The Steam Punk

(I hope this is in the right forum; if it's not, can a mod please stick it where it belongs?)

(Also, apologies in advance for the long post!)

Greetings, everyone; as you can see, I'm new here. I joined this forum in order to ask about what is, as the title indicates, a rather different use for a model aircraft engine.

First, a little background: My name is Andrew, and I am a senior-year Mechanical Engineering undergraduate at University of Maryland, Baltimore County. As part of our curriculum, each student is required to complete a Machine Design course, in which we work either together or in teams to build personal transportation devices; essentially, small one-person vehicles. Then, at the end of the semester, the completion of the project is celebrated with a big race, in which your vehicle's performance is analyzed to determine a portion of your final score, so speed, weight, and power are key.

For my vehicle, I decided to build a motorized skateboard, or more accurately, a motorized longboard; they are lightweight and corner very well, better than a bike or scooter traveling at identical speeds. I initially considered using belt-drive, chain-drive, or direct drive, but each of these methods involves incorporating a drivetrain which would be difficult to acquire parts for (I don't know where to get a miniature gearbox to turn several thousand RPMs at the engine into a couple hundred or less at the wheels, for example) and which would create a lot of additional potential failure points. So, I opted to use propeller drive instead. This serves three purposes; it eliminates the need for a drivetrain, it will improve my design grade (creativity and uniqueness are factors), and it will make the board look cooler, at least to me.

With that established, I set about figuring out what sorts of design criteria I should use. Obviously, my ride needs to be fast, but because UMBC's campus has several steep hills, I need it to be able to climb as well. With that in mind, I specified the following goals:
[*]The finished design will have a top speed of 25 mph (11.2m/s).[*]The finished design will be able to climb hills with up to a 12.5 degree slope without stalling.[*]The finished design will use a propeller no bigger than 22” in diameter, with smaller props being preferred.[*]The finished design will weigh at most 200 pounds (890N) including board and rider and whatever the rider is carrying.

Having decided what I wanted my ride to do, I began running calculations to see what sort of thrust output I would need. Applying some basic knowledge of physics and fluid mechanics, I derived the following equation to solve for velocity as a function of hill slope in a way that incorporates drag:



In this function, FG is the portion of the gravitational force (weight) of the loaded board acting downwards and parallel to the ground; FP is the thrust force of the propellor; CD is the Drag Coefficient of a standing person (1.15); A is the projected area of a standing person (1m^2); and p (rho) is the density of air (1.175 kg/m^3).

I then coded the above equation into SciLab and generated the following graphs:



The five lines plotted represent the velocity functions at, from lower left to upper right, 160, 170, 180, 190, and 200N of thrust. As the graphs clearly show, in order to maintain a respectable speed (Roughly 4.5m/s or 10 mph) at a slope of 10 degrees, I will need approximately 170N of thrust; at a slope of 12 degrees, I need 200N of thrust to maintain the same speed. Clearly, I am going to need a pretty prodigious powerplant/propeller pair to power my project.

Keeping in mind that I needed to produce between 170N (38.2lb) and 200N(45 lb) of thrust, I began playing around with the Static Thrust Calculator I found at

http://personal.osi.hu/fuzesisz/strc_eng/index.htm,

Inputting various diameter/rpm combinations in order to see what sort of hardware I would need. The results are charted below:



The following assumptions were made in the above calculations:
[*]All propellers were treated as “pull” propellers even though a prop-driven skateboard will require a pusher prop.[*]It was assumed that a “pull” prop mounted facing backwards on an engine which was also facing backwards would produce rearward-directed thrust equivalent to a correctly mounted pusher or pull prop.[*]Ambient air temperature was assumed to be 77 degrees Fahrenheit.[*]Ambient air density was assumed to be 1.175 kg/m^3.[*]The coefficient of Friction of the propeller blades was assumed to be 1.09

From this chart, I can see that in order to achieve the desired level of performance from my vehicle, I need one of the following:
[*]A 22x10 2-Blade Prop turning at 8000 RPMs.[*]A 20x10 3-Blade Prop turning at 8500 RPMs.[*]A 20x10 4-Blade Prop turning at 7500 RPMs.

