thrust vs RPM????

[BBCorvette18]

Ok I have a question. Is thrust and RPM directly related or is there some variance.
I think I know the answer but want to verify

Say I have two setups:
Setup 1. Makes 20oz of thrust and turns 10,000 Rpm.
Setup 2. Makes 20oz of thrust and turns 5,000 Rpm.

So my question is which setup will have a higher airspeed on the same airframe at the same weight?

[xmech2k]

I'm just throwing out my guess to see if I'll be right: the one that has a prop with a higher pitch regardless of RPM, unless they are the same pitch, then I vote for the higher RPM. I think there are SO MANY factors in this kind of equation.

[kyleservicetech]

Originally Posted by BBCorvette18

Ok I have a question. Is thrust and RPM directly related or is there some variance.
I think I know the answer but want to verify

Say I have two setups:
Setup 1. Makes 20oz of thrust and turns 10,000 Rpm.
Setup 2. Makes 20oz of thrust and turns 5,000 Rpm.

So my question is which setup will have a higher airspeed on the same airframe at the same weight?

There is a LOT involved with those thrust figures. The main concern is the blade pitch, and the resulting speed of the air being blown through the prop in flight. In fact, it would be possible to have very similar flight performance with both the 1K and 5K RPM props, just by changing the propeller diameter, and its blade pitch angle. That's one benefit of these electric power systems. Seems those bigger diameter propellers are a bit more efficient than a much smaller propeller running at the same power input at far higher RPM. I've got a number of Hacker motors, my A50 series motors are running 16 inch diameter props, pulling about 1100 watts. My A60 series motors are running a 19X12 prop, running about 2500-3000 watts. That 19 inch prop turns about 7300 RPM in the air, and it pulls a 16 pound model straight up out of sight.

With a given amount of watts (power or horsepower) you can have a prop that puts out 100 ounces of thrust at a blade pitch speed of 100 MPH, or a prop that puts out 200 ounces at a blade pitch speed of 20 MPH or there abouts. That 20 MPH represents a very low pitch on the propeller. Twenty MPH blade pitch speed would likely not be able to get your model off the ground, since most models have a stalling speed around that 20 MPH figure.

One good way to get a handle on all this stuff, is one of those computer programs such as www.motocalc.com, free for 30 days, then $39. With this program, you can put together a simple (or complex) model, outfit it with an appropriate motor/ESC/Battery, and try different propellers on it. Looking at motocalc's numbers and opinions, you can learn a lot without having purchased any expensive equipment.

Motocalc will spit out motor watts, motor current, voltage, efficiency, stalling speed, rate of climb, blade pitch speed, and a lot of other stuff.

[BBCorvette18]

I downloaded that program you always post (do you have a stake in that site lol) I need to play with it a LOT to try and get my head to understand what all its telling me lol. I don't see the specific motor I'm using so I dont know what all numbers to put in.

I played around with it a little bit and its interesting I was just wondering about a ballpark answer.

I've got a Scratch built foamy F-22 thats AUW is 10.2 oz. The motor and prop combo I have on it is supposed to make 12.5oz of thrust. with that combo it barely flies. I think I need to re calibrate the esc tho as I dont seem to be getting WOT out of it.

[hayofstacks]

My mini ultra stick, I played around with a lot.

A 10X5 prop gave it loads of thrust, it would take off in 5 feet, climb vertically all day, but probably only flew, i'd guess, about 45-50 mph without a dive. Throwing a 10X10 or 9X9 prop on it, it was very slow to take off, took forever to gain speed and height, but once it got there, it would shoot off like a rocket top end. Using a 11X5 or so pulled about the same amperage, but was reletively cruddy top end, but had very good verticle. Swapping to an 11X7, felt like you had no thrust and used quite a bit more amperage.

Same thing goes for when I swapped from a 1100kv to a 900kv on my alpha 450 sport. Verticle all day ong at 3/4 throttle with a 10X5 on the 110kv motor. With the 900kv motor, it flew well with a 10X5, but had very little differance in performance above that. Swapping it to an 11X5 or 11X7 gave unlimited verticle at 1/2 and maybe 13 amps, but lacked top speed.

[kyleservicetech]

Originally Posted by BBCorvette18

I downloaded that program you always post (do you have a stake in that site lol) I need to play with it a LOT to try and get my head to understand what all its telling me lol. I don't see the specific motor I'm using so I dont know what all numbers to put in.

I played around with it a little bit and its interesting I was just wondering about a ballpark answer.

I've got a Scratch built foamy F-22 thats AUW is 10.2 oz. The motor and prop combo I have on it is supposed to make 12.5oz of thrust. with that combo it barely flies. I think I need to re calibrate the esc tho as I dont seem to be getting WOT out of it.

