Prop Talk

[jap71173]

OK... so I’m a bit further along in my knowledge and skill level but I’ve hit a small stump when trying to understand props.

Here's what I think I understand: (feel free to correct my misconceptions)

1) increasing diameter will make your motor work harder but will push more air over your wing surface. (good for bigger planes).

2) a larger pitch will cause the prop to spin slower but bite more air (better for slower or 3D flight).

3) a general (but rough) rule of thumb is for every size in diameter you go up, you want to go down in pitch.

4) if your trying to keep your amp draw down because your motor out rates your battery & esc 3:1, then you use a smaller diameter prop with a larger pitch (ex: 10x4.7 could downsize to a 8x6) and this will lower the draw on the battery.

Here’s what I don't understand:

1) what is the difference between these props
APC 9x6e
9x6 SF
9x6 Direct Drive
9x6 Composite
9x6 GWS

2) what is the difference between a 9x6e vs. 9x6 thin

3) what method or rule do you use to determine what prop would work or not work with any given motor. (or what spec do you look at on the motor to answer this question)

4) and finally, whats the difference between "pitch speed" and "thrust"

Thx in Advance

[slipstick]

1. Difference is manufacturer and type.
2. Depends on manufacturer.
3. Either the manufacturer's recommendation or Motocalc or similar calculation program. It's a real pain working it out manually.
4. Pitch speed is the max. SPEED the prop will theoretically product. Thrust is the PULL it will theoretically produce. Thrust relates to acceleration and climb. Pitch speed relates to top speed.

BTW #2 of "what you already understand" is wrong. Higher pitch gives you more speed at the expense of lower thrust and so it's generally better for FAST planes. Slow flying and 3D need lots of thrust but little speed.

#3 also doesn't make much sense unless you include "if you want to keep the load on the motor more or less the same".

Steve

[CHELLIE]

Originally Posted by jap71173

OK... so I’m a bit further along in my knowledge and skill level but I’ve hit a small stump when trying to understand props.

Here's what I think I understand: (feel free to correct my misconceptions)

1) increasing diameter will make your motor work harder but will push more air over your wing surface. (good for bigger planes).

2) a larger pitch will cause the prop to spin slower but bite more air (better for slower or 3D flight).

3) a general (but rough) rule of thumb is for every size in diameter you go up, you want to go down in pitch.

4) if your trying to keep your amp draw down because your motor out rates your battery & esc 3:1, then you use a smaller diameter prop with a larger pitch (ex: 10x4.7 could downsize to a 8x6) and this will lower the draw on the battery.

Here’s what I don't understand:

1) what is the difference between these props
APC 9x6e is a thin prop made by APC for electric
9x6 SF is a prop with fat blades to draw lots of air at lower RPMs
9x6 Direct Drive is a prop ment to go directly onto the motor
9x6 Composite is a prop made out of Exotic strong Materials like Cf, made for High RPM and speed
9x6 GWS is a prop Made By Grand Wing Servo, normally with a 3mm hole, is ment for slow to medium speed planes, there are very inexpensive props, but do the job, not ment for high speed

2) what is the difference between a 9x6e vs. 9x6 thin not much difference if its a APC prop, 9x6e prop is a general prop for electric motors, thin props are normally used for speed and high RPM

3) what method or rule do you use to determine what prop would work or not work with any given motor. (or what spec do you look at on the motor to answer this question) KV, high KV motors use small Dia prop, Low KV motors use Big prop

4) and finally, whats the difference between "pitch speed" and "thrust" pitch speed is an estimate of speed given a prop pitch and motor RPM based on battery voltage, Kv and Pitch of prop, thrust is what a power system is able to deliver in real time, no guess work here, its best to use thrush ver Watts to determine a planes power requirements, because with thrust you know what you have, Watt is a little confusing for a newbie, because a small motor can be made to draw a lot of watts with a small prop and yet have very little thrust.

Thx in Advance

Look at the motor pages at Heads Up Rc, its a great guide for his motors and other motors too, based on motor size,kv and prop size, Jeff deals with Thrust, and thats the best indicator to use IMHO for the newbie just starting out, also his power systems are balanced, motor, prop esc and lipo, so the guess work has been taken out, but its best to always use a Wattmeter to check your set up, Hope that helps, Chellie

http://www.headsuprc.com/servlet/the...lectric/Detail

[Rockin Robbins]

I've gone down that same trial and terror road myself lately trying to put together a power system for a Slow Stick. Resolutely ignoring Chellie's advice (Heads Up was out of stock) I found two things that are immensely useful.

