
Using the needles to flatten the curve and to keep the curve profile mostly climbing from idle to wot works quite well for transition. I have found that leaning the lsn too much however results in points along the curve that become very insensitive to mixture changes. If the lsn is adjusted too lean I may have to move a mixture point on the curve all the way up to the top to detect an overly rich condition. So when that happens I richen the lsn to bring back more sensitivity to mixture adjustments. That's fine But.....
Thinking on this, I wonder how this effects the temp/barometric compensation. Is there a range of mixture sensitivity I should be shooting for? +/- Y movement of say 20% or something where too rich and too lean are detectable by rpm change?
Bottom line, I want to use the full capabilities of the system, not be out of range and end up having to adjust needles or the curve based on ambient temps or pressure changes.
Using the needles to flatten the curve and to keep the curve profile mostly climbing from idle to wot works quite well for transition. I have found that leaning the lsn too much however results in points along the curve that become very insensitive to mixture changes. If the lsn is adjusted too lean I may have to move a mixture point on the curve all the way up to the top to detect an overly rich condition. So when that happens I richen the lsn to bring back more sensitivity to mixture adjustments. That's fine But.....
Thinking on this, I wonder how this effects the temp/barometric compensation. Is there a range of mixture sensitivity I should be shooting for? +/- Y movement of say 20% or something where too rich and too lean are detectable by rpm change?
Bottom line, I want to use the full capabilities of the system, not be out of range and end up having to adjust needles or the curve based on ambient temps or pressure changes.
Last edited by Glowgeek; 10-10-2022 at 04:42 PM.
10-10-2022, 06:37 PM
#1028
The answer depends on the temperature and pressure you fly in and the reference values in the controller. Generally speaking, with the xiao controller, if you keep the mixture curve between 10-90% you're probably fine depending on how hot or cold your flying weather is.
With a 90% mixture, you'll run out of solenoid pulses when compensation reaches 111% which happens at around -30c with the standard 273k reference temp.
On the bottom end, you run out of resolution. With a mixture of 10%, compensation can change the mixture in steps of about .15% which feels adequate but setting lower mixture percentages comes with less resolution for both compensation and percent difference between adjacent steps.
With a 90% mixture, you'll run out of solenoid pulses when compensation reaches 111% which happens at around -30c with the standard 273k reference temp.
On the bottom end, you run out of resolution. With a mixture of 10%, compensation can change the mixture in steps of about .15% which feels adequate but setting lower mixture percentages comes with less resolution for both compensation and percent difference between adjacent steps.
Last edited by Raleighcopter; 10-10-2022 at 06:42 PM.
10-11-2022, 01:56 AM
#1029
The answer depends on the temperature and pressure you fly in and the reference values in the controller. Generally speaking, with the xiao controller, if you keep the mixture curve between 10-90% you're probably fine depending on how hot or cold your flying weather is.
With a 90% mixture, you'll run out of solenoid pulses when compensation reaches 111% which happens at around -30c with the standard 273k reference temp.
On the bottom end, you run out of resolution. With a mixture of 10%, compensation can change the mixture in steps of about .15% which feels adequate but setting lower mixture percentages comes with less resolution for both compensation and percent difference between adjacent steps.
With a 90% mixture, you'll run out of solenoid pulses when compensation reaches 111% which happens at around -30c with the standard 273k reference temp.
On the bottom end, you run out of resolution. With a mixture of 10%, compensation can change the mixture in steps of about .15% which feels adequate but setting lower mixture percentages comes with less resolution for both compensation and percent difference between adjacent steps.
I may be asking about mixture scaling vs. correction factor scaling? Not sure. It may take a phone call to communicate the question but I'll try again.
With the engine running at 3000 rpm let's say I have to increase mixture strength at that point on the curve by 5% to cause an overly rich condition i.e. a drop in rpm. Now, let's say I lean the lsn a touch in an attempt to flatten the curve, all fine, but now it takes a mixture strength increase of 25% to see to an ovely rich condition. That's 5 times more upward movement on the curve to produce the same overly rich condition i.e. the same mixture enrichment.
