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F8F Bearcat - 125 MPH for $250

Old 02-07-2011, 03:10 PM
  #26  
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Originally Posted by 57sailplane View Post
quote "For the analytical part, start by analyzing your objectives. Do you want to go as fast as possible, as slow as possible, stay airborne as long as possible, minimize expense, maximize handling? What are you trying to achieve? Then we can start scaling a motor/prop to the performance criteria."

yes we already discussed this. and you can keep changing your neglected beating around the bush answer as many times as you like but i will still keep on asking the pertinent question at hand.

again what kind of tools do you use to do this, or formulas, or do you just go by what the manufacturer says or do you go by what others have done in the past to start scaling a motor/prop to the performance criteria?
First tool: define your objectives for a specific model. Without objectives, nothing of value can be offered. Think of it like walking into a car dealer and saying, "I would like a car please." That statement leaves 100% of the trade space open to the certain mistakes of those who pretend to know what's in your mind.
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Old 02-07-2011, 04:57 PM
  #27  
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ok,

lets say this plane. the one you have on this blog how did you go about setting up a power system for it. oh and by the way what motor is it you are using. or is it a secret.
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Old 02-07-2011, 09:49 PM
  #28  
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Originally Posted by 57sailplane View Post
ok,

lets say this plane. the one you have on this blog how did you go about setting up a power system for it. oh and by the way what motor is it you are using. or is it a secret.
It's not a secret, it's on my blog, linked int he first post in this thread. This plane was actually picked for an engine concept, one that would fit in a 40" class warbird and turn a big honkin' 4-blade prop. I picked the GWS 10x8x4 since it is well balanced and inexpensive, as 4-blades are so easy to break. Power will come from an inline twin engine, both turning the same shaft, one motor to turn one 10x8, the other motor to turn the other 10x8 in a 4-blade arrangement. Torque will be crazy, but that is true to scale.

I haven't completed it yet; so much to do, so little time.
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Old 02-08-2011, 02:12 AM
  #29  
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so the question remains how did you go about picking the 10 size motor for this plane? your blog says 10 size motor is this an eflite?
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Old 02-08-2011, 03:06 AM
  #30  
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Originally Posted by 57sailplane View Post
so the question remains how did you go about picking the 10 size motor for this plane? your blog says 10 size motor is this an eflite?
The planned motor will be a twin .10, but realistically I have a long way to go on that pending invention. I have been unable to get the Super Tiger .10 to spin a different 4mm shaft, even one that is about the same length--I've tried several. Obviously, the final in-line twin will need a longer shaft that the one supplied (though I might be forced to couple two). I'm not exactly sure why the motor won't spin a different, apparently identical shaft without faulting. More testing to follow.

As far as motor choice, this plane used a different process than you are using - I started with the motor in mind for a specific set of objectives: to build an inline twin engine warbird with enough power to swing a scale-looking 10x8x4-blade, treating that prop as two 10x8x2-blades on a single shaft.

My flying objectives are none, but I would like at least 100mph, preferably 125, to fit with my hopped up warbird theme. To fit the airframe to the engine, I decided that aroung 80% wood sheeting would be a requirement just to hold the thing together in bullet-like flight. I preferred a small wing to keep wing loading very high and handling predictable, as the plane goes transonic (seriously, high speed stability is my only concern).

Point is, this plane was matched to my objectives for a set of motors which were conceived in advance, so it went motors, objectives, airframe. That is probably not representative of how most hobbyists might think, which might go more like: airplane, motor, objectives. The best way is most likely objectives, airframe, motor, but this example shows there are valid exceptions.

As for the interim motor, it is one I had laying around unused: a Turnigy 36-35. It is a very heavy motor with a poor thrust to weight ratio, but running on 4-cells it matches up to my inferred objectives reasonably well until I get the twin running.

Since the Turnigy starts to wreak of acrid smoke after 2-3 seconds at full throttle turning the objective 10x8x4-blade, pulling around 62 Amps, I propped it with a custom 8x7.5 for now. To build the prop, I clipped a PZ 9.5x7.5 prop to get a wide chord propeller all the way out to the tips. To use a normal 8" twist profile, wouldn't develop sufficient thrust out around the radial cowl boundary. That prop draws about 45 amps right now, in the air moving flat out.