The last step in this phase of the design process is to select an engine to satisfy one of the above conditions. In order to keep my final design as compact and maneuverable as possible, I would like to use a 3- or 4- bladed prop in order to keep my design as compact as possible by allowing for a smaller prop diameter; with this in mind, I began researching engines online.

The first page I looked at was RC Hobbies Online, http://h1071118.hobbyshopnow.com/ser...egory=EAG&tag=. From their selection of gas-powered airplane engines, it looks like I would need something along the lines of an Evolution 58GX2 ($599.99) or Evolution 80GX ($749.99), or a Zenoah G62 ($449.99) or GT80 ($879.99).

Checking several additional webpages just resulted in finding the same engines over and over, so those look like my best bets.

So, based on my research and calculations, my current plan is to use a Zenoah G62 engine (http://h1071118.hobbyshopnow.com/pro...p?prod=ZENE62A) (facing backwards) and a Zinger 20x10 4-Blade Propeller (http://www3.towerhobbies.com/cgi-bin...?&I=LXYNM4&P=7) (mounted facing backwards on the engine shaft) turning at 7500RPMs in order to produce the 45lb of thrust my calculations show I will need to maintain a speed of 10mph while climbing a 12 degree slope with a propeller-driven skateboard that weighs 200lb fully loaded, including rider.

That's as far as my research has gotten me. Before I start fleshing out my design project writeup and sending out emails to companies to request sponsorship, though, I wanted to ask some people who have more experience than I do with model aviation (read: any experience at all). So, here are the questions I would like to get resolved before moving to the next phase of development:
[*]Will the Zenoah G62 be able to turn the 4-blade 20x10 Zinger prop at 7500 rpm? I just kinda guessed based on the fact that it will turn a larger 22x10 2-blade prop at a higher speed, 7800rpm.[*]Will mounting a pull prop backwards on an engine shaft which is also facing backwards produce thrust in the same direction as if they were both facing the right way?[*]Are there any other engine/prop combinations that will produce the necessary 40 to 45 lb of thrust but which I may have overlooked (such as an 18x10 3-Bladed Prop turning at 10,000rpm)?[*]I used a Static Thrust Calculator to determine my thrust values; I understand that Static Thrust is different from the Dynamic Thrust an engine produces while it is in motion, but is the difference significant enough to alter my calculations, especially since my skateboard will be moving considerably slower than a plane in flight?[*]Do the thrust values I got from that calculator look more or less accurate?[*]Based upon what you know, will 45 lb of thrust really be enough, especially for climbing hills? It seems like it should be, but I don't know anything about model aviation.[*]Do you guys have any other advice, or anything I should know?[*]Do you think my design is feasible?

Thanks in advance for any and all assistance!

JohnBuckner

My Feedback: (1)

This will not work. Mounting a propellor backwards on an engine will not change the direction of the airflow. To do this you will require a "pusher Propellor'" Usually designated with a P on the size specs. They are not avaliable in nearly the same variety as conventional tractor propellors.

Your project may be feasible however I seriously doubt you will be able to acquire a test pilot for a skate board with a hugh prop or heck even a small one spinning at high rpm.

John

The Steam Punk

I was afraid that the double-reversal wouldn't work. I couldn't find a 20" 4-bladed pusher prop ANYWHERE, just 20x10 2-blade version manufactured by Zinger. I wonder if they'd do a one-off 4-bladed model just for me if I asked really nicely...

Or, would it be possible to mount two 20x10 pusher props at 90 degrees from one another on the same driveshaft in order to get the thrust of a 4-bladed prop? I doubt it.

As for the safety concerns... I plan to build a shroud similar to that found on oscillating household fans, except made from 1/8" steel rod welded together. I already ran some test models through a finite element analysis program and I'd have to be jumping on the cage as hard as I could and then some in order to get it to flex enough to possibly connect with the prop, so it should be safe.

Thanks for the input!

Crash Campbell

Hi Steam Punk,

I think you may be re-inventing the wheel. Powered para-gliders use a back pack mounted motor with a pusher prop turning in a shroud (like a house hold fan). Some years ago when beach fishing I was passed by a young bloke riding a large skate board type device with wide inflateable tyres, he had the power unit from a para-glider on his back and was motoring along the hard sand at the water's edge at about 40kph.

http://www.google.com.au/images?hl=e...ed=0CEUQsAQwAw

Notice the guy on the scooter?????

