Not that old question
|Dave Hess||19/12/2018 10:12:09|
|301 forum posts|
Electric motors make maximum torque at zero rpm, while as IC motors make it at high rpm, so you get a different torque reaction when you open the throttle.
There's loads of other factors that affect the way a plane handles. How much side-thrust and down-thrust did each motor have? Were they different sized propellers? How did those factors change the prop-wash?
|Martin McIntosh||19/12/2018 10:28:54|
2776 forum posts
One of my models which I have tried with both is a small Dalotel. I used the same APC i/c 9x6 prop., so this cannot have contributed to the difference in feel. Revs were about the same with the E version having a slight edge. The cg was the same in each case as an i/c with a full tank. The model would seem to yaw a little as it pleased and generally did not quite go where intended.
I can only really put this down to the large gyroscopic effect caused by the spinning motor mass; a small control input in one direction causing the `gyro` to precess at 90 deg. to this. Does not explain why EDF`s and turbines do not do this though, unless it is because their masses are closer to the cg.
Anyway, the Dalo soon got converted back to i/c as did the others.
Edited By Martin McIntosh on 19/12/2018 10:30:10
|J D 8||19/12/2018 11:05:31|
1118 forum posts
I have seen many a take off go squirrely or worse with IC and electric just because the power stick has been slapped open to full in an instant. You are right in thinking that more spinning weight will result in more gyroscopic progression, something that several WW1 types with rotary engines are well known for.
I have a small electric Polykarpov I16 RATA which is a handfull at times.Once on the approach to land you are committed,any attempt to open up and go around it will swing left and roll in even if you open up slowly.
|Nigel R||19/12/2018 11:09:08|
2722 forum posts
The biggest effect of all?
|Jon - Laser Engines||19/12/2018 11:18:31|
|4562 forum posts|
I saw a full size I16 at duxford once. From the moment the engine started to the moment it stopped it just looked like it was trying to kill itself at every opportunity!
|Piers Bowlan||19/12/2018 11:50:10|
1753 forum posts
Nice model JD8 even if it is a handful. Was it scratch or plan built and what size?
349 forum posts
To a first approximation, an ic engine is a constant torque device so, to get more power out of it you need to get it to rev higher, generally by fitting a smaller prop.
An electric motor by contrast, is essentially a constant rpm device so, to get more power out of it you need to increase the torque, generally by loading it up with a larger prop.
As well as confusing the converting i.c. pilot when it comes to prop selection, this difference also affects what happens when the model flies at different airspeeds. As the model accelerates, the propeller 'unloads' causing the i.c. motor to rev higher and so produce more power. However, reducing the load on an electric motor simply reduces the torque demand so the power output goes down. I believe this may be behind the feeling expressed earlier of electric models feeling gutless - i.e. they seem to have limited top speed. The answer of course is often to use a higher pitch prop on the electric model to limit the power drop off as the model accelerates.
Well, it's a theory!!
|Peter Miller||19/12/2018 12:21:11|
9836 forum posts
JUst a comment on the fact that ic engines do not have a linear response to the throttle stick.
On My Futaba Field Force 8 I can set exponential on the trottle which does make the r esponse linear.
The lack of exponential on the throttle is my only complaint against my Spektrum Tx
|Simon Chaddock||19/12/2018 13:02:32|
5370 forum posts
I think you are probably right.
not only are the power curves of IC and electric (particularly brushless) different but with electric the change in motor torque is created the instant the throttle is moved.
Probably not that significant in normal flying but it will produce dramatically different effects opening the throttle when 'low and slow' particularly on light and powerful electric planes!
|Nigel R||19/12/2018 13:31:28|
2722 forum posts
"The lack of exponential on the throttle is my only complaint against my Spektrum Tx"
Probably no surprise, but most spektrum tx models have a throttle curve you can use in aero mode.
As another dodge, if you have a dx6i, you might get away with using the heli mode, which has a curve, although that's not much use if you have more than 4 channels in use on the RX.
|Jon - Laser Engines||19/12/2018 14:04:21|
|4562 forum posts|
Power...ah yes, that all inclusive word
This is where the waters muddy as 'power' is not the be all and end all. An overloaded electric motor can draw massive power due to its high current draw, but you wont get much actual useful power out of it.
Again, an ic engine can generate more power at higher rpm, but if you use the wrong prop you might not get the performance you want and the performance on a different propeller, at lower revs, will give you more feeling of power.
And then we enter the really complicated issue of propellers and their efficiency as its propeller thrust and not engine/motor power than actually makes the model move. I am sure a bent butter knife will generate great power if you look at the engine RPM side, but its efficiency will be woeful so the model will feel like it has no...power.
In the end, we use a power system for our models and there is a balance to be had between maximum power output of the power plant, the efficiency of the propeller, and the drag of the model.