Surely that has to effect how much bearing temp and pressure corrections have on the running behaviour of an engine in varying climates?
So what I'm looking for is a recommended mixture range in the Y axis (Too lean to too rich) to adhere to, at any given point on curve, so that the mixture strength corrections provided by the temp/pressure sensor work effectively.
Even compromise has limits.
Last edited by Glowgeek; 10-11-2022 at 03:06 AM.
10-11-2022, 03:20 AM
#1030
Senior Member
Thread Starter
I knew my question was not going to be an easy one to communicate without knowing the technical terms to use. I believe you answered a different question, the one about recommended min/max mixture settings on the curve as they relate to temp/pressure correction. I get that, and that does come into to play as well, but that's not exactly what I was asking.
I may be asking about mixture scaling vs. correction factor scaling? Not sure. It may take a phone call to communicate the question but I'll try again.
With the engine running at 3000 rpm let's say I have to increase mixture strength at that point on the curve by 5% to cause an overly rich condition i.e. a drop in rpm. Now, let's say I lean the lsn a touch in an attempt to flatten the curve, all fine, but now it takes a mixture strength increase of 25% to see to an ovely rich condition. That's 5 times more upward movement on the curve to produce the same overly rich condition i.e. the same mixture enrichment.
Surely that has to effect how much bearing temp and pressure corrections have on the running behaviour of an engine in varying climates?
So what I'm looking for is a recommended mixture range in the Y axis (Too lean to too rich) to adhere to at any given point on curve, so the mixture strength corrections provided by the temp/pressure sensor work properly.
Even compromise has limits.
I may be asking about mixture scaling vs. correction factor scaling? Not sure. It may take a phone call to communicate the question but I'll try again.
With the engine running at 3000 rpm let's say I have to increase mixture strength at that point on the curve by 5% to cause an overly rich condition i.e. a drop in rpm. Now, let's say I lean the lsn a touch in an attempt to flatten the curve, all fine, but now it takes a mixture strength increase of 25% to see to an ovely rich condition. That's 5 times more upward movement on the curve to produce the same overly rich condition i.e. the same mixture enrichment.
Surely that has to effect how much bearing temp and pressure corrections have on the running behaviour of an engine in varying climates?
So what I'm looking for is a recommended mixture range in the Y axis (Too lean to too rich) to adhere to at any given point on curve, so the mixture strength corrections provided by the temp/pressure sensor work properly.
Even compromise has limits.
First of all, if you for example have a needle setting at that 3000 RPM such that the fuel curve is at 10%, then a change in curve of 5% to get it extremely rich, that is a change of is more or less 50% of its original value,
Now if you close the LS needle such that it forces you to set the fuel curve at 50%, and you need to go to 75 to get that same rich condition, then that ALSO is a change of 50% of its original value.
The other thing is, that the corrections as done by the controller (or LUA script) based on pressure and temperature, are relatively very minute. they are, during a normal day, in the order of magnitude of 1 or 2% tops of the original value. You can not directly translate that into a number for the curve.
At the top end of the curve (WOT) you want to have about 20% clearance, to make 100% sure that the correction of the controller will never hit the end of its range.
At the lower end, you generally do not want the curve to dip significantly below 20%, and that has very little to do with the corrections of the controller, but everything with the resolution of the curve.
If you have a Taranis or similar, the minimal adjustment you can make to the curve is 0,5% of total range. At a fuel opening of 80% of total range, one step adjustment is 1/160th of the momentary fuel delivery. That is a step size of 0,625% of "momentary"
At the low end of the curve, at 20% fuel opening, that same 0,5% step is 1/40th of momentary fuel delivery. That is a stepsize of 2,5% of "momentary".
The controller-caused corrections due to pressure and temperature are not affected by this, those have a much greater resolution. If the controller reduces or increases 1%, then that will be 1% of momentary, regardless of whether momentary is big or small.
Also: Things are not lineary. They are within small ranges of deviation. That is not due to the solenoid being not linear, but because the engine responds to being richer or leaner.
