z-8

Fly-away cost.

http://z8rc.blogspot.com/2010/11/125-mph-for-250.html

57sailplane

i have built somthing simular. a gws 190.. my plane is way more dragy.. so i am thinking of up grading to somthing like this. i read your description on this bird and wonder what kind of motor you are runing if you dont mind me asking?
also does it come with retracts?

later andrew K.

rcers

Andrew - Z-8 is taking a bit of a break from Wattflyer. Just wanted you to know he would be unable to answer you for a while.

Mike

TDisaster

"Lastly, I suppose I might go easier on ignorant toy testers if they weren't mostly slimy salesmen. Slimy is to be expected in sales, but I really hate when I know a gazillion times more than a salesperson who's only job is to learn about what they are selling."

http://z8rc.blogspot.com/2010_12_01_archive.html

That about explains it...

57sailplane

great article TDisaster,

i like the part about how much 1 single ounce of weight can do. man i am going to build as light as possible from now on. i was already finding this out on my own flying fast planes but did not understand why.

i use motorcalc on all my motor set ups. it is not perfect but it gets me in the ball park. i hack and hack until my finding come close to real life. like flight times and max speed and so forth on a specific plane that i own. i type all these features into motorcalc so that my ending results are the same as real life findings with my existing motor. then i can change motors hoping to get a close guesstimate. it has worked well for me.

one thing i would also like to mention that this article does not delve in is prop sizing. i have found for my style of flying and most sport flying that the higher pitch props seem to be more efficient then lesser pitch. of coarse this is all to do with my set up and findings. as for every set up i try different props test flying and recording the times. my thing is this if you are ruining a 9x6 or a 10X7 don't be scared to try a 9x7.5 or 9x9 and a 10x8or a 10x10. the bad thing about this is if you are at the limits with the smallness of your motor take offs can become difficult so there is a trade off. also i have found on my bigger planes that landings can be slowed by using the lesser pitch props. so i guess it all comes down to that just right set up. that why i love to fly electric. making the best of what you can on a certain plane.

anyways relay like the article and i do think it to be very feasible to make this plane fly 125 mph with the right set up.

TDisaster

That's not my article.

That's Z8's blog. Honestly, I don't like how he goes about posting, but that's just me.

57sailplane

oops sorry my bad..

Rc247

I love the Bearcat!!! Wish FMS or PZ would come out with one!!!

z-8

Thanks for the nice words about the article. The problem with using a thrust calculator is the same problem that all the ground test motor/prop data bases have:

Thrust is not the force the motor produces, far from it. In fact, there is no relationship whatsoever to the un-installed force a given motor produces and aircraft thrust, which is...

Thrust = (Uninstalled thrust @ airpseed - installation error @ airspeed) - ((Induced Drag + Parasite Drag) @ airspeed)

The only thing Thrust Calc or even an actual uninstalled static test run attempts to estimate is the red term @ 0 knots. That doesn't yield any useful info about thrust.

kyleservicetech

Agreed:
www.motocalc.com does give an indication of the prop thrust versus flying speed of your power system setup. That program will quickly let you know that, for example a 16X4 propeller would be rather useless on just about any model you can find. (That is, if you could even find a 16X4 prop!)

57sailplane

whatever.. seems funny how this is all i use and done rather well.. motorcalc that is.

z-8

MotoCalc is nice in that it at least tries to calculate/model actual installed aircraft thrust at an airspeed, based on the weight of the airframe and weight of the total power system including battery, theoretical main wing lift and main wing drag (though it has no idea about installation error or total drag)--unlike people who test uninstalled motors on a stand, which never yields any data of any value.

So I checked MotoCalc against some actual, installed thrust, static tests I have recorded.

T-28 w/Power 10, 10x7, 3-cell/35C static thrust:
MotoCalc = 38.1 oz
Actual = 45.4 oz
Error = 20%

Unfortunately MotoCalc didn't have the other 3 motors for which I have actual installed data. But at only 20% off, it was closer than I thought it would be. Although I could have probably estimated/guessed closer than that.

57sailplane

awe,

yes i can just throw some numbers at motorcalc and come up with some funny figures also. if you dont believe in the tools you use and hone them to your already fixed result then that tool will give you bad results. i dont just look at thrust using motor calc i look at flight times, amps, voltage drop, watts in, full thorttle flight times i use the comparison charts and also the opinons given by motorcal. and use this so that the results are close to live this way when i throw in a differant motor i am not sadly disapointed.

thats the differance between a beginer, journyman and a master machinest and if you don't understand this last statment then there is no reason to discuss this any longer.

later andrew K.

kyleservicetech

Out of curiosity, did you check the RPM, current,volts,watts against motocalc?

I've found that at least for the Hacker line of outrunner motors those figures are pretty close, within 10% or so. And, another modeler had a well known motor I set up for him, and found that the KV numbers for that motor were way the heck off. He had to go from a 6S2P A123 pack to an 8S2P A123 pack.

z-8

That was with the specific aircraft model, span, wing area, empirical weight, motor, and battery.