LOL, wish I had a stake in motocalc, Hacker, A123 cells, Spektrum radios, and who knows what else

As for the specific motor, you might luck out by googling that motor and see what shows up. Unfortunately, many of the very low cost motors don't provide the motor winding resistance, IMHO, one of the more important specifications on those motors.

As indicated, 12.5 ounces of thrust is nice on a model that weighs less than 12 ounces. But if that motor is using a big diameter, low pitch prop, the prop pitch airspeed might actually be lower than the flying speed of your model. That makes for a "lousy" setup!

Should this be the case, programs such as motocalc will flag it.

[Larry3215]

If I am remembering correctly, thrust varies with the square of the RPM and the cube of the diameter.

Prop pitch does not enter into the calculations - in theory. In practice, it does make a difference in flight, but RPM and diameter dominate.

So, if you double the RPM, and keep the diameter the same, you get 2 x 2 = 4 times the thrust.

On the other hand, if the RPM stays the same and you double the diameter, you get 2 x 2 x 2 = 8 times the thrust.

So, if you want more thrust, diameter is your best friend. However, its going to take more power no matter how you do it - a lot more power!

Power required varies at a different ratio that is one order of magnitude greater, so, if you double the rpm (at the same diameter), it will take 8 times as much power to get 4 times as much thrust - roughly.

If you double the DIAMETER (at the same RPM), it will take 16 times as much power to get 8 times as much thrust - roughly.

There are other factors to take into account but thats the gross formula and will get you in the ball park.

[BBCorvette18]

I'm running a 8x6 prop and its a HURC Emax 2805 1700kv on 2s it was barely flyable on 3s it flew decent but not like I expected. I'm going to play with it some more on Monday or Tuesday and see if I can get more RPM out of the setup via end points and/or recalibrating the esc.

Also thinking about running it on 4s, and or dropping the prop down to a 6 or 7 inch prop.

[Larry3215]

If you're after speed, then you want more rpm and a smaller diameter - up to a point. You need enough thrust to safely launch the model and to over come the drag in flight, so a super small prop wont work.

Keep in mind, that the cheepest way to increase speed is to reduce drag. Fair in linkages, sand corners, thin down trailing edges, wheel pants, etc.

Increasing speed follows the RPM formula as far as power required. So to double your speed takes roughly 8 times as much power if you are using the same size prop.

In your case dropping diameter sounds like a good option - just be sure your esc, motor, packs can handle the extra power load

[hayofstacks]

If you don't have good specs for the motor, and a watt meter, i'd recommend not doing that too much.

The plane also plays a large part in how the plane flys. Let's look at my power up 450 sport motor from heads up rc.

http://www.headsuprc.com/servlet/the...0-Sport/Detail

With my slowtcik, basically stock and a 1800 3cell, it flew a good 40 maybe even 45mph. Wasn't much performance increase playing with prop sizes. A 10X5 was the best prop for this. Good verticle(read: thrust) and a good top speed. Going to a 10X3.8sf prop, the amps were about the same, top speed was quite a bit lower, but I had more thrust with lower amp draw below 1/2 throttle. This could almost double my battery if I went wot a lot.

On my mini ultra stick, the 10X5 gave me a good 50, maybe even 60mph. The 10X7 felt the same, maybe a bit higher top end, and a 10X10 it felt like it had no power, unless you got it moving level first. With the 10X10 it could creap up to maybe even as high as 70 mph in level flight. All the props seemed to get about the same flight time, and were fairly close to the same draw, the 10X10 or course being highest.

On my alpha 450 sport, the 10X5 again worked best. Anything bigger didn't seem to change thrust much, and hurt verticle a bit, but not as obvious as the mus. A 10X3.8sf again got me quite a bit more flight time and lower draw at 1/2 or less.

Now my slow stick built as a bipe, my favorite prop was the 10X3.8sf. The plane had so much drag and would bubble up so greately that it would not go faster then 30mph in a dive. The 10X5 actuallt had the same thrust, but needed more throttle to fly top end. My "prop hanging" used quite a bit more throttle and amprage then the 10X3.8, and gave me the same verticle and thrust.

So this same motor on 3 planes, all weiging reletively closeto the same weight, flew completely different on all of them. The bipe slow stick was actually the second worst for flight times because it required so much throttle to over come drag.