For example, the case you mentioned, setting the engne to 3000 RPM, then causing an overly rich situation. At the optimal setting (let's assume that to be 10% curve value), the solenoid passes an amount of fuel X. If you now increase the curve value, and thus the solenoid opening to 15%, an increase of 50%, then the new fuel flow will NOT be 1,5 x X. Simply because the overly rich condition will cause the engine to drop RPM, and because of that, carb vacuum will drop and thus fuel flow, while increased, will not be 1,5 times as much.
Airflow will drop due to the lower RPM, but also this will NOT be linear to RPM drop, because the lower vacuum means that the filling grade per intake stroke has altered.
All in all this causes it to be extremely hard to "mathematically" work with the numbers here. It is largely a matter of trial and error.
The only way to be able to do that, is when we would have extremely accurate air and fuel flow instruments. Those do not exist in a form that returns realtime info AND are suitable for the relatively small pressure differences we are working with. (It is possible to make a rotometer for small flows, but the nature of the rotometer is that it creates a pressure difference in order to measure, and that will in turn influence the engines ability to draw fuel, so it will distort the relation between curve value and fuel draw),
It can be done, but it is extremely time consuming.
10-11-2022, 04:13 AM
#1031
I'll add...
The controller reports correction factor as telemetry and also on the display. Let's assume your reference temp and pressure are 273 and 1000mb. That means your correction will be 100% at reference conditions. If the maximum mixture you have is 80%, the controller can increase the correction to 125% before running out of headroom at the top of the mixture where the solenoid would be fully open. A correction of 125% happens around 218k (273/218) which is about -55c (neglecting pressure variations which are much smaller). On the bottom end, as Bert and I said, the issue is resolution and the delta in fuel delivery per step. You won't run out of headroom on the bottom but you will run out of resolution. With my controller having 6145 solenoid steps and a mixture setting of 10%, each step is about 0.15%. at some point the steps become too big for meaningful adjustments. I suspect it happens somewhere below 5% mixture. Be safe, stay below 90% mixture (80% is even better) and above 10%, although I believe you can go lower than 10%. Also, like I said, sometimes, as in the case of my borked airbleed needle, you cannot change the bottom or the curve shape so they wind up being whatever they are.
The controller reports correction factor as telemetry and also on the display. Let's assume your reference temp and pressure are 273 and 1000mb. That means your correction will be 100% at reference conditions. If the maximum mixture you have is 80%, the controller can increase the correction to 125% before running out of headroom at the top of the mixture where the solenoid would be fully open. A correction of 125% happens around 218k (273/218) which is about -55c (neglecting pressure variations which are much smaller). On the bottom end, as Bert and I said, the issue is resolution and the delta in fuel delivery per step. You won't run out of headroom on the bottom but you will run out of resolution. With my controller having 6145 solenoid steps and a mixture setting of 10%, each step is about 0.15%. at some point the steps become too big for meaningful adjustments. I suspect it happens somewhere below 5% mixture. Be safe, stay below 90% mixture (80% is even better) and above 10%, although I believe you can go lower than 10%. Also, like I said, sometimes, as in the case of my borked airbleed needle, you cannot change the bottom or the curve shape so they wind up being whatever they are.
10-11-2022, 04:17 AM
#1032
Senior Member
Thread Starter
I'll add...
The controller reports correction factor as telemetry and also on the display. Let's assume your reference temp and pressure are 273 and 1000mb. That means your correction will be 100% at reference conditions. If the maximum mixture you have is 80%, the controller can increase the correction to 125% before running out of headroom at the top of the mixture where the solenoid would be fully open. A correction of 125% happens around 218k (273/218) which is about -55c (neglecting pressure variations which are much smaller). On the bottom end, as Bert and I said, the issue is resolution and the delta in fuel delivery per step. You won't run out of headroom on the bottom but you will run out of resolution. With my controller having 6145 solenoid steps and a mixture setting of 10%, each step is about 0.15%. at some point the steps become too big for meaningful adjustments. I suspect it happens somewhere below 5% mixture. Be safe, stay below 90% mixture (80% is even better) and above 10%, although I believe you can go lower than 10%. Also, like I said, sometimes, as in the case of my borked airbleed needle, you cannot change the bottom or the curve shape so they wind up being whatever they are.