First, there is a free program, Drive Calculator with a medium large database of motors, ESCs, battery packs and propellers. It has a sorta large database of test numbers for the combinations and it is a moderated community project to ensure volume and quality of data. It will match motors to propellers, toss out the ones that won't work and let you see the numbers on all the legitimate options. It's really good about telling you what is concrete test values and what are calculated values too. This combination, for instance, I chose to do without as sometimes Turnigy motors are overrated and this prop exactly matches maximum rated watts for the motor. I'd feel more comfortable at 18 watts or so. Time to back off on the pitch and/or diameter. My best numbers came from an APC 11x4.7 SF. Seems like too much difference, but the large blade area of the slow flier prop makes more difference than the reduction in both diameter and pitch. Manufacturer of prop and prop type is very important in expected effect. You're not going to figure all that our with pen and paper.



Secondly, there is a little-used option in ESCs--the Turnigy Super Brain series. For about a $10.00 premium, which takes the place of about $100 in test equipment you can have an "Integrated flight data logger, records data for the ENTIRE flight such as:
- Voltage
- Current
- Temperature
- Motor RPM
- Throttle travel"

Here's the graph from my first motor run with a 10x8 Zinger three blade that I had lying around from 1983:



Now this is test stand data but nothing takes the place of in-flight data and that's what these puppies do, letting you download the info to your computer and save it. Between these two and paying close attention to manufacturer's information, reviewers and customer feedback you can pick with confidence and verify that it works as expected. Did you notice that totally unvetted prop that can't be purchased any more had a peak draw of 18.8 watts? Talk about dumb luck! I might run it later but I'll start out with the slow flier prop.

[kenchiroalpha]

Hi
Read over the enclosed PDF lots of good info on Model props
Hope this helps
Take care dear friend
Yours Hank

[dkrhardy]

Waaaay over my head! The calculators are very good, I'm sure, but I really don't want to spend days/weeks trying to learn all of this. I spent 40+ yrs in a changing tech field and I'm tired of absorbing material. That is why I ask questions that are valid for this one time kind of thing. That is also why I call the manufacturer or hobby shop to get the correct "fit" for my project.

I am getting lazy as I age. I pay people to do jobs I know I am not qualified to do. And I ask questions of the folks that sure seem to know what they are doing. I am jealous of the time and effort that folks put in to gain that knowledge! It is good to see you passing on your skills.

BTW, when I rode motorcycles I only rode on nice sunny days!

[CHELLIE]

Originally Posted by dkrhardy

Waaaay over my head! The calculators are very good, I'm sure, but I really don't want to spend days/weeks trying to learn all of this. I spent 40+ yrs in a changing tech field and I'm tired of absorbing material. That is why I ask questions that are valid for this one time kind of thing. That is also why I call the manufacturer or hobby shop to get the correct "fit" for my project.

I am getting lazy as I age. I pay people to do jobs I know I am not qualified to do. And I ask questions of the folks that sure seem to know what they are doing. I am jealous of the time and effort that folks put in to gain that knowledge! It is good to see you passing on your skills.

BTW, when I rode motorcycles I only rode on nice sunny days!

X2 Yes those Motor Calculators are way over my head too, they are great if your a Geek/Nerd Type in to all that Gismo stuff I am a baby boomer, i like things That are Simple Thats why i use tried and proven specs used by Heads Up Rc, Jeff has motor, prop, esc and lipo recomendations on his motor pages, with thrust measurements, amp draw Etc already done for us not so Sabby with computer motor programs, Jeffs motor charts also work with other motors of similar size and KV, that has made it easy for me and others to get a power system with in a safe amp draw, Hope that helps, Chellie

[FlyWheel]

Originally Posted by Rockin Robbins

<snip>
I've gone down that same trial and terror road myself lately trying to put together a power system for a Slow Stick. Resolutely ignoring Chellie's advice (Heads Up was out of stock:-)) I found two things that are immensely useful.