Cheers,

Colin

MinnFlyer

My Feedback: (4)

A 2-blade prop will be more efficient that a 4-blade. When 4-blade props are that small, each blade will run into the wake of the blade before it.

jester_s1

Neat idea and good work on the theory. Two things that come to mind for me on this is that the rider is going to be in the way of airflow to the prop, possibly reducing the thrust it can produce. The second thing is that with a pusher configuration, it looks like the vehicle is going to be very unstable with the rider's weight up front. Given that, you might consider a tractor setup, which will also make your prop selection a lot easier.

ChuckW

If you can't find an appropriate pusher prop then I would just make it a tractor set up. It could still be behind the rider. All it would require is that the engine be behind the prop (i.e. like a seaplane engine pod or nacele mounted above the fuselage) or you find some sort of engine that will turn the appropriate direction for the tractor propeller if it were mounted in front of it.

The engines mentioned will obviously work but there are plenty of lower cost alternatives from DLE, RCGF and others. There are also other premium alternatives like DA, Fuji, etc.

This is exactly the sort of contraption I would build to ride around and annoy my neighbors.

kdc

Frankly the idea is madness from the safety point of view. Any collision with a person would risk their long hair or fingers etc being drawn into the prop if it is at the front. If at the rear the rider might also get their hair or clothes drawn in.

In any case its been done before on a car.....see the one in the Petersen Auto museum in Los Angeles. A failure! Or of course the hovercraft which works well but is not suitable for running near people. Remember also that large model aircraft can easily be restrained by 1 person even with the engine running full throttle.

You are an engineer yet cannot design and make a suitable gearbox?

The Steam Punk

kdc - I know that from the safety PoV it's probably a terrible idea, but then again, well-executed bad ideas are what I live for. As I said in an earlier reply, I do plan to shroud the prop, and I have actually been working on a design that would turn it into a ducted-fan engine for increased performance AND safety.

As for the failed car... *shrugs* Oh well. XD

Oh, and it's not that I can't design a gearbox; gear reduction calculations are easy and designing the gears themselves would be tedious but doable. The problem is that I don't know much about where to actually get the parts to build one, and having them all custom-made would be prohibitively expensive.

ChuckW - Can you point me to some suitable DLE, RCGF, or Other-brand lower cost engines? I remember running into DLE in my Googling but didn't see a model with enough power; I'll go check some more when I finish this post.

jester_s1 - I am aware of the stability issues and have designed a custom super-longboard with an almost 5-foot deck what features a "drop-down" center section which will actually sit BELOW the axels of the longboard's trucks. What's more, I plan to use special mountainboard trucks with 18" axels and 8" air-filled rubber tires in order to maximize stability. The hardware will be similar to that seen here: http://www.mbs.com/productcart/pc/vi...?idProduct=318

MinnFlyer - I know the 2-bladed prop will be more efficient but it will also necessitate a larger diameter, which I was trying to avoid for the sake of practicality.

Crash Campbell - Actually, that sort of board is exactly what I have been looking at; I am just looking to stick the engine and prop onto the board itself rather than on my back because, well, why not?

Thanks for all the input! *goes to research DLE and RCGF engines*

Update: The DLE 111 is quite possibly the sexiest piece of hardware in the history of ever. That would give me MORE than enough power.

Update 2: The RCGF GF65 is damn fine too, and retails for $280 instead of $450. It also produces more power and weighs less than the Zenoah G62 I was looking at. It almost seems too good to be true; can anyone weigh in on the quality/reliability/power of this engine? I am seriously intrigued, since it would probably be a lot easier to get my hands on one of these.

One more question: Most engines seem to be benchmarked with 2-blade props. Since I'm looking to use a 4-blade, how can I know if the engine will be too little, too much, or just right for the props I am looking at? For example, the DLE 111 is benched with a 27x10 2-blade according to http://jasmine.ecrater.com/product.php?pid=7935105; how does that compare to a 20x10 4-blade? I would think that the masses of the two props would be roughly the same and so it would work, but I don't know and don't feel like breaking hundreds of dollars of hardware to find out. So, any advice?