The controller reports correction factor as telemetry and also on the display. Let's assume your reference temp and pressure are 273 and 1000mb. That means your correction will be 100% at reference conditions. If the maximum mixture you have is 80%, the controller can increase the correction to 125% before running out of headroom at the top of the mixture where the solenoid would be fully open. A correction of 125% happens around 218k (273/218) which is about -55c (neglecting pressure variations which are much smaller). On the bottom end, as Bert and I said, the issue is resolution and the delta in fuel delivery per step. You won't run out of headroom on the bottom but you will run out of resolution. With my controller having 6145 solenoid steps and a mixture setting of 10%, each step is about 0.15%. at some point the steps become too big for meaningful adjustments. I suspect it happens somewhere below 5% mixture. Be safe, stay below 90% mixture (80% is even better) and above 10%, although I believe you can go lower than 10%. Also, like I said, sometimes, as in the case of my borked airbleed needle, you cannot change the bottom or the curve shape so they wind up being whatever they are.
10-11-2022, 04:26 AM
#1033
I wouldn't say it's seriously limited below 10% (assuming we're talking about my controller. At 10% it's sending an uncompensated solenoid pulse of 615/6146. Compare that to the number of rc channel steps I can read (1024 steps). Compensation can still adjust the mixture fairly finely at 10% mixture, it's just that the uncompensated solenoid steps have less resolution due to the RC channel being at 102 of (maximum) 1024 pulses. Here you can see that an rc channel change results in a 1% change in fuel mixture which is becoming a bit large.
10-11-2022, 05:30 AM
#1034
Senior Member
Thread Starter
I wouldn't say it's seriously limited below 10% (assuming we're talking about my controller. At 10% it's sending an uncompensated solenoid pulse of 615/6146. Compare that to the number of rc channel steps I can read (1024 steps). Compensation can still adjust the mixture fairly finely at 10% mixture, it's just that the uncompensated solenoid steps have less resolution due to the RC channel being at 102 of (maximum) 1024 pulses. Here you can see that an rc channel change results in a 1% change in fuel mixture which is becoming a bit large.
that's exactly what I said? The controller has no issues, the curve has. on a -100 to +100 scale, at 10% opening (-80 as curve value) there is only a resolution of 5%, smallest step possible is 5% of base value.But the controller still can do those much smaller steps.
Side note, just out of interest: did your boxer allready fly? Ben's LT40? Just curious.
10-11-2022, 06:24 AM
#1035
The ft40 was underpowered on the lt25. I need to find a larger engine but life got in the way. The ft120 was swapped out for the ft160 but I haven't had an opportunity to start the 160. Hopefully life gets a bit more settled soon.
10-11-2022, 11:45 AM
#1037
I have a recently overhauled Saito FA-50 Golden Knight I could send your way Dave. The internal parts are A+ but the outside could use a coat of ceramic black paint.
10-11-2022, 04:22 PM
#1038
The spec's on that LT25 are a little misleading... 4.5 lbs, 63" wing span, and flying with a .25 - .32 2 stroke / .20 - .26 4 stroke, seems a bit under powered... The sig kadet MK1 was about the same wing span and flew very well with a .40 2 stroke, and .50 4 stroke.
10-11-2022, 04:50 PM
#1039
I did train fellow with an LT-25 back in the day and can confirm the plane was a little light on power with a 25 two stroke (decent one).. I remember thinking it needed a bit more. It flew well but no excess of power like was typical of the .40 size trainers of the day. I designed a trainer to fly on a 25 and it had the same issue. I then redesigned it (smaller/lighter) and it was a great success on only a .15 - Not a powerhouse but a great flying training machine - taught my oldest to fly RC on one and kitted a bunch for local modellers who all had great success with it.
10-11-2022, 05:21 PM
#1040
I did train fellow with an LT-25 back in the day and can confirm the plane was a little light on power with a 25 two stroke (decent one).. I remember thinking it needed a bit more. It flew well but no excess of power like was typical of the .40 size trainers of the day. I designed a trainer to fly on a 25 and it had the same issue. I then redesigned it (smaller/lighter) and it was a great success on only a .15 - Not a powerhouse but a great flying training machine - taught my oldest to fly RC on one and kitted a bunch for local modellers who all had great success with it.