With the heavy Turnigy motor and big 2200mAh 4-cell required to feed it, the 60A ESC plane weighs about 41 oz--way too much for anything but speed with only a 35" span and balsa sheeted, tapered F8F wings. Wing loading is cool 25 oz/ft, for in-the-hundreds-handling.

It is definitely a solid wood bullet, and it lands like one too.
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Old 02-08-2011, 05:12 AM
  #31  
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Originally Posted by z-8 View Post
As far as motor choice, this plane used a different process than you are using - I started with the motor in mind for a specific set of objectives: to build an inline twin engine warbird with enough power to swing a scale-looking 10x8x4-blade, treating that prop as two 10x8x2-blades on a single shaft.

My flying objectives are none, but I would like at least 100mph, preferably 125, to fit with my hopped up warbird theme. .
125 Mph on a 10X8 prop, that would require some 15,000 RPM on that prop. (Per motocalc.com)
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Old 02-08-2011, 06:43 AM
  #32  
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Originally Posted by z-8 View Post
............Power will come from an inline twin engine, both turning the same shaft, one motor to turn one 10x8, the other motor to turn the other 10x8 in a 4-blade arrangement.
Why go to all that complexity when you could just go for a larger motor capable of generating twice the torque of the Super Tiger 10's with a similar kv
One large motor should work out lighter than two small ones and will certainly be cheaper and simpler. Or is it just for the sake of experimentation and to utilise hardware that you already have?

The real Bearcat used a double row radial but that was to keep the diameter down for better aerodynamics.. Cowl diameter is rarely a design constraint for electric motors selection and certainly isn't in the case of a Bearcat.

Steve
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Old 02-08-2011, 07:46 AM
  #33  
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Originally Posted by z-8 View Post
...I have been unable to get the Super Tiger .10 to spin a different 4mm shaft, even one that is about the same length--I've tried several. Obviously, the final in-line twin will need a longer shaft that the one supplied (though I might be forced to couple two). I'm not exactly sure why the motor won't spin a different, apparently identical shaft without faulting. More testing to follow.
You are not by any chance trying to drive two motors from one ESC are you? If so that would explain your problems.
Easiest solution is to use two ESC's.. It 'should' also work on one ESC if you build in some adjustability to allow the timing of both motors to be precisely synchronised.

There is no reason at all why a motor would fail to run if all you did was change it's shaft (unless something was not assembled correctly or was damaged, obviously).
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Old 02-08-2011, 10:00 AM
  #34  
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Originally Posted by kyleservicetech View Post
125 Mph on a 10X8 prop, that would require some 15,000 RPM on that prop. (Per motocalc.com)
Sounds about right.. A motor of about 1200kv running on 4 cells would spin at about that speed under load.
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Old 02-08-2011, 10:37 AM
  #35  
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Originally Posted by kyleservicetech View Post
125 Mph on a 10X8 prop, that would require some 15,000 RPM on that prop. (Per motocalc.com)
Well I can tell you that's wrong in about 2 seconds, without using a computer:

8" x 15000 rpm = 120,000 inches per minute = exactly 10,000 feet traveled in a minute. Just reaching 120 mph needs 2 miles a minute, or 5280 x 2, or 10,560 feet traveled. 125 mph would need about 4% more, or 11K feet traveled.

So ThrustCalc is off by 10%.

However, that is using an empirical methodology, not analytical. In other words, the only possible way to have the luxury of actually knowing the RPM achieved, in a particular model, at speed, in the air, is to measure the rpm of that model, at speed, in the air. By that time, you are already flying that model, at speed, in the air. That is hardly a help.

Anyone can convert miles/minute to inches/minute on the back of an envelop. That is all these "complex" computer programs are doing.
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Old 02-08-2011, 12:51 PM
  #36  
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Originally Posted by z-8 View Post
Well I can tell you that's wrong in about 2 seconds, without using a computer:
Or perhaps not..