First, there is a free program, Drive Calculator with a medium large database of motors, ESCs, battery packs and propellers. It has a sorta large database of test numbers for the combinations and it is a moderated community project to ensure volume and quality of data. It will match motors to propellers, toss out the ones that won't work and let you see the numbers on all the legitimate options. It's really good about telling you what is concrete test values and what are calculated values too. <snip>

Is Drive Calculator the same as MotoCalc (it sounds like it), or is it something else entirely?


Posted via Mobile Device

[kyleservicetech]

Originally Posted by jap71173

OK... so I’m a bit further along in my knowledge and skill level but I’ve hit a small stump when trying to understand props.

Here's what I think I understand: (feel free to correct my misconceptions)

1) increasing diameter will make your motor work harder but will push more air over your wing surface. (good for bigger planes).

2) a larger pitch will cause the prop to spin slower but bite more air (better for slower or 3D flight).

3) a general (but rough) rule of thumb is for every size in diameter you go up, you want to go down in pitch.

4) if your trying to keep your amp draw down because your motor out rates your battery & esc 3:1, then you use a smaller diameter prop with a larger pitch (ex: 10x4.7 could downsize to a 8x6) and this will lower the draw on the battery.

Here’s what I don't understand:

1) what is the difference between these props
APC 9x6e
9x6 SF
9x6 Direct Drive
9x6 Composite
9x6 GWS

2) what is the difference between a 9x6e vs. 9x6 thin

3) what method or rule do you use to determine what prop would work or not work with any given motor. (or what spec do you look at on the motor to answer this question)

4) and finally, whats the difference between "pitch speed" and "thrust"

Thx in Advance

There are a number of computer programs that predict performance of your models and various motor/prop/battery combinations. The one I've settled on is www.motocalc.com, free for 30 days, then $39.

You can plug in a typical motor and battery, then put in a range of props from say 8X3 to 10X6, with one inch increments in blade and pitch specifications. Then Motocalc will spit out a whole lot of info on those props, starting with 8X3, then 8X4, 8X5, 8X6, then 9X3, 9X4 and so on.

Pitch speed is an indication of the velocity of the air passing through the propeller as the motor is spun up. The faster the RPM of the prop, and the higher the blade pitch, the faster that air velocity will be. Propeller thrust is just that. If you put your model on the driveway, tie a spring scale to the back of the model and fire up the motor, the spring scale will show the thrust value of your setup.

Things to watch for is the current and watts, and on the far right of the printout, the blade pitch air speed. That pitch speed needs to relate to the type of model you are flying. A small diameter high pitch prop could have the same wattage as a large diameter, low pitch prop. But as an extreme, that large diameter, low pitch prop might not fly your model fast enough to get over the models stalling speed. That's something modelcalc will warn you about.

Modelcalc also predicts rate of climb, useful information. But this program is only as accurate as the specifications of the motor you are using. And, sadly, some of the real inexpensive import motors provide specifications not even close to their actual values.

The attached screen dumps shows motocalc on the data entry screen, and printouts showing on the left side different props ranging from a 12-8 to a 16-12 prop. If you just specify one propeller size, motocalc spits out information on your model, and gives an opinion on how it will fly.

[jap71173]

Originally Posted by CHELLIE

Look at the motor pages at Heads Up Rc, its a great guide for his motors and other motors too, based on motor size,kv and prop size, Jeff deals with Thrush, and thats the best indicator to use IMHO for the newbie just starting out, also his power systems are balanced, motor, prop esc and lipo, so the guess work has been taken out, but its best to always use a Wattmeter to check your set up, Hope that helps, Chellie
http://www.headsuprc.com/servlet/the...lectric/Detail

Although i greatly appriciate any and all answers to my question, Chellie wins the cake... I'm not just saying that because she's my favorite Wattflyer... it's because as usualy, her answer is short and simple yet very effective... I can actually understand that langauge... whereas the motor program and charts are too much info to absorb.

Thanks again Chiellie...

BTW "Heads Up RC" is really a great place to learn alot AND to buy the parts i need... super fast shipping at great prices! Awesome!!! Thx

[Rockin Robbins]

Drive Calculator is entirely free and seems to be very accurate as it uses tested data on the different combinations of ESC, prop, engine and battery pack from testers all over the world. It is also VERY easy to use and doesn't tax your brain in the slightest. It is very clear on what numbers are actually tested numbers and which ones are calculated. I don't think MotoCalc makes any such distinction. The only frustration is when the motor you're considering isn't in the database. MotoCalc is a proprietary, commercial piece of software.