Stock slow stick with ideal prop could get me 45min flight times regularly
Mus was 8-12 minutes, reguardless of prop or flying style. Some of this may havve had to do with me liking the zooming sounds it made on low level passes. Sounded like a pylon racer when it zipped by.
Alpha 450 would do 30-45 mins depending on prop and throttle
Slowstick bipe was about 20 mins on the 10X5 and about 30 on the 10X3.8sf.

There is actually quite a bit that goes into getting the ideal flight times, amp draw, verticle and top speed. All of these things need to be properly matched to get a good efficient combo. A prop change can make a drastic differance in performance. I actually have to prop up a bit for some planes due to my 4600ft elevation to get the same peak thrust and amp draw as you would at sea level.

[BBCorvette18]

The plane is a F22 scratch built dollar store foamy. AUW of 10.2oz and wingspan of 22"ish lol

[hayofstacks]

http://www.headsuprc.com/servlet/the...0kv/Categories

This is your motor correct?

I think you could get a lot of performance back by going to a slightly larger motor. I can't really comment much on motor and prop sizes for your particular setup. I have delt mainly with 450 sized motors and 9-11" props.

As a profile "jet", a lot of your power will be from speed rather then thrust. For instance, a higher speed prop will get the plane moving quicker, but it will not have as much thrust. It would take a lot more level flight to gain speed, the you would use the speed to gain altitude, rather then just going verticle at wot.

Basically, you want more thrust, and speed. Its likely that you need a good bit of airspeed to keep the plane in the air. If you can't get enough speed, then the plane will feel very sluggish and under powered because it won't be able to gain altitude without stalling, even though you have a lot of thrust.

[hayofstacks]

Think of a boat. It needs to get "on the plane" before it will get to its top speed. It has to overcome the drag of the boat and get the boat moving fast enough for the prop to become efficient. If you have too little pitch on the prop, it will never get enough speed to get on plane. Too high of a pitch, and the motor will not move enough water.

Although the prop produces thrust at every rpm, it will not make much at low speeds. It is basically stalling because it isn't moving enough air. When it moves fast enough to become efficient, its moving more air, and that's why the amp draw goes up. You might only have 3 or 4 oz of thrust until 1/2 throttle or so, then it will start moving more air and the amp draw will go crazy and by full throttle you should get to the 12oz of thrust and full "pitch speed". With a lower pitch prop, you will have more thrust at lower rpm's, maybe 10 oz at 1/2 throttle., but at full throttle it might only make 12. The air on the lower pitch prop will also have less air speed, even though the prop is moving the same speed, the air isn't moving as fast.

Pretend you have 12oz of thrust at 12 amps,this would be the low pitch prop.
12oz of thrust at 20 amps. Even though the thrust is the same, the 20amp setup is moving the air faster. Giving you a higher pitch.

You need to find the fine line between have enough thrust for good verticle, and enough speed to keep the prop from stalling and enough airflow over the wings.

Hope that makes sense, and I didn't just confuse you worse.

[BBCorvette18]

Nope thats pretty much what me and my father concluded I just wanted to make sure we were going in the right direction.

I was stuck on thrust is thrust if I have 20oz of thrust the object will move at Xmph. Which might be true for a turbine or something but when its a prop its different.

I think Im really over thinking everything lol.

Also when researching the build I didn't see that they used the 2800kv motor with (i think a 6" prop) I went with the 1700kv after looking at the thrust numbers and assuming that it would be sufficient to fly. Which technically it is it just doesn't fly good lol.

On the plus side this is the first Motor we put into a plane that wasn't WAY over powered. The other 2 we did fly at 1/4 to 1/2 throttle lol.

a watt meter is in our future as well.

[slipstick]

You need to look at both thrust and pitch speed.

In really simple terms for props, thrust = acceleration and pitch speed = top speed and you need to balance the two.

If you have low thrust but high pitch speed it will never manage to accelerate up to the theoretical top speed.

If you have high thrust but low pitch speed it will tear off like a shot but only up to the (low) maximum speed.

Steve

[CrimzonRider]

Originally Posted by BBCorvette18

I'm running a 8x6 prop and its a HURC Emax 2805 1700kv on 2s it was barely flyable on 3s it flew decent but not like I expected. I'm going to play with it some more on Monday or Tuesday and see if I can get more RPM out of the setup via end points and/or recalibrating the esc.

Also thinking about running it on 4s, and or dropping the prop down to a 6 or 7 inch prop.