Prop pitch is the distance a prop would 'screw' through the air if a prop worked exactly like the thread on a bolt screwing through a nut. That's not really how a prop works of course, props arent screws and the air is not a nut! A prop blade is actually a rotary wing. The prop has an airfoil shape that features significant camber. What that means is that even when the prop is moving through the air at it's full 'pitch speed' it still can generate thrust. This is exactly like a cambered airfoil that still generates lift even when it's chord line is at zero deg angle of attack.

Typically the 'zero lift angle of attack' might be -3 degrees measured from the chord line (depends on amount of camber). If we assume that zero lift angle of -3 degrees for the prop blade then we affectivly add 3 degrees to the blade angle which increases effective pitch by about 1" for the example we are looking at.. calculating out that means at 15,000rpm the prop still generates thrust up to a speed of abour 128mph.

I've no idea if motocalc considers the above in it's thrust/speed calculation but it possible that it does.

How fast the plane would actually go is much more complicated of course, drag needs to be considered which Motocalc only make a very rough approximation of..... Motocalc is far from perfect but it's a handy little tool to get in the right ballpark.. It's better than guessing!
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Old 02-08-2011, 02:05 PM
  #37  
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Originally Posted by kyleservicetech View Post
125 Mph on a 10X8 prop, that would require some 15,000 RPM on that prop. (Per motocalc.com)
Yea and I wonder how many blades will still be on the prop at half that number.

I have used that propeller before and can say it is a piece of junk, other than looking cool.
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Old 02-08-2011, 04:33 PM
  #38  
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motorcalc definatly considers loss of thrust from fuse and wing drag. if anyone that uses it notices at top speed the amount of thrust the motor is producing this is the thrust lost by drag of the plane. in a perfect sanario a plane with out drag would show an end result of 0 thrust being used as the plane is moving in the oposite direction like a bolt and nut. it also has options on eficiency of the prop as jetplaneflyer was talking about the prop is not a true bolt and nut in the sky. motorcalc also looks at airfoil design, angle of attack and camber. it also dose a basic look at fuse structure. be it very simple and basic and in no means is a wind tunnerl but if the user hones his skills using this instrument you can get dang close.

one of the biggest problems i have found with motorcalc is that most manufacturers of motors and exspecially batteries and even esc's dont put out enough info to use this calc. so the user either has to guess or use somthing simular. the problem is in the battery industry they are advancing so quickly its hard to determine what true life specs really are. i really need to get a meter to read the internal impedance of a battery. this would make a big differance in results. because just by changing this criteria alone by a minamal amount can make all the differance. so this is why i say you need to no your tools. i have gathered enough info to know were my batteries stand.


what kv of motor is your turnigy 3536 z-8?
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Old 02-08-2011, 05:05 PM
  #39  
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[QUOTE=57sailplane;783788]
one of the biggest problems i have found with motorcalc is that most manufacturers of motors and exspecially batteries and even esc's dont put out enough info to use this calc. so the user either has to guess or use somthing simular. the problem is in the battery industry they are advancing so quickly its hard to determine what true life specs really are. i really need to get a meter to read the internal impedance of a battery. this would make a big differance in results. because just by changing this criteria alone by a minamal amount can make all the differance. so this is why i say you need to no your tools. i have gathered enough info to know were my batteries stand.

QUOTE]

That's why I like those Hacker motors. No, they are not cheap, but, their specs are accurate, and you can run them at the power levels listed on their specifications with out worry about overheating issues. My new Hacker A60-16M motor, on a 12S2P A123 pack runs with in about 4-5% of www.motocalc.com predicted RPM on a 19X12 APC-E wide blade prop.

It will be interesting to see how that motor and power system compares with Motocalc when this model is put into the air, since my 80 Amp ICE HV ESC records the current, voltage, RPM and so on. (That will be awhile though, we still got two feet of snow on the ground around here!)