But yes, info from a seller like Heads Up RC who care about their products and tests them in different combinations on various airframes can short-circuit the process and give you pretty dead on and dependable information. And without checking Drive Calculator against their recommendations I wouldn't trust the computer program.

But without the program I wouldn't have had the confidence to whip over to Hobby King, find out my chosen motor wasn't in stock and pick the Turnigy 2217-20, knowing it was a great match for the Slow Stick. I probably would have ended up going over to Eflight and paying three times more for a motor that I knew would work. I doubt that I could have obtained personal advice on that little known motor. But it's a very impressive piece of machinery.

I really think being intimidated by a little piece of software that works well and logically is a bit silly. Now if it were made by Microsoft where the functions you need every day are buried three menus deep and the ones you might use once in a lifetime are a one button proposition, THEN I'd be intimidated!

[jap71173]

Originally Posted by Rockin Robbins

Drive Calculator is entirely free and seems to be very accurate as it uses tested data on the different combinations of ESC, prop, engine and battery pack from testers all over the world. It is also VERY easy to use and doesn't tax your brain in the slightest. It is very clear on what numbers are actually tested numbers and which ones are calculated. I don't think MotoCalc makes any such distinction. The only frustration is when the motor you're considering isn't in the database. MotoCalc is a proprietary, commercial piece of software.

But yes, info from a seller like Heads Up RC who care about their products and tests them in different combinations on various airframes can short-circuit the process and give you pretty dead on and dependable information. And without checking Drive Calculator against their recommendations I wouldn't trust the computer program.

But without the program I wouldn't have had the confidence to whip over to Hobby King, find out my chosen motor wasn't in stock and pick the Turnigy 2217-20, knowing it was a great match for the Slow Stick. I probably would have ended up going over to Eflight and paying three times more for a motor that I knew would work. I doubt that I could have obtained personal advice on that little known motor. But it's a very impressive piece of machinery.

I really think being intimidated by a little piece of software that works well and logically is a bit silly. Now if it were made by Microsoft where the functions you need every day are buried three menus deep and the ones you might use once in a lifetime are a one button proposition, THEN I'd be intimidated!

RR...sorry if it sounded like I’m dismissing the software approach... I’m really not... as i stated "too much info to absorb"... simply means that even though the program is very useful and gives tons of great information... it's useless to me if i still don't know what it means. To clarify even further... so the program tells what motor works with what prop... that’s great, but what did i learn exactly... i still don't even know what "2217-20" means on a Turnigy motor... it looks like a incorrect phone # to someone just starting as myself.

But when you speak newbie language like "high KV motors use small Dia prop, Low KV motors use Big prop" or "9x6 SFis a prop with fat blades to draw lots of air at lower RPMs" then that is stuff i can understand and actually use right now when i go to the hobby shop to by props for my plane.

Don't get me wrong RR... I have already downloaded the program and installed it... it looks like a very useful tool and I'm sure I will use it "eventually" but not until i understand "HOW" props work and their differences.

thank you for all your answers... the more the merrier

[Rockin Robbins]

I'm sorry if you misunderstood me too. For me it's fun to play with the numbers and fool around with computer software. For some reason I was born with the ability to sense how things should work there, even though at age 56 I'm from the wrong generation for that.

You think I'm going to look down my nose at someone like Chellie, who has more experience in her little finger than I have in my entire body? If my computer program says a prop will work and she says it won't, I'm listening to her.

Math is only a language that describes things. Contrary to popular opinion and the opinion of many cosmologists, math can describe things that are and things that are not. The math will be equally valid in both cases, but in one case it is verified by reality and in the other case it is falsified by reality. Have you ever thought how obvious it is that heavy things fall faster than lighter objects? It's easy to construct a mathematical formula to reflect that. But if you actually drop them off a tall building they will insist on ignoring your elegant math and falling at exactly the same speed.

So for me it's fun to play with programs like Drive Calculator and see how they stack up against reality. If they don't it's fun to figure out why. If you get a program you can trust, then it can be used (sometimes) to enable you to try that prop that nobody in the group has any experience with, or that Turnigy 2217 motor nobody has ever talked about aside from Flite Test's flying cow.