To give a few examples from my own models that are total opposites but gave the same result:
saito 45 in Nieuport biplane. I wanted better takeoff/cIimb performance so went went up from 13x5 @ 8500rpm to 15x5 @ 6500rpm. This lost me about .2 of theoretical maximum engine power, but the model felt as though it had more power, especially at low throttle, due to the higher efficiency and thrust of the larger prop.
The other was my Sea Fury. This was a little slow in a straight line and lacked vertical performance. I went from 22x8 @ 6800 to 21x8 @ 7400. This was a deliberate move on my part to bring the engine RPM up to get more power from the engine, and it worked nicely with the greater RPM allowing more vertical performance and greater top speed.
A friend also went from a 9000rpm 16x6 to an 8400rpm 16x8 and felt the model had considerably more 'power' even though maximum theoretical HP had dropped.
|Chris Walby||19/12/2018 14:06:26|
907 forum posts
Thanks guys, all very interesting and I appreciate comments regarding the differences with taking off and landing.
Noise I am not to sure about as even though I was flying IC if you have two or three other people on the flight line with IC I can't hear mine anyway.
Regarding the prop size would a inch in diameter make that much difference to flight characteristics and it seemed quite tolerant with fuel levels.
Come to think about it the prop was further forwards with IC, but again would 3/4 inch make a difference.
I still think it tracked better, was easier with rolls, loops (big ones) and stall turns with IC. It was no acrobat although generally a nice model to fly post A cert.
|J D 8||19/12/2018 14:34:11|
1118 forum posts
Hi Piers, the model is all balsa from aTDM models plan,span is about 32 inches,weight a tad under 700 grams [ my scales are metric] with battery. Recon it could have done with being built a bit lighter.
I prevously posted about this litle monster under the title " Rata by name rat by nature" Cheers John.
|Jon - Laser Engines||19/12/2018 14:35:34|
|4562 forum posts|
An inch of prop is quite a lot nd will make a significant difference.
|Geoff Sleath||19/12/2018 14:51:41|
3238 forum posts
I must be singularly insensitive because I've never noticed much difference between similar models powered with electric or ic. My DB 58" Tiger Moth (as in my icon) is electric and I propped with a 13x4 initially because I didn't need any speed so a fine pitch was OK. It had a lot of acceleration on the take-off run a flew OK. I tried a 12x6 and the only real difference I noted was the need for a longer take off and slightly lower current when flying straight and level. I kept the the 12x6.
I'm the same with bikes - pedal or motor. I read reviews which described all sorts of effects of tyres/gearing/'feel' but mostly they don't register when I ride. I've always thought they were nonsense but perhaps it's me
|Frank Skilbeck||19/12/2018 15:57:00|
4358 forum posts
One thing I have noticed is that an electric motor at a constant throttle setting will draw more current when the model is climbing, this is because as the model slows down the prop is doing more work and as the electric motor is trying to maintain the rpm it will draw more current. It's most noticeable when doing a loop and will often trigger an over amp sensor alert at a constant throttle setting. Effectively the throttle position is trying to maintain a constant motor rpm, whereas on an IC model it's a constant throttle position which is regulating the air/fuel throw and hence power of the engine, so when a prop is loaded up the engine will slow down unless the throttle is opened to a greater degree than an electric motor.
But having said that, apart from a different throttle response and ability to cut all power on an electric model on landing and use the prop windmilling as drag, on similar sized/weight models I don't notice any real flying differences. My electric models range from 100 to 1800 watts and IC from 30 2 stroke through 90 fourstoke to 32cc Petrol. The other big difference is the longer flight time on IC.
|Peter Christy||19/12/2018 17:02:28|
|1375 forum posts||
I wonder how much of that was due to the increase in the "un-screened" area of the prop in a model with a large frontal area?
Many years ago, I had a "Stringalong", a large-ish (by the standards of the day!) biplane. It was designed by Frank Knowles, for what would now be called F3A aerobatics. It was powered by a Webra Blackhead 61, and on a wooden 13x6 had very sprightly performance for such a large and draggy model. On a 12x6, it wouldn't even take-off! I always put it down to the relatively small amount of prop protruding beyond the fuselage.....
|Don Fry||19/12/2018 17:18:40|
3437 forum posts
The competition aerobatic people used to overcome this difficiency with IC engines by using a tuned pipe, slightly off tune (too short)in straight and level. Put the nose up, and it comes on tune.
|Frank Skilbeck||19/12/2018 17:22:48|
4358 forum posts
too short or too long?, if it was too short then it would only come on pipe at higher rpm, whereas too long and it would drop back into the tuned pipe optimum rpm
|Don Fry||19/12/2018 17:48:24|
3437 forum posts
Frank, I claim knowledge that they used to do it. Overlong, or overshort I bow to a better source.
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