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Thrust vs Power to Weight Ratios for Prop Planes

JDConover

Junior Member
#1
I'm scratch building the FT Cruiser and would appreciate all the help that anyone can provide in designing the power system from the ground up. I know that I can use the parts specified by Flite Test but I really want to learn the fundamentals for future builds.

I'm not into 3D flying yet so I want to keep the performance to a realistic setting. I imagine it would be difficult to fly a model in the same fashion as a full scale aircraft and would think that most would want more power/thrust to weight in their models. I say this because in a full scale aircraft you don't typically climb out at 45-60 degrees of pitch angle but you probably would with a model just to accommodate field size and maintain a visual with the model.

That all being said I have quite a few questions. I'll start with just a couple and hopefully keep the thread going until I'm out of questions. I know this isn't the first time for this type of question so if you know of any links please post.

1. When designing a prop/motor combination for a prop plane do you base it off of thrust to weight, power to weight or a little bit of both?

2. I have seen were some people like to use a 1:1 ratio for power or thrust to weight. Is this a typical goal for non 3D planes?

3. Do you typically size the motor to run at 100% for the initial climb-out then for the rest of the flight maintain 70-80%?

Thanks in advance to all who can help.
JC
 

pgerts

Old age member
Mentor
#2
Go for what is suggested for the FT Cruiser.
1 There are no general ratio. A slowflyer might have a 1:4 T/W ratio and an extreme racer a 4:1 ratio. You must know the type of plane and the size and weight before thinking of the power and speed you need from the motor/prop.
3 Many of my planes take of at less than 50% and fly normal at very low throttle. 100% is used short time in special parts of a maneuver. On the other side - a racer (pylon) plane might use 100% all the time.
 
#3
Most people start with Watts/lb.
100W/lb for modest flying and 250 or more for crazy aerobatics.
This where a Watt meter is a essential tool
a) so you can work out the Watts/lb by direct measurement (with a set of weight scales).
b) so you can avoid 'frying' anything with an inappropriate motor/ESC/battery combination.

Thrust is a rather a nebulous figure as it depends very much on the motor and prop (or fan) combination and whether it refers to static thrust or flight thrust. Flight thrust is the important figure but it is almost impossible to measure!
 

JDConover

Junior Member
#4
Thanks quorneng. I pulled the following of the internet as a starting point see if you agree.

50-70 watts per pound; Minimum level of power for decent performance, park flyer/slow flyer models
70-90 watts per pound; Trainers and slow flying scale models
90-110 watts per pound; Sport aerobatic and fast flying scale models
110-130 watts per pound; Advanced aerobatic and high-speed models
130-150 watts per pound; Lightly loaded 3D models and ducted fans
150-200+ watts per pound; Unlimited performance 3D and aerobatic models

Do you typically climb out at full throttle then maneuver at less than full or just constantly back and forth out of full? I'm curious what would give you the most life out of motor and battery. Looks like if you primarily operate somewhere around 80% or less it should save the motor and keep heat to a min. Perhaps this is over thinking it.
 

JDConover

Junior Member
#5
Go for what is suggested for the FT Cruiser.
1 There are no general ratio. A slowflyer might have a 1:4 T/W ratio and an extreme racer a 4:1 ratio. You must know the type of plane and the size and weight before thinking of the power and speed you need from the motor/prop.
3 Many of my planes take of at less than 50% and fly normal at very low throttle. 100% is used short time in special parts of a maneuver. On the other side - a racer (pylon) plane might use 100% all the time.
Thanks for your response. I agree that it would make sense and be a lot quicker to just order the parts as suggested by FT, but I'm trying to get a little more out of the project. The why and how are just as important to me as getting the plane in the air and flying around.

You mentioned that there were a couple items that I needed to know, type, size and weight.

The type is Multi-Engine.
As for size and weight I will have those numbers upon completion of the build minus the motor.

I was considering an allowance of 1.3 oz on the motor if i go with the suppo 2208/14 1450kv at 36g.
 

earthsciteach

Moderator
Moderator
#6
JD - I very much respect your desire to learn the design of power systems "from the ground up." The rules of thumb given above are very helpful and get the job done pretty well. But, as you can see, there is a lot of "wiggle room" within those rules. What seems like it should be a fairly straightforward design exercise becomes pretty convoluted given the lack of motor rating standards by manufacturers and inconsistencies in motor nomenclature. In a nutshell - figuring out this electric power stuff can be really frustrating!

Here is a decent place to start: Selecting Power Systems
 

JDConover

Junior Member
#7
JD - I very much respect your desire to learn the design of power systems "from the ground up." The rules of thumb given above are very helpful and get the job done pretty well. But, as you can see, there is a lot of "wiggle room" within those rules. What seems like it should be a fairly straightforward design exercise becomes pretty convoluted given the lack of motor rating standards by manufacturers and inconsistencies in motor nomenclature. In a nutshell - figuring out this electric power stuff can be really frustrating!