But everybody has different likes and dislikes, different skills and abilities. That's why a place like Wattflyer is great because people of different outlooks like me and Chellie and you and gdhampton can toss around different opinions and most of the time agree that there are always at least several ways to do something right. Here you get to be exposed to a lot of them!

As for understanding how props work, Hank uncovered a gem in post #5. For the math challenged, just pretend the math is no big deal, admire the pretty greek letters and mathematical symbols in there and you'll fake the thing out. Just plow through the stuff you don't understand and pluck out the good stuff. It does a good job of explaining without benefit of the formulae. You're interested in learning general theory and language and that's all in there in plain english.

[SkyCadet]

I always suggest getting to the REAL numbers, using drivecalc as a guide. When I coverted a Lancair ES to brushless, I wanted some more power and yet decent flight times. I went with a Park 450 BL, 890Kv motor, and 25A ESC, running 3S 2100 20C Lipos. What I did before converting the plane was to get a good meter [Watts up or similar is great!], and a RPM meter [GP]. I planned to use a 9x7x3-blade on it, and wanted to evaluate the static performance under various throttle and WOT times.

I designed a simple static mounting scheme, and then bench tested the power system. By doing so, it not only told me what to expect on a worst-case scenario [static testing is the worst case, since a plane in the real world moves through the air once it gets going], but it also showed what I was to expect from a thrust perspective, so I could estimate what I'd see when done.

The power system I used works VERY well - nearly 3D, definitely sport performance, and with average flying, 20-22 minutes on a battery, say 15-18 with more aggressive flying.

BTW, for all here, my REAL WORLD testing has shown that in general, if chosen well, a 3-blade can provide more thrust for a given motor, despite drawing more current. My MAS 3-bladers only draw 12-15% more power, but yield ~30% or more pull. Okay, that's an estimate, but pretty close. Also, none of us generally fly WOT all the time. Under moderate throttle settings, I get nominal performance to the 2-blades I have used in the past. In fact, the extra pull helps out in near-stall conditions or severe climbouts during landing aborts.

So, in conclusion, if you plan to fly electric, do a little more to understand WHAT drives the plane/heli, so you can effectively fly safely, and with more enjoyment.

[PS - I also have a PZ SuperDec BL flying a MAS 8x6x3 and was told both by the LHS and PZ themselves that I'd overdrive the power system. Testing showed this not to be the case, and with over 300 flights to date, still going strong - on ONE original 3-blade prop!]

SkyCadet

[SkyCadet]

By doing the testing and evaluating RPMs at different throttle settings, one can also estimate [using the theory for thrust estimate, and pitch speed for top speed estimates] how fast the plane will fly, too. Also, using the REAL current draw data, and voltage/wattage in, one can evaluate th efficiency of their systems, and the flight times to be expected.

Good stuff to know...

SC

[Rockin Robbins]

Originally Posted by SkyCadet

........<whatdya mean I can't quote the siggy? All right I'll type it myself>

3-blades are better!

Well, by fortuitous circumstance and dumb luck I will have the opportunity to check that out, comparing the Zinger 10x8 3 blader against the APC 11x4.7 SF two blader. That's a bit in the future but you've given me hope that the comparison will be interesting!

[CHELLIE]

Originally Posted by Rockin Robbins

I'm sorry if you misunderstood me too. For me it's fun to play with the numbers and fool around with computer software. For some reason I was born with the ability to sense how things should work there, even though at age 56 I'm from the wrong generation for that.

You think I'm going to look down my nose at someone like Chellie, who has more experience in her little finger than I have in my entire body? If my computer program says a prop will work and she says it won't, I'm listening to her.

Math is only a language that describes things. Contrary to popular opinion and the opinion of many cosmologists, math can describe things that are and things that are not. The math will be equally valid in both cases, but in one case it is verified by reality and in the other case it is falsified by reality. Have you ever thought how obvious it is that heavy things fall faster than lighter objects? It's easy to construct a mathematical formula to reflect that. But if you actually drop them off a tall building they will insist on ignoring your elegant math and falling at exactly the same speed.

So for me it's fun to play with programs like Drive Calculator and see how they stack up against reality. If they don't it's fun to figure out why. If you get a program you can trust, then it can be used (sometimes) to enable you to try that prop that nobody in the group has any experience with, or that Turnigy 2217 motor nobody has ever talked about aside from Flite Test's flying cow.