ChuckW

Iwouldn't put too much faith in adveritsed engine power. Some engine are over-rated and some are under-rated. Just use it as a rough guideline.

I know you are talking about a large cage around the prop for safety. Airboats and para-gliders obviously have something like that. If you are still concerned about prop safety though, maybe some sort of turbine or ducted fan power would be better. Search youtube for jet or turbine powered bicycles and motorccyles. People have actualy taken model jet engines and even some full sized ones and put them on these things. I'm not condoning some of their lax safety measures of course. Some of the more serious electric ducted fans, while expensive, are very impressive too. Not sure if any of these would provide enough thrust but you never know.

Whatever you do, just think safety. Props can mangle or even kill, engines get hot, etc. Be sure to take adequate safety measures and operate in a controlled environment.

ChuckW

Ihave not owned the RCGF-65 but I do have one of their 50cc engines. Ihave been extremely pleased considering the price. It may not be quite on the level as a DA, 3W or Zenoah but it's still very good and Ididn't pay nearly as much for it as those others.

The Steam Punk

I was already looking into building a ducted-fan design using the kind of wooden hoops used in drum making, such as these: http://www.precisiondrum.com/html/st...ple_hoops.html That should increase performance and safety at the same time, which would be nice.

Also, I did look at hobby-scale jet turbines, but dayum, they retail for like $2000. No way I can afford that and no way any manufacturer will sponsor me with one.

Also, both the RCGF GF65 and the DLE 55 and 111 are reed valve/electronic ignition engines, meaning I can adjust the timings to run them in reverse, thus allowing me to use a standard prop as a DIY pusher; because of that, and price, it looks like the above three are my new favorite engines.

Now I just need to figure out whether any of them will turn a 20x10 4-blade at the 7500 to 8000 RPMs I need; any advice there?

jimmyjames213

My Feedback: (2)

duct fans are a lot less effiecent than props so that wouldnt help

what i would do is buy a pocketbike http://www.minipocketrockets.com/ and modify the engine/transmission/ect to fit your longboard. that will have the power to do what you want to achieve while the prop engine wont.

The Steam Punk

Those bikes are actually a lot less than I thought they were; I would have pegged them at $800+, not $180. I could definitely pull the drive train out of one of those and modify it for my needs if I can't find a sponsor for an actual r/c aircraft engine. Thank you for that; even if I don't use one for this project, it is almost inevitable that I will tear one apart at some point!

Also, your signature has been my life motto for years. I approve!

kdc

Safety of other people is paramount! A huge fan down near the sidewalk will probably 'hoover' up all sorts of debris including cigarette ends, waste paper & dust etc even if it does not catch vital parts of pedestrians.

A V belt drive to the wheels might prove better and a V groove could easily be machined in one of the wheels. If the belt is loose and tensioned by a jockey pulley a simple clutch arrangement is available perhaps using pressure from your heel to operate. Check out lawnmowers to see that V belts and jockey pulleys work well in practise at these sorts of power. Modern V belts are much narrower than the old type and made of modern materials. If reduction gear is needed then another belt could be used, with the larger pulley slightly higher up to clear the ground.
I would have thought an electric motor would be easier to control and more in keeping with modern noise and pollution trends.

There are specialist suppliers of gears ready made for any purpose, ( such as Muffett Gears ) but a competent engineer should be able to machine his own with just a lathe. But getting something from a scrapped machine is cheaper for experiments. Consider an epicyclic gearbox as a neat solution too, maybe a bicycle hub of the Sturmey Archer type.

jester_s1

On the idea of reversing the engine to be able to use a standard prop as a pusher- it won't work. The airfoils on props are directional, so reversing them won't give anywhere near the thrust as running them forward. Would probably result in the prop coming apart too, since they are designed to take the bending stress in the normal direction of rotation.