Here is a decent place to start: Selecting Power Systems
Hey earthsciteach! Thanks for the link. I'll read through it soon. Yeah I know it's going to be a challenge and at some point I will just have to allow some variable for the unknown. I was just trying to get my design as close to a science as possible. Some of the articles I read sound a lot like the way my buddies and I worked on our hot-rods during high school. "Here, put this 750 double pumper and 392 cam in and see what happens". While trial and error usually give you experience of what works and does not there is a science of determining the best starting point for your engine then fine tuning it.

That being said I can imagine myself eventually at the park asking the person next to me, "Hey that's cool, which motor and prop are you using".

By the way, awesome video on heat shrinking water/soda bottles. I plan to use that idea. I was curious if you had tried connecting a shopvac to the pvc pipe in the top while using the heat gun.
 

earthsciteach

Moderator
Moderator
#8
I have not tried the shop vac. That is a great idea - thanks!

The selection of the power system components would be easy given adequate and consistent info from manufacturers. Sadly, that does not exist. Or, at least its not published. It shouldn't be very different than selecting a pump impeller and motor.
 

squishy

Pirate ParkFlyer
#9
Building a power system the right way involves knowledge of the complete picture, how everything works and how it all interacts, then using the manufacture's recommendations as a starting point for your real world testing, even then, your bench testing is static. I like to say "there's only one way to find out" and that is when your airplane hits the wind. But you don't want to just be guessing and throwing on different props or batteries thinking it doesn't make much difference, it makes a HUGE difference, so much of a difference in fact that you could be flying right now at half power, half speed for the half the time you could be...The best thing to do is learn learn learn about how motors, ESC's and batteries interact, what all the numbers mean and not just on paper, the physicals of it. Buy a cheap watt meter and test test test...It took me a very long time, like two years of scratch building, before I truly grasped it enough to confidently help others with their systems, and I am still learning everyday...
 

earthsciteach

Moderator
Moderator
#10
JD - not to sound like a shill, but my favorite vendor is headsuphobby.com for many reasons. One of those is that the owner tests and publishes data for various prop/battery combos for a given motor.

For instance, this is the motor I use on my Surfer/Bixler clone: http://www.headsuphobby.com/Power-Up-450-Zoom-Outrunner-Brushless-Motor-H-225.htm

This is the data available on his site for the motor based on his bench tests:

Battery Size

Propeller Size

Thrust (oz.)

Amps

Notes

3-Cell 11.1v

EMP 6 x 3E

34 oz.

28 amps

 

3-Cell 11.1v

EMP 6 x 4E

33 oz.

29 amps

 

3-Cell 11.1v

APC 6 x 4E

30 oz.

29 amps

 

3-Cell 11.1v

APC 6 x 5.5E

31 oz.

35 amps

400 watts (avoid constant high throttle)

3-Cell 11.1v

APC 5.5 x 4.5E

22 oz.

28 amps

 

3-Cell 11.1v

APC 5.1 x 4.5E

25 oz.

30 amps

 

3-Cell 11.1v

EMP 5 x 5E

21 oz.

26 amps

 

4-Cell 14.8v

EMP 4.5 x 4.5E

23 oz.

25 amps

400 watts (avoid constant high throttle)

4-Cell 14.8v

APC 4.5 x 4.1E

21 oz.

23 amps

 


This is SO much more helpful than the usual amount of info given on other sites. Also, if you are unsure, send them a question and they respond within a day, usually within a couple of hours. I still run my setups past Jeff at headsup if I am unsure. More often than not, he has a better idea that costs the same, and in some cases, less than what I was thinking. Great resource!
 
#11
Your question about how much throttle to climb out is not really answerable.
Obviously if you have an unlimited vertical ability (200+W/lb) you don't need full power but you may use it to gain height very quickly.

On my Endurance plane I set the power (less than 1/4!) so it just climbs but then I am not trying to go anywhere fast. Only to stay up as long as possible.

For maximum overall efficiency as earthsciteach points out it does get rather complicated. Finding the very best motor/prop/battery arrangement can take quite a bit of trial and error and will vary depending on what sort of flying you want the plane to do.
 

JDConover

Junior Member
#12
EarthSciTeach,

That will be most helpful.


Squishy,
I appreciate your input and do understand there are a multitude of variables that can alter performance. However I plan to take the approach of understanding one concept or component at a time. Many of the hobbyists have pointed out that the numbers provided by the manufacturers are optimistic at best, and that's OK because I'm not really concerned about that at this point. All I'm really concerned about is designing around what the manufacture states that their product should perform. In other words lets say that I want a plane with a 1:1 thrust to weight ratio, my design might be for 1:5 just so that I know it will fly anyway. Standard barometric pressure of 29.92 and temp of 59F or 15C rarely exist at sea level, but that's what I intend to design around.

Let me give you and others an idea where I'm headed with this.

I've been kinda wobbling back and forth on taking the easy route of using the watts per pound guidelines shown above, and thrust to weight. I think I really want to attempt calculating the T/W ratio.