But everybody has different likes and dislikes, different skills and abilities. That's why a place like Wattflyer is great because people of different outlooks like me and Chellie and you and gdhampton can toss around different opinions and most of the time agree that there are always at least several ways to do something right. Here you get to be exposed to a lot of them!

As for understanding how props work, Hank uncovered a gem in post #5. For the math challenged, just pretend the math is no big deal, admire the pretty greek letters and mathematical symbols in there and you'll fake the thing out. Just plow through the stuff you don't understand and pluck out the good stuff. It does a good job of explaining without benefit of the formulae. You're interested in learning general theory and language and that's all in there in plain english.

Yea Another baby Boomer I am 56 Too, I am Old School, I agree that 10 Different people will have 10 different ways of doing something, as long as the out come is correct, thats all that matters This E Power stuff is Hard enough to understand, I know when i first started with e Power, i did not think i was going to like it, I have flown Glow For Many years, My first e power plane was a GP profile with a geared brush motor, and a lipo, Dang i struggled with trying to find out how to charge my lipo, I finally got a triton charger with a Hyperion Balancer, ok that worked ok, then my power supply was not up to par, and i kept getting trouble codes, that drove me crazy, the GP profile plane was flimsy it had CF braces, it flew but not very well, I was happy when a Cell Phone tower Jumped out in front of it i used the geared brush motor from that, and put it in a 25 nitro Cub from Hoppy people with a 9v car battery it was to heavy to get off the ground, it taxied real fast those were my Humble beginings with E Power I learned fast, Everyone here has helped me to understand this E Power stuff, I struggled for about 1 year to fully Grasp E Power, So I know what the Newbie is going Through, Been there Done That, So I try to Explain this E Power in as Simple Way as possible, so others can Grasp it, I am no Expert by any means in E Power, but I have Learned enough to help others just Starting out to be Successful and have fun and to try to take out the Flustration of trying to set up a power system, that seems to be the hardest part of this hobby with e Power, Jeff @ Heads Up Rc has been a Great help to me when first starting out, i learned a ton from his Motor Site, and so I pass that on to others, after being in this E Power Hobby for a little while, then a person can Advance to more powerful set ups and use Useful tools Like a Motor Calculator to achive that, but for a newbie just starting out, and i know that when i first started with e Power, Motor calculators would have been Way over my Head, because i did not have a good understanding of e Power systems Yet, Take care and have fun, Chellie

[kyleservicetech]

Originally Posted by CHELLIE

and put it in a 25 nitro Cub from Hoppy people with a 9v car battery it was to heavy to get off the ground, it taxied real fast those were my Humble beginings with E Power I learned fast,

Put me with the old folks on electric power. My first electric models (in 1982) used motors "I'd borrowed from work", rewound for more power, made gear boxes for them. Even made up home made brushes for the motors from parts out of an automotive alternator. It was a GOOD day when one of those motors hauled my 8 foot sailplane up to about 200 feet before the motor burned out. Many many more rewinds on many more motors and I came up with something that actually worked.

Until one of those Astroflight Geared 40 motors showed up at my house. That was the end of rewinding motors and wondering if it would burn up.

Now with the high powered brushless outrunner motors, ready to go out of the box, at reasonable prices, those old fun days are long gone.

And, you can use your computer and this ugly model software to get you into the ball park on the motor/ESC/battery setup before buying something that is either to big, or to small for your model.

Things have really improved from the "good old days"!

[Rockin Robbins]

I say copy someone else's success exactly! That way you have a plane that flies good while you go through the trial and terror of figuring out what works and why (or why not). When a project won't fly right, pull out the copycat plane and work out your frustration!

Holy cow! I'm not even taking my own advice. I'm doing it wrong!

[JetPlaneFlyer]

Originally Posted by SkyCadet

By doing the testing and evaluating RPMs at different throttle settings, one can also estimate [using the theory for thrust estimate, and pitch speed for top speed estimates] how fast the plane will fly, too.

I'm curious to know how you would estimate a planes speed based only on pitch speed and thrust i.e. not considering the aerodynamics of the plane itself?

I've always considered top speed to be a difficult thing to estimate because IMHO it depends very much on the drag of the airframe. For instance take the same powertrain and put it in:

1. A large WWI triplane with big draggy cowl, undercambered wings, plus lots of struts and wires.
2. A small sleek pylon racing model.

