Power Vs. Speed........

[rumme]

These 2 elements seem to commonly get confused or get reffered to as being the same thing when in reality I just learned they really are not when it comes to r/c planes.After purchasing a sky scooter pro2 and flying it stock,it had very good power aspects {good in the wind..ascended easily from a hand launch,abiltiy to do a loop easily from straight flight with no downward desent beforehand..etc.}..with this said..I certainly had the desire for an increase in speed from its stk form.The recommendations to help increase this speed at a economical and simple procedure was to upgrade to a 3:1 gearbox and go with a smaller prop {9x6}.Unfortunately I never realized that some of the power aspect would be given up to see this increase in top end speed.Now the plane has a much more difficult time flying in 10-20 mph winds,doesnt ascend as fast or easily as before and doesnt loop as easily from level flight.I kind of feel this is very important information for any newbie who may not realize the tradeoff when it comes to wanting to go faster.I can honestly say I liked the sky-scooter alot better in its stock configuration.ecspecially since its winter where I live and quite windy...

[vintage1]

Normally I find that climb and loop ability are traded of for wind handling and top speed.

If it geared 3:1 on a 9x6 prop, you should gt a good maictire of all of those.

But a 3s LIPO battery may do better than 7 cells. And maybe an 8x6,.

What was its stock power...?

[Frank B]

If you want more speed with the same power (amp draw), you will typically get less thrust. I think that the thrust is what most people "feel" as power (climb rate, acceleration, loops etc.). Usually you can't increase one without decreasing the other, unless you go to a bigger prop and gearing, which raises the overall efficiency of the system.

Also speed is very expensive. If I'm not mistaken, it takes 16 times more power to fly twice as fast.

[vintage1]

something like that.

[Vindicator!]

If you want more speed with the same power (amp draw), you will typically get less thrust. I think that the thrust is what most people "feel" as power (climb rate, acceleration, loops etc.). Usually you can't increase one without decreasing the other, unless you go to a bigger prop and gearing, which raises the overall efficiency of the system.

Also speed is very expensive. If I'm not mistaken, it takes 16 times more power to fly twice as fast.

I see power as the battery amp and voltage or watts. Use of the power results in either high thrust, high speed or a mixture of the two trading one for the other.

Frank - you can setup a motor to move a plane 75 mph at 10.8 volts using 10.8 volts and 8.2 amps (89 Watts) with a thrust of 390 grams on a very small motor. Or using the same motor and similar Watts (91 watts) provide 586 grams of thrust and 15.5 MPH speed with a gearbox. So your statement about 16 times more power to fly twice as fast may be true if you don't change the configuration of the prop and gearing. In RC flying - you would change those things to modify what you wanted.

I hope this is helpful.

Vinnie

[rumme]

thanks for your replys...I consider this subject to still be one of the most confusing topics in r/c...I see so many people saying they want more power..when in reality they mean they want more speed..stk setup on sky-scooter pro2 was a 10x7 folding prop with a 3:1 gearbox {non-ballbearing}......the most recommended modification from this stk setup {not including the change from the 7-cell nicad pack that comes with the plane to a 8-cell nimh which does give better performance all around} is to go with a 9x6 -non folding slow flyer prop..and a ball bearing 3.1:1 gearbox thats sold at servo city {or anything similiar}.Even though these mods were quite inexpensive ($35) from the reviews/comments I had read online,I was expecting a whopper of a difference in the speed arena.So I was a bit dissapointed when I noticed the power loss and really havent been able to see the speed difference in my most recent flights because of the windy conditions weve had here all week.Also,I dont recall being told about this loss in power before doing the modification in any posts I read or even by any of the online stores..I guess its just a learning expirience.BUT I DO LOVE THE SKY SCOOTER..the best rtf I know of..I just think its better stk then changing the prop/gearbox like ive read so much about..

[Frank B]

... So I was a bit dissapointed when I noticed the power loss ...

That is what I ment, when I said that thrust is what most people "feel" as power. You didn't actually notice a loss in power, but a loss in thrust. You can't feel the power (Watts) when flying the plane.

I do a lot of calculations myself on my different setups, but it is not easy to visualize how the plane is going to handle in the air. I usually try with different props when I'm out in the field, to find the one that feels right, and even the same sized prop from a different manufacturer can make a difference.

[Andy W]

I lot of people put too much faith in calculations and, especially, "thrust" figures. They have little to no bearing on how well a model will perform in the real world (exception - it takes a certain amount of thrust to hover and accelerate from hover with a 3D model). Calculations are useful because you can ensure the power system components are correctly matched, and you can take the results and compare to the in-flight performance for future reference. However, you need to tune the system to your liking, and once you have the motor and battery selected, the prop is the easiest component to change out in the field.
..a

[Frank B]

Another thing to notice is that the raw data from the calculations don't tell the whole story. If you compare a setup that has a lot of thrust with one that has a lot of speed, the first one will show a better climb rate, but in real life the fast one may actually climb faster, because you don't start from zero speed. When you pull the stick back on a fast plane it doesn't need much of a climb rate to shoot into the air.

[fly4fun]

Andy is that a Picture of you in your Avator. I think you need a Hair Cut :)

[FLL]

Up till now, the thrust vs speed issue had to be a compromise since we didn't have the ability to change pitch in flight. That appears to be changing since variable pitch setups are now appearing. Looks like we will soon have readily available the same ability to change pitch in flight that our full scale big brothers have enjoyed for many years.

Frank

[fly4fun]

What Change Pitch in Flight! Am I missing something here? Where can I learn more about this.

[DeaninMilwaukee]

Drag goes up with the cube of speed, therefore to go 2x as fast requires 2 x 2 x 2 = 8x the output power to the prop.