As far as being off by "10%", holding accuracy to within 10% is rather good, when you've got all the variables involved. Like first, assuming your propeller pitch is actually close to its specified value. I worked in a company that manufactured 34,000 volt, 800 amp three phase circuit breakers and their controls. For decades, holding that control's accuracy to plus/minus 10% was a real problem, given that this accuracy applied over the full temperature range of minus 40 to plus 140 degrees F. We were not able to hold plus/minus 1% until the control was computerized. And, that control's price STARTS at about $5000.00, and can easily go to several times that value with custom designs. Add to the controls cost, the $15,000 for the circuit breaker.

And, I suspect that flying our models in different ambient temperatures will affect the accuracy by far more than plus/minus 10%.
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Old 02-08-2011, 06:09 PM
  #40  
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Originally Posted by JetPlaneFlyer View Post
Or perhaps not..

Prop pitch is the distance a prop would 'screw' through the air if a prop worked exactly like the thread on a bolt screwing through a nut. That's not really how a prop works of course, props arent screws and the air is not a nut! A prop blade is actually a rotary wing. The prop has an airfoil shape that features significant camber. What that means is that even when the prop is moving through the air at it's full 'pitch speed' it still can generate thrust. This is exactly like a cambered airfoil that still generates lift even when it's chord line is at zero deg angle of attack.

Typically the 'zero lift angle of attack' might be -3 degrees measured from the chord line (depends on amount of camber). If we assume that zero lift angle of -3 degrees for the prop blade then we affectivly add 3 degrees to the blade angle which increases effective pitch by about 1" for the example we are looking at.. calculating out that means at 15,000rpm the prop still generates thrust up to a speed of abour 128mph.

I've no idea if motocalc considers the above in it's thrust/speed calculation but it possible that it does.

How fast the plane would actually go is much more complicated of course, drag needs to be considered which Motocalc only make a very rough approximation of..... Motocalc is far from perfect but it's a handy little tool to get in the right ballpark.. It's better than guessing!
All the "thrust calculators" only do a simplistic conversion from inches to miles, including MotoCalc. For this, we need a computer? Not.

That is one of the many points I make in my blog article to illustrate why such simplistic stuff is useless (and almost always counterproductive) in any discussion about real world aircraft/model propulsion.
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Old 02-08-2011, 06:13 PM
  #41  
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[QUOTE=kyleservicetech;783792]
Originally Posted by 57sailplane View Post
one of the biggest problems i have found with motorcalc is that most manufacturers of motors and exspecially batteries and even esc's dont put out enough info to use this calc. so the user either has to guess or use somthing simular. the problem is in the battery industry they are advancing so quickly its hard to determine what true life specs really are. i really need to get a meter to read the internal impedance of a battery. this would make a big differance in results. because just by changing this criteria alone by a minamal amount can make all the differance. so this is why i say you need to no your tools. i have gathered enough info to know were my batteries stand.

QUOTE]

That's why I like those Hacker motors. No, they are not cheap, but, their specs are accurate, and you can run them at the power levels listed on their specifications with out worry about overheating issues. My new Hacker A60-16M motor, on a 12S2P A123 pack runs with in about 4-5% of www.motocalc.com predicted RPM on a 19X12 APC-E wide blade prop.

It will be interesting to see how that motor and power system compares with Motocalc when this model is put into the air, since my 80 Amp ICE HV ESC records the current, voltage, RPM and so on. (That will be awhile though, we still got two feet of snow on the ground around here!)

As far as being off by "10%", holding accuracy to within 10% is rather good, when you've got all the variables involved. Like first, assuming your propeller pitch is actually close to its specified value. I worked in a company that manufactured 34,000 volt, 800 amp three phase circuit breakers and their controls. For decades, holding that control's accuracy to plus/minus 10% was a real problem, given that this accuracy applied over the full temperature range of minus 40 to plus 140 degrees F. We were not able to hold plus/minus 1% until the control was computerized. And, that control's price STARTS at about $5000.00, and can easily go to several times that value with custom designs. Add to the controls cost, the $15,000 for the circuit breaker.