Off course, MotoCalc doesn't even attempt to measure the most important factors, like installation error - for example - you can put a 8" prop on the T-28 model and it shows 100% thrust, even though the large radial cowl will occlude more than 80% of that prop arc.

So yes, MotoCalc is a novelty aimed at stark beginners, but at least the author demonstrated a basic understanding that uninstalled thrust is but a single term (which is extraordinarily difficult to calculate in its own right) in a sea of exceptionally complex aerodynamic interactions to figure the resultant force known as Thrust - like attempting to very crudely estimate induced and parasite drag at a given airspeed. Obviously, you need a state-of-the-art wind tunnel to gauge any particular model's form, skin friction, interference, and induced drag characteristics across a very broad range of flying weights and angles of attack. And that enormously complicated empirical data set is still going to hold true only in the very rare case of level unaccelerated flight.

Heck, just installation error resulting from aircraft form pulling back on any given tractor prop, alone, can range from a rather small % to 100% error, just from thrust occlusion resulting from a particular model/prop match.

57sailplane

so the question arises.. what tools do you use to set up a plane and its power system?

z-8

Unfortunately, or fortunately depending on if you are an artist, it is mostly art not science. It can be scientific given extraordinary resources, like access to empirical or high fidelity analytical airframe modeling. But... as the comparison above showed... a smart tool like ThrustCalc can get you in a the vague ballpark since it crudely models weight and a few pieces and parts of drag.

What most people don't seem to understand, certainly those who make it their life hobby to to measure uninstalled thrust of thousands of props and motors while on a test stand, is that uninstalled thrust numbers, alone, are totally unrelated to the achieved thrust of any flying solution. It is similar to measuring the horsepower of a car engine on a test stand, then concluding that any car or truck or tractor trailer that uses that engine will achieve the same 0-60 time. Although believing that for ground transpo can make one look like a genius compared to believing the same is true as it relates to aviation propulsion systems.

So the answer to your question is there is no easy answer, sometimes counter-intuitive solutions work best.

For example, avoid using a motor/prop that uninstalled e-motor testers recommend because they believe it to be very efficient, and thus the best answer in their ground-influenced minds. Electric motor efficiency in isolation is actually a metric of poor motor T:W ratio. Motors need to strain to a degree to be light when viewed through the lens of the overall flying solution, making the least efficient e-motor in isolation often the most efficient overall.

kyleservicetech

Do you have a specific plane in mind? If so, let us know, there is usually someone with that same exact model, along with how they powered it up. And, as indicated before, programs such as motocalc will get you in the ball park with the power system.

As z-8 suggests, you could select a motor capable of perhaps 400 watts. The propeller, battery and motor would be very different between a very fast racing model, or a sailplane, both running 400 watts on the prop. The racing model would need a fairly small prop with high pitch numbers, turning over at a very hgh RPM. The sailplane would need a fairly large diameter prop with low pitch numbers turning over at a much lower RPM. All depends on what your models power needs are.

57sailplane

so your answer is

sometimes counter-intuitive solutions work best.

yes i read all about how the thrust calcs are only good for ball park figures, and the guys doing the static test have no idea what real life flying statistics are achieved.

i got it. i understood you the very first post..

but still i want to know what you use to set up that plane that you have never flown before that is like no other you have ever owned to achieve the goals you have in mind for it.

z-8

That's exactly right, and even watts per pound is semi-meaningless without objectives. The key is to define your goals before talking about powering them, without falling short and without much waste.

z-8

The key is in the end of your last sentence. What if someone came up to you and asked what's the best engine for a car?

I can make a car that will go from here to Alaska on a AA battery, how much time do I have? I can even make a car that will do 200 mph on a AA battery given enough time to build potential energy. I can make a car that will barely go, but will be great on fuel (efficient) or I can make a car that will be great fun to drive but is no fun at the fuel pump (effective). The overall technology level and my mastery of how it plays against my specific objectives is what dictates the level of compromise I must accept.

Powering an airplane is much more complex than a car, because it has to fight gravity in the Z axis while translating in the X/Y/Z axis, and there are multiple fuel sources in play at the same time. Both electricity and air are used as fuel. Time can also be viewed as a fuel to sip or broadly consume to move through the 4th dimension.

So on one hand, it is too simplistic to ask "how do I power an airplane?" And even more simplistic to suggest an answer. On the other hand, it is also too complex a set of interacting problems to go searching for a solution using a purely analytical method. On the third hand, it can be too costly to determine the best solution using a purely empirical method.

So the answer is, you get in the vague ballpark analytically, then you probably need to go empirical or draw on previous empirical solutions. This is why the biggest defense contractors in the world with huge resources constantly produce crappy airplanes than fly horribly or accomplish no mission, or in other cases, do a great job.

57sailplane

ahh,

yes the variables are great. exactly what you explane in your example about the car. me and you both experience this when setting up a knew plane.

so you do some kind of setting up the plane analytically.

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.

i think i covered all of them.

z-8

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.

Gohmer

Have you sold any yet? Did they go that fast?

57sailplane

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?