10-11-2022, 06:34 PM
#1041
Let me have a look Lonnie - it was a highly "prefab" kit and assembled without plans - just a booklet. But I might have a complete "kit" laying around here still - Would love to have someone put together another now for "nostalgia". The Offshore "ARF" craze took over and put me out of the "kit" business and left me with boxes of precept parts.. Had to lay off my entire workforce ( me 
)...
)...
10-12-2022, 01:37 AM
#1042
Senior Member
Thread Starter
It still surprises me, because AA5BY over on RCGroups reported a Sig LT25 to fly very well with a Saito 30 on gas...
But it might also simply be me, having a different definition of "underpowered". As long as a trainer can take off from a lawn-quality grass runway, maintain a climb at a 20 degree angle or do a loop from level flight, I consider it adequately powered.
But it might also simply be me, having a different definition of "underpowered". As long as a trainer can take off from a lawn-quality grass runway, maintain a climb at a 20 degree angle or do a loop from level flight, I consider it adequately powered.
10-12-2022, 04:09 AM
#1043
Well true, the definition of underpowered would very from pilot to pilot... the sig kadet MKII has an engine rating for a .25 - .40 2 stroke, flying weight 5 lbs... so if the LT25 weighed less, 10oz weight reduction can make a substantial difference in flight performance... I'm sure the saito .30 would be well balanced power to weight on gas.
10-12-2022, 04:29 AM
#1044
I think power is very relative to the Pilot - and to what they typically fly. I throughly enjoy planes you have to fly and that are not simply pulled along with an excess of power. When I started flying glow again the "electric only" guys would always asked what was wrong with the engine as it seemed to not have enough power..Flying a Moki 61LS powered Kaos - I thought it had tons of power for an old school glow (even with a rich breaking in setting) but it can not pull vertical at 1/2 power like many electrics can. I think the LT-25 flew well but it was not the same power feeling as the MK2 .40 powered kadet ( which I always had the students fly at 1/2 power or less.) The MKII could climb out and get to "safe altitude" quickly and kept training on task after takeoff or a boo-boo. It also offered a more advanced student the ability to Play around and get them better prepared for the next step. The LT-25 (LT-40/Goldberg Eagle/Other newer trainers) flew different and more "on the wing". They could feel a bit "underpowered" but I think it was a relative thing. I'm sure the 25 would fly well with a smaller 4 stroke but perception is everything sometimes.
10-12-2022, 08:47 AM
#1045
It still surprises me, because AA5BY over on RCGroups reported a Sig LT25 to fly very well with a Saito 30 on gas...
But it might also simply be me, having a different definition of "underpowered". As long as a trainer can take off from a lawn-quality grass runway, maintain a climb at a 20 degree angle or do a loop from level flight, I consider it adequately powered.
But it might also simply be me, having a different definition of "underpowered". As long as a trainer can take off from a lawn-quality grass runway, maintain a climb at a 20 degree angle or do a loop from level flight, I consider it adequately powered.
OTOH, I also like to watch my 60 size WWll birds zip by at 70+ mph.
Last edited by Glowgeek; 10-12-2022 at 09:46 AM.
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10-12-2022, 12:15 PM
#1046
I think power is very relative to the Pilot - and to what they typically fly. I thoroughly enjoy planes you have to fly and that are not simply pulled along with an excess of power. When I started flying glow again the "electric only" guys would always asked what was wrong with the engine as it seemed to not have enough power..Flying a Moki 61LS powered Kaos - I thought it had tons of power for an old school glow (even with a rich breaking in setting) but it can not pull vertical at 1/2 power like many electrics can. I think the LT-25 flew well but it was not the same power feeling as the MK2 .40 powered kadet ( which I always had the students fly at 1/2 power or less.) The MKII could climb out and get to "safe altitude" quickly and kept training on task after takeoff or a boo-boo. It also offered a more advanced student the ability to Play around and get them better prepared for the next step. The LT-25 (LT-40/Goldberg Eagle/Other newer trainers) flew different and more "on the wing". They could feel a bit "underpowered" but I think it was a relative thing. I'm sure the 25 would fly well with a smaller 4 stroke but perception is everything sometimes.