Output is the key word here as most ( any?) motors can't come anywhere near putting out 8x the output power without their efficiency dropping way off, therefore requiring much more than 8x the input power to create that 8x output power.

If you want to go faster, its usually far more effective to decrease drag than increase power, and on the skyscooter, there are lots of easy opportunities to reduce drag.

1) flatbottom the wings. Those stock undercamber wings are like dragging a parachute around with you. When I had my skyscooter, I used foamcore board from the hobby shop to sheet the wing bottoms. I just hot glued and taped them into place.

2) tape over all the holes that cover the skyscooter fusulage.


Those two changes made my otherwise stock skyscooter approx. 20% faster, and had the unexpected side benefit of slightly longer flight times as well.


Dean in Milwaukee

[FLL]

A friend told me about a 3-D guy at a show in Ohio who adapted a heli tail rotor on his fixed wing plane and could reverse pitch while in a dive and make the plane go back up in a nose down attitude. I think the current issue of model aviation has an article about how it's done. Also heard there may be setups of this type coming to market.

Frank

[fly4fun]

Thats nuts Frank.

[everydayflyer]

What Change Pitch in Flight! Am I missing something here? Where can I learn more about this.


See this thread;
http://www.rcgroups.com/forums/showthread.php?t=270730

Found a better one:
http://www.rcgroups.com/forums/showthread.php?t=285023

Charles

[thacherd]

I believe the lastest issue or maybe the one before it of FlyRC had an article about the variable pitch prop.

[FLL]

Sorry, it wasn't the AMA mag but of course here on EZONE where I saw the setup. It's the second thread Everydayflyer posted.

Frank

[peterangus]

Frank
Was it the April issue?

[50+AirYears]

There've been several variable pitch props on the market over the last 30 years, some of which had to be adjusted on the ground, and a couple that used a wobble plate device in the built on spinner which used a separate servo to change the pitch in the air. You could use a low pitch to get yourself off the ground faster, and go to high pitch in the air. They never really caught on because they were fairly fragile, and often needed a high power servo.
The most common use of variable pitch today is in the International rubber powered competition field, especialy in the indoor microfilm, where 36 to 48 inch wingspan models weighing 2 to 5 grams fly endurance times of around an hour, and in outdoor Wakefield, where heavier models use the power that can be wound into sixteen strand 1/4" rubber motors. These props respond to the torque loading as the rubber winds down, usually. These competition models are actually super high tech, especially in the application or materials.

[FLL]

Peterangus,
Sorry, I trusted my memory and of course was wrong, saw it here.

Frank

[banktoturn]

These 2 elements seem to commonly get confused or get reffered to as being the same thing when in reality I just learned they really are not when it comes to r/c planes.After purchasing a sky scooter pro2 and flying it stock,it had very good power aspects {good in the wind..ascended easily from a hand launch,abiltiy to do a loop easily from straight flight with no downward desent beforehand..etc.}..with this said..I certainly had the desire for an increase in speed from its stk form.The recommendations to help increase this speed at a economical and simple procedure was to upgrade to a 3:1 gearbox and go with a smaller prop {9x6}.Unfortunately I never realized that some of the power aspect would be given up to see this increase in top end speed.Now the plane has a much more difficult time flying in 10-20 mph winds,doesnt ascend as fast or easily as before and doesnt loop as easily from level flight.I kind of feel this is very important information for any newbie who may not realize the tradeoff when it comes to wanting to go faster.I can honestly say I liked the sky-scooter alot better in its stock configuration.ecspecially since its winter where I live and quite windy...

rumme,

The distinction you are describing is a valid one. Rather than referring to it as a distinction between power and speed, I would describe it as the difference between thrust at high airspeed and thrust at low airspeed. Thrust is what you need to fight wind, or climb fast, and thrust is also what you need to overcome drag and acheive high airspeed. The main difference is whether the prop is well suited to generating thrust at high airspeed or low airspeed.

banktoturn

[rumme]

Thank you..I actually find myself in the same situation once again..I am putting a brushless himaxx 2812-1080-dirct drive motor in my scooter now..with my nimhs..the plane has a total weight of about 23 ozs..im looking to get about 40 mph top speed while still maintianing enuff thrust to allow the plane to do some decent vertical from level flight..not 3d vertical..but good enuff that it doesnt look sloppy in the air..and wont stall after about 20 ft like the stk setup does...since its mostly trial and error out in the field//I was informed to start with a 9x5 prop..then try a 10x6 or 8x6..to try and achieve my ecpecatations with out burning up the motor/.batteries..id assume that geting a thrust ratio of 3/4 the weight of my plane..would allow for enough torque to do the type vertical climb im looking for...

[vintage1]

To go straight up requires a minmum of 70watts per lb on a decent brushless, and then thats only by comprmising top speed. 100W/lb is better, and static thrust needs to be at least 50% more than the model weight for really hair raising climbs.

Even so, I have a model that has pulled over 150W/lb input (inefficient can motor) and that still has not got unlimited vertical, though I can get 5 rolls vertically before it runs out of speed and flops over. It does fly on the level at about 60mph though. :D

You get what you tune for. I could gear it and go straight up, but limit top speed to under 50mph I suppose...

[Sail 'n Soar]

If you want more speed with the same power (amp draw), you will typically get less thrust. I think that the thrust is what most people "feel" as power (climb rate, acceleration, loops etc.). Usually you can't increase one without decreasing the other, unless you go to a bigger prop and gearing, which raises the overall efficiency of the system.

Also speed is very expensive. If I'm not mistaken, it takes 16 times more power to fly twice as fast.

Eight times - goes up with the cube of speed.