And, I suspect that flying our models in different ambient temperatures will affect the accuracy by far more than plus/minus 10%.
A motor calc can't really be "off" by any percent until you measure the specific installation. For example, if a radial cowl obsures the entire prop diameter, thrust calcs will be off by 100% by definition. Any aircraft form in front of or behind the prop will have a proportional effect (see my blog article for a more detailed discussion). Or, a large battery or heavy motor will spike induced drag, changing the thrust requirements themselves by a massive amount. Etc, etc.

The 10% MotoCalc error was simply converting miles to inches, which is all any of these programs attempt to do.
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Old 02-08-2011, 06:55 PM
  #42  
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[QUOTE=z-8;783804]
Originally Posted by z8rc View Post

A motor calc can't really be "off" by any percent until you measure the specific installation. For example, if a radial cowl obsures the entire prop diameter, thrust calcs will be off by 100% by definition. Any aircraft form in front of or behind the prop will have a proportional effect (see my blog article for a more detailed discussion). Or, a large battery or heavy motor will spike induced drag, changing the thrust requirements themselves by a massive amount. Etc, etc.

The 10% MotoCalc error was simply converting miles to inches, which is all any of these programs attempt to do.

no kidding ya its a program. the program is only as good as the user. of course you or i am not going to put a 4 inch prop on the plane in this blog. would not make any sence. the user must dial what he persievs as being the best estimation. the calculater is only there to increase your chances of sucess. its physics you can only use the tools to get a more calculated guess.

not just shoot from the hip and say well this motor looks like it will work. or the manufacture says so.

how do think any of these model company's figure out which motors work best in which plane. some may just simply do trial and error but i bet the ones the give you specifics do so because they no the truth why. because that there are laws of physics that will get you close off the get go, why else would they give the specifics.

your arguments make no sense what so ever. why have physics teachers or people that study this criteria. yes there are flaws but you need to learn to work around them. even the flight simulates don't get it exactly right but people learn to fly off of these planes because they are close enough to the real life situation.

i understand that you want to disolve the use of motor calculators but it sure seems funny how the big dogs even provide calculators. I for one think motorcalc is one of the best.

i have even gone as far as taken my calculation from motor calc and the specifics from my aircraft and have jamed them into realfight sim and have gotten amazing results.

thats how well i know my tools. i get to fly my plane before i build it. and it might not be exact but its way closer than just shooting from the hip.

you keep on guessing which motor combo to put in your planes and i will stick with the physics and more scientific approach and i know one of us will most always be happy in there new build.

oh ya still want to know what kv your turngy motor is.

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Old 02-08-2011, 07:08 PM
  #43  
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oh and by the way you can build this plane and i could take this exact same setup and build it just like you do and be farther off of your results then any calculater just because of the inconsistancies of the cheaper motors, differances in batteries and what not. . so you see a person needs to do calculations and testing to come up with the best results. and why not use a caclultor that take a lot of the time, and guess work out of the equation.
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Old 02-08-2011, 09:14 PM
  #44  
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Originally Posted by 57sailplane View Post
oh and by the way you can build this plane and i could take this exact same setup and build it just like you do and be farther off of your results then any calculater just because of the inconsistancies of the cheaper motors, differances in batteries and what not. . so you see a person needs to do calculations and testing to come up with the best results. and why not use a caclultor that take a lot of the time, and guess work out of the equation.
I'm certainly not arguing the need to do some calculations before trying a motor, but it doesn't take a computer to convert miles to inches, either, which is all any "thrust calculator" does in the above example. The greater point is that you have to know both the chosen airframe form, the entire power system (the motor being one part), and most importantly, the user's chosen objectives before any calculations of value can be made.

Simply asking (and perhaps even more crazy: answering), "What's the best motor for a car?" is really not a starting point. Same concept, times ten, for zero-sum aircraft systems.