10-12-2022, 07:44 PM
#1047
Bert, (or others) - Ever convert a OS 37SZ H - Found a "prop converted" one up here for a decent price and might press the buy button on it. Im thinking it would make a good little "Small Block" two stroke gasser...
Comes with a sweet MACS mini pipe Muffler...
Comes with a sweet MACS mini pipe Muffler...
10-12-2022, 11:43 PM
#1048
Senior Member
Thread Starter
I would expect that engine to be an excellent candidate for conversion, Chris. It is ringed, and it has a heatsink head. The carb might be even very suitable "as is" because heli carbs are in general tuned for a leaner midrange compared to aero engines (as a glow, that engine probably did not do all that well in aero conversion).
I would most definitely try it.
I converted an ASP 36 heli engine once (pre-solenoid), and it was fairly decent with the stock carb:
The OS 37 is a bit filigrane in design (piston is very thin walled and everything is built rather compact to get that .37 cu.in. in the same footprint that was originally designed for a .32 but other than that is is an excellent engine.
I would most definitely try it.
I converted an ASP 36 heli engine once (pre-solenoid), and it was fairly decent with the stock carb:
The OS 37 is a bit filigrane in design (piston is very thin walled and everything is built rather compact to get that .37 cu.in. in the same footprint that was originally designed for a .32 but other than that is is an excellent engine.
10-13-2022, 12:03 AM
#1049
Senior Member
Thread Starter
IMHO, that is a misconception. The plane WILL behave different, and it won't even be close. It is a bit like fitting a yugo with a Chevy small block and limiting the throttle and fixing the gearbox in 2nd gear, then expecting it to behave the same as the original engine and the full gearbox.
An overpowered plane in my experience behaves different, despite the pilot limiting himeslf to not using that power. It is not only in the additional weight, but also, and mainly, in the different prop size.
I mean, a floater plane with a small engine, 9 x 4 prop and a throttle range of 2500 to 12500 RPM behaves distinctly different than the same plane with a large engine and a 10 x 6 prop where the RPM is limited to 9600 (exact same output). Airspeed envelope and prop braking are different, landing speed is different.
It does not have to be a BAD thing, but it most definitely WILL be different. Not the same, and usually the plane with the larger, slower prop WILL fly easier than the plane with the smaller higher spinning prop. More thrust because of the larger propdisc means a faster acceleration from a near stall despite the same power.
It is one of the reasons why I additionlly like the gasser conversions: it reduces power output (AKA invites to install a larger engine) and thus the use of larger, slower spinning props compared to the original "glow alternative". My SpaceWalker runs a 14 x 6 at 9500 RPM, where the originally intended powerplant would have a 13 x 6 spinning 10500.
The speed and thrust characteristics are quite different.
An overpowered plane in my experience behaves different, despite the pilot limiting himeslf to not using that power. It is not only in the additional weight, but also, and mainly, in the different prop size.
I mean, a floater plane with a small engine, 9 x 4 prop and a throttle range of 2500 to 12500 RPM behaves distinctly different than the same plane with a large engine and a 10 x 6 prop where the RPM is limited to 9600 (exact same output). Airspeed envelope and prop braking are different, landing speed is different.
It does not have to be a BAD thing, but it most definitely WILL be different. Not the same, and usually the plane with the larger, slower prop WILL fly easier than the plane with the smaller higher spinning prop. More thrust because of the larger propdisc means a faster acceleration from a near stall despite the same power.
It is one of the reasons why I additionlly like the gasser conversions: it reduces power output (AKA invites to install a larger engine) and thus the use of larger, slower spinning props compared to the original "glow alternative". My SpaceWalker runs a 14 x 6 at 9500 RPM, where the originally intended powerplant would have a 13 x 6 spinning 10500.
The speed and thrust characteristics are quite different.