On 3s, with a 8x6, that 10.2oz a/c should be blazing! What 3s batt you running? C value??

cr

per Headsup:

Propeller data for 3 cell Lipo batteries:

The GWS EP 8040 prop produces 19 ounces of thrust at 12 amps

The GWS EP 7035 prop produces about 14 ounces of thrust at 7.5 amps

The GWS EP 7035 x 3 prop produces about 15 ounces of thrust at 9 amps

The APC 7x3.8WSF prop produces about 16 ounces of thrust 12 amps

The APC 7x4E prop produces about 16 ounces of thrust 10.5 amps

The APC 7x5E prop produces about 15 ounces of thrust 11 amps


Propeller data for 2 cell Lipo batteries:

The GWS EP 9050 prop produces about 14 ounces of thrust at 9.5 amps

The GWS EP 8040 prop produces about 11 ounces of thrust at 7 amps

The GWS EP 8043 prop produces about 13 ounces of thrust at 8.5 amps

The GWS EP 8060 prop produces about 12.5 ounces of thrust at 10 amps

The GWS HD 8060 prop produces about 11 ounces of thrust at 9.5 amps


Please be aware that the battery used can make a tremendous amount of difference in the performance of brushless motors. The above data was obtained using batteries in good condition that were fully charged. Thrust and amp draw may be less with the use of batteries rated at lower amp output, and slightly more using batteries rated for higher amp output. Other factors, such as ESC timing and variations in actual motor KV can affect the performance of brushless motors as well. We recommend the use of a Watt Meter to test the current draw of your particular power system, especially if you plan to use a prop that pushes the motor near it's maximum amp rating.

[JetPlaneFlyer]

What Larry said.. thrust is proportional to RPM squared. if you have the same prop and you double the RPM then thrust quadruples.

As an interesting asside....In theory you can have massive thrust for very little power providing your prop diameter is enormous. In the real world you have practical limitations that prevent you running huge props but as an example of what can be done here is 200lb of thrust from about 500W of power

Human-Powered Helicopters: Straight Up Difficult on Vimeo

[BBCorvette18]

Running a DBY 1000mah 30c 2s. its from HURC and is brand new. the 3s is a turnigy 1000mah 20c IIRC. Like I said I dont believe I am getting WOT for some reason will be looking into that tomarrow and also trying out the 8040 prop I also have.

I'm trying to play with motorcalc to get a better prop setup but am not sure on how to obtain the proper motor spec. I know its a 140w 1700kv motor but I dont know how to turn that into what motorcalc wants.

[kyleservicetech]

Originally Posted by BBCorvette18

Running a DBY 1000mah 30c 2s. its from HURC and is brand new. the 3s is a turnigy 1000mah 20c IIRC. Like I said I dont believe I am getting WOT for some reason will be looking into that tomarrow and also trying out the 8040 prop I also have.

I'm trying to play with motorcalc to get a better prop setup but am not sure on how to obtain the proper motor spec. I know its a 140w 1700kv motor but I dont know how to turn that into what motorcalc wants.

What motocalc needs is the following:
Motor KV
Motor no load Amps
Motor winding resistance
Motor weight in ounces.

Generally the motor suppliers will provide everything but the winding resistance. The motor no load current can simply be measured with a wattmeter, and no propeller on the motor shaft.

The winding resistance is a bit more involved, here is how its measured:
http://www.wattflyer.com/forums/showthread.php?t=50740

[kyleservicetech]

Originally Posted by Larry3215

If I am remembering correctly, thrust varies with the square of the RPM and the cube of the diameter.

Prop pitch does not enter into the calculations - in theory. In practice, it does make a difference in flight, but RPM and diameter dominate.

Prop thrust is affected somewhat by the propeller blade pitch. This is easy to checkout in www.motocalc. Power input to the propeller is roughly proportional to the blade pitch, double the pitch results in near doubling the watts going in.

As others have indicated though, these are static measurements. In flight measurements can vary widely from the static measurements, again can be shown with that same www.motocalc.com program.

[solentlife]

Originally Posted by slipstick

You need to look at both thrust and pitch speed.

In really simple terms for props, thrust = acceleration and pitch speed = top speed and you need to balance the two.

If you have low thrust but high pitch speed it will never manage to accelerate up to the theoretical top speed.

If you have high thrust but low pitch speed it will tear off like a shot but only up to the (low) maximum speed.

Steve

Now I'm hearing better !

Diameter gives thrust ... pitch gives speed.

Mix these 2 with RPM and you get the result. So what I'm saying is props combine all 3 to give you a result.
If you have a slowflying WW1 Biplane - you want plenty of prop wash over surfaces as well as airflow. But you don't want speed that tears the wings of ... so low pitch, large diameter ... she'll pootle around all day.
Go for speed then its pitch for speed and then diameter to overcome airframe drag etc. ... with RPM to get the prop driving ... which usually ends up with small props.

Each person will have their preference even with same model as another ... it's a matter of trial to get what you want.

Nigel