One perfect F8F Bearcat power solution maximizes time aloft, another perfect F8F solution maximizes top speed, another perfect F8F solution gives best handling, another perfect F8F solution yields a chosen level of compromise between some, another perfect F8F solution yields a chosen level of comprimise between all of the of above, and so on....
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Old 02-08-2011, 10:42 PM
  #45  
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you are correct as far as is dosnt take a computer do to the few calculations that you do to set up a power system but you are wrong in thinking that motorcalc only does a few simple calculations or even just take the simpelest factors into effect as i said before the most minute changes can change the whole outcome. not simple not simple at all,

yes we definatly need to now what we are looking for as an outcome.. lol come on man. your kidding right this really dosnt determine weather or not we use caculations or motocalc that the choice of the builder.

i agree with you in your simple example of that there is not a best motor to put in a car for all these differant out comes. well you keep on proving my point of corse there isnt that best motor but there is a best motor combo for each instance and by using a tool that uses multiple calculations you can arive at each instance faster and easier and i think a better oveall understanding of what you have then just doing a few caculations.

no need to go way off the subject as if this is some kind of car sale advertisement. we really no what we are after in each instance and and i guess most of us understand the vital need for such great tools.

you want senarios, lol well here you go.

its like this we have this dang tree in the yard and its leaning toward the house and the city says it needs to go. not hitting the house is the outcome we are after. are you with me here.. you see the house is of real interest not the car parked next to it. not the little kid walking by and not the wife standing next you.. lol

so you can use your few calculations this is like using a hatchet. go ahead take a wack. wack away man it takes a while lots of time and energy. wack, wack, wack man are you getting tired yet. come on wack some more.

now here i stand same senario but i got this great tool a program. very simular to what i like to call a chain saw.. wow man i got it cut down and the dang program threw in a couple of ropes to keep the tree from hitting the house. It could have hit the car or it may have it the kid or mayby my wife. i think it hit the car not sure oh ya i am way off the subject here lol..

are you done with the wacks yet.. go ahead take another one you no you want to..

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Old 02-09-2011, 12:45 AM
  #46  
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Originally Posted by 57sailplane View Post
you are correct as far as is dosnt take a computer do to the few calculations that you do to set up a power system but you are wrong in thinking that motorcalc only does a few simple calculations or even just take the simpelest factors into effect as i said before the most minute changes can change the whole outcome. not simple not simple at all,

yes we definatly need to now what we are looking for as an outcome.. lol come on man. your kidding right this really dosnt determine weather or not we use caculations or motocalc that the choice of the builder.

i agree with you in your simple example of that there is not a best motor to put in a car for all these differant out comes. well you keep on proving my point of corse there isnt that best motor but there is a best motor combo for each instance and by using a tool that uses multiple calculations you can arive at each instance faster and easier and i think a better oveall understanding of what you have then just doing a few caculations.

no need to go way off the subject as if this is some kind of car sale advertisement. we really no what we are after in each instance and and i guess most of us understand the vital need for such great tools.

you want senarios, lol well here you go.

its like this we have this dang tree in the yard and its leaning toward the house and the city says it needs to go. not hitting the house is the outcome we are after. are you with me here.. you see the house is of real interest not the car parked next to it. not the little kid walking by and not the wife standing next you.. lol

so you can use your few calculations this is like using a hatchet. go ahead take a wack. wack away man it takes a while lots of time and energy. wack, wack, wack man are you getting tired yet. come on wack some more.

now here i stand same senario but i got this great tool a program. very simular to what i like to call a chain saw.. wow man i got it cut down and the dang program threw in a couple of ropes to keep the tree from hitting the house. It could have hit the car or it may have it the kid or mayby my wife. i think it hit the car not sure oh ya i am way off the subject here lol..

are you done with the wacks yet.. go ahead take another one you no you want to..
Sure, the point is you have to define user objectives as the overriding consideration before starting on a solution. MotoCalc is at least a little more sophisticated than the static motor test graph builders, in that it at least acknowledges that user objectives, total power system weight, and aircraft form are important. Simply testing the motor component alone, on a static stand that has no relationship to any props design airspeed or installation error, or any of the myriad of errors and backwards assumptions I document in my blog article, is so useless as to actually be funny. I grant you MotoCalc is better in that respect, as previously stated.