You will be likely to see a huge difference in top speeds, despite having the same power, same static thrust and same pitch speed.

Steve

[kyleservicetech]

Originally Posted by JetPlaneFlyer

I'm curious to know how you would estimate a planes speed based only on pitch speed and thrust i.e. not considering the aerodynamics of the plane itself?



I've always considered top speed to be a difficult thing to estimate because IMHO it depends very much on the drag of the airframe. For instance take the same powertrain and put it in:

1. A large WWI triplane with big draggy cowl, undercambered wings, plus lots of struts and wires.
2. A small sleek pylon racing model.

You will be likely to see a huge difference in top speeds, despite having the same power, same static thrust and same pitch speed.

Steve

Good point, put a large diameter low pitch prop on a model designed for 100+ MPH, and it won't get off the ground. Or put a small diameter high pitch prop on a Piper Cub model, again, it won't perform well.

I've got one of those "How fast is it" modules that indicates the flying speed of my models. Most of my Kilowatt powered models came out in the 60-80 MPH area, straight and level.

In the late 1990's I had a giant scale Piper Cub that came in at about 50 MPH, by watching it pass over the ground with a stop watch. That model had a brush type motor running about a Kilowatt, turning a 20x10 wood prop at about 5900 RPM. Sold the motor and model, still got the 20x10 props.

[SkyCadet]

Originally Posted by Rockin Robbins

Well, by fortuitous circumstance and dumb luck I will have the opportunity to check that out, comparing the Zinger 10x8 3 blader against the APC 11x4.7 SF two blader. That's a bit in the future but you've given me hope that the comparison will be interesting!

Glad to help RR!

The big thing is to check the current when running close to the rails. I find that the 3-blade props DO draw a bit more on WOT, but so long as you're not overdriving the power system, you should be fine.

Not all 3-bladed props are equal: Some have poorer performance, due to the aspect ratio of the blade, blade shape, etc. Just watch out for the 10x8x3 Zinger - that's more pitch than the 4.7 pitched APC, and may not run quite as close. If your motor can easily handle it, then you're fine.

SC

[SkyCadet]

You're right, pitch speed is NOT top speed. Airframe drag and propeller inefficiency don't permit a plane reaching pitch speed. However, if one attempts to get in the 3x stall speed rule of thumb or higher as a pitch speed, the model will fly nicely. Without actually evaluating the WOT RPM value, you'll never know this value for sure. So, checking it before flying is a necessity if you want to make sure everything will work.

SC

[kyleservicetech]

Originally Posted by SkyCadet

You're right, pitch speed is NOT top speed. Airframe drag and propeller inefficiency don't permit a plane reaching pitch speed. However, if one attempts to get in the 3x stall speed rule of thumb or higher as a pitch speed, the model will fly nicely. Without actually evaluating the WOT RPM value, you'll never know this value for sure. So, checking it before flying is a necessity if you want to make sure everything will work.

SC

Agreed:
These "Outrunner" motors usually turn a big diameter prop at a low RPM, at least as compared to a comperable Glow engine.

So, I've got a Hacker A50-16S motor with a 16X12 APC-E prop that flys very well with a Showtime 50, with a measured airspeed on the order of 65 MPH. The prop turns at about 6000 RPM.

If you put a 4 stroke 70 glow engine on it, you'd need a prop with perhaps a pitch of 8 or so, due to the higher RPM of the glow engine.

It's interesting, one of my club members does have a 4 stroke 70 on his Showtime 50, mine outperforms his in climbing ability and general acrobatic flying. but he can fly for 15 minutes, mine is limited to 8 minutes.

[jap71173]

Ok... so I was flying my Multiplex Funjet yesterday since it was the only thing i have that will fly in 15+mph winds.

I've been flying it all this time with a APC 6x5 Prop and have gotten skilled enough to fly it 5ft from the ground at WOT...

So I decided it was time to experiment and so i changed the prop to a 5x5. Well, I tossed it up no different than all the other times and it basically did a nose dive into the ground!?!?

So i tried it again, this time tossed it much higher and harder and it struggled but once it got some pitch speed it was up high. It didn't seem to fly any faster than with the 6x5 prop, but it was highly obvious it did not have the initial thrust...

So someone tell me please, if i want to make it go faster just by changing the prop, which prop should I use next? I'm guessing a 5.25 x 4.75 would increase the speed considerably... am I wrong?