Edwin

My Feedback: (2)

https://sdp-si.com/eStore/
http://www.pololu.com/catalog/category/22
http://www.mcmaster.com/#
http://www.rinomechanical.com/
http://www.wmberg.com/


Edwin

The Steam Punk

SDP-SI.com, heh, I remember them from my CAD design course a couple years ago; I ordered some acetate bevel gears from them for a children's toy we had to design using solid modelling.

I do like the idea of the toothed belt drive, but it would still have to be gas-powered since another group claimed "electric belt drive" as their powertrain of choice already; we're not allowed to do designs that are too similar to another group's work. However, on the bright side, I emailed the chair of the mechanical Engineering department and got permission to use the school's manufacturing resources for my project, even though they are usually mostly the domain of the grad students; that gives me the capacity to make gears, axles, and pullies using a 5-axis CNC milling machine that costs almost as much as my house did when it was first built. With a pocket-bike engine and a toothed belt drive (I don't trust DIY chains; I've watched too many episodes of Junkyard Wars and the damn things always break) using custom-milled parts, I should be able to put together something quite speedy and capable of some serious hill-climbing.

Of course, that still depends on finding a company to sponsor my project with a motor, but since direct drive would require less power, I would also need a smaller, cheaper motor, so my odds of finding a freebie go up too.

I may still buy a propellor anyway just for fun.

Now I'm all excited. I bet I could give the existing commercially-available motorized skateboards and longboards a serious run for their money, if not outright outperform them...

kdc

Why expect someone else to pay for ( sponsor ) your project? You should look for scrap items that could be utilised, look for a scrap lawnmower engine ( especially one with an electric start ) and scrap washing machines could provide a free suppy of pulleys and belts. A trip to a firm servicing invalid scooters, bicycles or lawnmowers might provide useful info on what commercial parts might suit, maybe they have old parts to dispose of too.
If you use a glow or gas engine look for ones with a pull start as used in model cars, boats etc. Design for easy belt changing!

jester_s1

It's not hard to take the engines out of weed eaters and put a pulley on the front. It might take two of them though to get the horsepower you need.

The Steam Punk

We're encouraged to seek sponsorship; the official budget for each team's vehicle, which is covered by lab fees, is $200, but beyond that anything we use has to be either sponsored or paid for by group members. What's more, even if group members pick up parts at reduced prices (say, yard sales or whatever), we still have to use the retail cost of those parts when calculating our total build cost. So basically, a part gotten for free from a company sponsor adds as much to our total cost as the same part gotten for pennies on the dollar at a yard sale. So, it only makes sense to try for freebies first.

This class is about more than just the project design; it's about writing design reports, planning, organizing, presenting, researching, communicating, persuading, etc. The goal is for us to recieve our design challenge, research solutions, choose a solution, run all the necessary calculations, adjust our design til it meets design goals, finalize the design, identify what hardware we need in order to build our final design, generate a bill of materials, calculate all our costs, budget appropriately, write up reports detailing all our work, use our reports to prepare presentations to companies who might be persuaded to sponsor us, assemble our final designs, draft final reports, present those final reports to any companies that DID sponsor us so they can see the end result of their contribuitons, and finally we race our vehicles to see how well we did. It's intended to teach us not just how to design something, but how to interact with people the way we will in the real world after graduation. It's a lot of work, but it's really rewarding to see a project through from beginning to end.

Plus we get to race against the campus police force's Segway unit; they agreed to enter the race this year since it's being set up as a charity event, with seats at the finish line being paid for in canned food that gets donated to a local food bank.



Regarding easy belt changine - already on it. Longboard wheels are easily removable so they can be changed and customized, making access to the drive pulley a breeze.

jimmyjames213

My Feedback: (2)

lol thanks [8D] i love designing stuff from scratch and going crazy making it work, theirs nothing like it when it gets moving, its a feeling you dont forget.
ive designed a bunch of stuff on autodesk inventor that im just waiting to get hold of a cnc and get cutout. cant wait to go to college where they have all the equiptment i need (just got out of high school)
good luck with everything, hope it all works out

The Steam Punk

I actually just tried to install the ProE Student Version on my laptop today only to discover it doesn't work with Vista SP2 and my processor is too slow to run it at anything greater than the speed of lag. I will be spending a lot of time in the campus modelling lab this summer, methinks...