That said, no simple thrust calculator can capture even mildly complex user objectives, or the actual aircraft type and form, and it disregards the iterative nature of aerodynamic impact. The iterations account for roughly 85% of each variable's total impact, i.e. the 6:1 rule. Correctly powering a plane for a mission is a massive computational undertaking, even if you have extensive data to model the airframe and desired flight regime using the most powerful super-computers that $multi-billion aircraft companies can muster on $multi-billion budgets. And still, planes rolling into service routinely return botched performance compared to expectations, sometimes by a VERY wide margin. So put me down as "skeptical" that a simple program, that, for example, converts miles to inches to figure achieved speed, can get within any useful percentage of anything much.

Anyway, just for grins, I ran the F8F numbers thru MotoCalc with "race" as my oversimplified objective (I really wanted to examine a wing loading range vs. top speed trade space, with all other variables minimized, then chose a winner). Of note, MotoCalc suggested an 8x6" prop, each blade of which twists AoA to generate equalized lift as a function of redial velocity squared from hub to tip. So even testing nothing else about it's suggestion we know, from prop diameter alone, that the calculated thrust will be 60% obscured by the F8F's large radial cowling. Quite a large miss.

Last edited by z-8; 02-09-2011 at 01:11 AM.
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Old 02-09-2011, 02:26 AM
  #47  
57sailplane
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so you ran the wizard this of course is just a starting point. you must refine the data as such putting in a prop no smaller then the one you require say 8.75 inches and put one somewhat larger say 10inches. motor calc will give you result for these and in between. you may find that the motor that the wizard picks may not be the best one as i hardly ever use the wisards choice and can come up with a better alternative giving me better results out of motorcalc. the wizard is only the begining of this program. it just gives an all around starting point filling in the actual calculator for you. as you stated it can not see that the fuse is going to eat up the props area. there are way more tools to look at also. as you use motor calc and begin to understand why your result differ then what you actually have you will be able to put in stats so that the ending results are ritght on the money. then when you make a change such as prop size or motor size your new rusults are much closer to real life. thus getting you a much better starting point with your new plane or motor or what not.

anyways i am glad you can see that at least one of these caclulaters can be helpful and i hope we can stand on good humer and good terms.

later andrew K
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Old 02-09-2011, 06:58 AM
  #48  
JetPlaneFlyer
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Originally Posted by z-8 View Post
....each blade of which twists AoA to generate equalized lift as a function of redial velocity squared from hub to tip.
That's incorrect. The twist on the blade is to achieve constant pitch (and therefore approx constant blade angle of attack when in flight) not constant lift. The outer part of the blade will always lift MUCH more than the inner part due to it's velocity being higher, and lift being proportional to velocity squared.
In inner 1/3rd of the blade actually produces only a tiny proportion of overall thrust, probably 5% or less (which is why you don't lose any appreciable thrust when you fit a spinner). So losses due to the cowl will be vastly less than you might expect just based simply on occluded area.

Steve

Last edited by JetPlaneFlyer; 02-09-2011 at 07:45 AM.
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Old 02-09-2011, 01:43 PM
  #49  
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Originally Posted by JetPlaneFlyer View Post
That's incorrect. The twist on the blade is to achieve constant pitch (and therefore approx constant blade angle of attack when in flight) not constant lift. The outer part of the blade will always lift MUCH more than the inner part due to it's velocity being higher, and lift being proportional to velocity squared.
In inner 1/3rd of the blade actually produces only a tiny proportion of overall thrust, probably 5% or less (which is why you don't lose any appreciable thrust when you fit a spinner). So losses due to the cowl will be vastly less than you might expect just based simply on occluded area.

Steve
That is an old wives tale with no aerodynamic basis. Both the twist and the chord variation of given prop blade is designed to equalize lift distribution from hub to tip, as a function of radial velocity squared, as I said. Lift is distributed in accordance with the structural ability of the material to carry it, which means the area near the hub carries the same or more force than the tip, or the cantilever would break under load.

It is simple aero that tapering-to-0 or elliptical wing tip (= 0 chord at the tip), twisted to neutral AoA (= 0 alpha at the tip), generates 0 lift at the very tip. That is exactly why props both twist and taper: Cl is not of the same degree and V^2, S is not of the same degree as V^2, as multiple terms taken together, they are.
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Old 02-09-2011, 02:20 PM
  #50  
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