In the perfect world, and all things considered equal, 10oz of Thrust from the prop SHOULD suspend a 10oz craft from the prop with no aid from the wings. You and I know that's not going to happen, but theoretically it should.

So instead of shooting for 10oz of thrust lets go for 15oz (1.5:1 T/W) so that when we are done I should have something that will make it off the ground.

Assuming that I can find a motor that given it's efficiency limitations can provide enough power to swing a prop at 12,000 RPMs.

Without getting too deep yet lets start with calculating the static thrust for a 7x5 prop at http://personal.osi.hu/fuzesisz/strc_eng/

Static Thrust = 15.87oz
Required BHP = 0.149 or 0.149HP*745W/HP = 111.005W

Now lets look at the chart posted earlier to see what the suggested watts/lb is for a 10oz or 0.625lb plane would be.

"150-200+ watts per pound; Unlimited performance 3D and aerobatic models"

150*.625=93.75W to 200*.625=125W

This would become my point of reference. From here I can play around with prop sizes and pitches published motor ratings and speeds etc. Then go back and forth to the watt per pound schedule as a comparison.

What do you think?
 

JDConover

Junior Member
#13
EarthSciTeach,

I took a second to compare measured performance APC Style Props from Heads Up Hobby with predicted performance on http://personal.osi.hu/fuzesisz/strc_eng/ . While the results weren't consistent the differences did fall within a 20% margin. I thought it was very interesting. Noting the smaller the diameter the closer the predicted result.


JD - not to sound like a shill, but my favorite vendor is headsuphobby.com for many reasons. One of those is that the owner tests and publishes data for various prop/battery combos for a given motor.

For instance, this is the motor I use on my Surfer/Bixler clone: http://www.headsuphobby.com/Power-Up-450-Zoom-Outrunner-Brushless-Motor-H-225.htm

This is the data available on his site for the motor based on his bench tests:

Battery Size

Propeller Size

Thrust (oz.)

Amps

Notes

3-Cell 11.1v

EMP 6 x 3E

34 oz.

28 amps

 

3-Cell 11.1v

EMP 6 x 4E

33 oz.

29 amps

 

3-Cell 11.1v

APC 6 x 4E

30 oz.

29 amps

 

3-Cell 11.1v

APC 6 x 5.5E

31 oz.

35 amps

400 watts (avoid constant high throttle)

3-Cell 11.1v

APC 5.5 x 4.5E

22 oz.

28 amps

 

3-Cell 11.1v

APC 5.1 x 4.5E

25 oz.

30 amps

 

3-Cell 11.1v

EMP 5 x 5E

21 oz.

26 amps

 

4-Cell 14.8v

EMP 4.5 x 4.5E

23 oz.

25 amps

400 watts (avoid constant high throttle)

4-Cell 14.8v

APC 4.5 x 4.1E

21 oz.

23 amps

 


This is SO much more helpful than the usual amount of info given on other sites. Also, if you are unsure, send them a question and they respond within a day, usually within a couple of hours. I still run my setups past Jeff at headsup if I am unsure. More often than not, he has a better idea that costs the same, and in some cases, less than what I was thinking. Great resource!
 

JDConover

Junior Member
#15
I used 11.1 x kv and 14.8 x kv where applicable in the chart you posted. Something I'm really curious about is how actual rpm varies with larger diameter props as actual thrust diminishes compared to predicted thrust. If its a consistent curve actual thrust could be predicted.
 

xuzme720

Dedicated foam bender
Mentor
#16
The larger moment arm of a larger prop could be skewing the actual results by differences in environmental factors such as air density...

I don't know, just shooting in the dark here.
 

JDConover

Junior Member
#17
The larger moment arm of a larger prop could be skewing the actual results by differences in environmental factors such as air density...

I don't know, just shooting in the dark here.
If RPM remains constant or close to I would bet the loss in thrust is due to prop flexing.
 

xuzme720

Dedicated foam bender
Mentor
#18
If RPM remains constant or close to I would bet the loss in thrust is due to prop flexing.
Flex is a more likely culprit I would say. I was also thinking about, probably over-thinking, the scale effect of air as a fluid body and not scaling.
 

earthsciteach

Moderator
Moderator
#19
Well, we could undertake a little group project. Measure thrust, rpm, and amp draw at various throttle settings for various props. Say throttle settings at 1/2, 3/4 and full. We can then generate curves.
 

JDConover

Junior Member
#20
Well, we could undertake a little group project. Measure thrust, rpm, and amp draw at various throttle settings for various props. Say throttle settings at 1/2, 3/4 and full. We can then generate curves.
That would definitely be a beneficial project for this endeavor. Using a power supply instead of a battery, motors from a single manufacturer, and props from a single manufacturer would give the best data.

I also think recording the air density at the time of the test would be equally important. Barometric pressure in Hg, Relative Humidity, Temp and elevation. Then try to run the tests in a short time frame.