drew110307

Hello, I am a senior engineering student at Cedarville University. For my senior design project we are building a RC airplane to compete at the SAE Aero Design East advanced competition in March. We have already purchased the O.S. 46AXII ABL engine. The design team last year was having trouble getting enough thrust out of the engine and we are limited to a 0.46 in^3 displacement so getting a larger engine is out of the question. An idea we had was to get a gearbox to create more thrust. The crankshaft size on this engine is 1/4 inch. From what I am seeing most gearboxes are designed for much smaller motor shaft sizes, for electric engines. We are also not allowed to use electric engines. I found other pinion gears at: https://www.servocity.com/html/32_pi...l#.VjEi7HqrS72 and was wondering if one of these 1/4 inch gears could be substituted for a gear on one of the gearboxes below that was designed for a smaller motor shaft. If you had any sort of information that would possibly help with this I would greatly appreciate it.

Gearboxes:
http://www.rcecho.com/RC-Model-Big-S...ane-GB090.html
http://www.rcecho.com/RC-Model-Metal...otor-Gear.html

GREG DOE

Depending on what you have access to in regards to fabrication, you might consider a belt drive. Don't forget that any reduction device will come at the cost of added weight. Also a single two gear reduction will dictate a left hand rotation prop, which may limit your prop choices. A belt drive does not change the direction of rotation.
You don't say where you are located. You might try a local hobby shop, or ask for contact information for a local model club. Most modelers are willing to give advice, or even help. Over the years we have seen many teams build models for these events, only to see them destroyed by inexperienced pilots. If you don't have a qualified pilot, it's not too soon to look for one. Good luck. Greg

R8893

My Feedback: (20)

I was involved several years ago in this competition and reduction units were specifically disallowed. OS produced engines with a gear reduction way back when. They did not last long on the market. Speculation that failures were due to the fact that the same set of teeth take the impact of the combustion pulse every time. And then the teeth break. If you can use the reduction unit, the belt drive is the way to go. The belt can stretch to take the impulse load. Components readily available from McMaster-Carr.

If you really want to go after more thrust you could consider some 30's technology: a constant speed propellor that would let the engine run near its peak horsepower.

Chuck

Lnewqban

Welcome, drew110307!

For any choice, consider the tremendous centripetal effect of the high rpm's of the engine combined with the small diameter of pulleys or gears.
Many belts, and even chains, have a minimum radius of turn.

Any metal gear will need lubrication; hence, a case that contains it (again, the centripetal effect).
Your shaft for the propeller will need two bearings (radial-axial) and anchoring points.
The maximum HP delivered by the engine will dictate how robust (and heavy) the elements of your transmission need to be.

Consider these:
https://en.wikipedia.org/wiki/Epicyclic_gearing

https://en.wikipedia.org/wiki/Friction_drive

https://en.wikipedia.org/wiki/Contin...e_transmission

52larry52

My Feedback: (1)

An OS 46 AX is a good engine in it's size class. I would consider adding a tuned exhaust pipe and experimenting with various prop sizes and pitches. After that, work on imnproving whatever airframe you are using. A model airplane engine tachometer may be of some value to you, they don't cost much ($25-$30).

j.duncker

My Feedback: (2)

From a purely theoretical standpoint gearing the engine down to use a larger propeller is the way to go BUT overcoming the engineering difficulties is really difficult.

I have been involved in the UK version of this competition in the past and the successful models were almost always direct drive engines on simple models with competent pilots.

The only gear drive that I saw working reliably was based on the OS 61 commercial unit and adapted to the Irvine 40 engine they were required to use.

The two attempts to use belt drives seemed to have very significant friction losses and seemed to be less capable than the direct drive units.

RCVFR

My Feedback: (7)

drew110307, congratulations on participating in the SAE Aero Design competition. It is a wonderful event, I wish you well in it. I think you are trying to solve the wrong problem. The rules stipulate the power source to provide a "constant" for all the design efforts. Last year's "inadequate thrust" is more likely due to excessive drag rather than an engine problem. That is at the heart of your design project: to design and build an aircraft that will be capable of carrying a maximum load within the design parameters. Selecting the airfoil will tempt you to go for the most lift, but you have to keep in mind more lift=more drag. More wing area (lifting surface) = more lift = more drag. So what is the best compromise? That's where the fun begins!

Best of luck to you.

j.duncker

My Feedback: (2)

TOP TIP It is well worth your while to scout out any bumps on the runway. Every little helps when you are at the limit.

Carlos G

Hi drew110307,

If a reduction system is allowed you might want to look at an RC helicopter rotor drive system. There were a number of glow heli's that used the engine size you are talking about. A side benefit is the collective pitch rotor could be used as a quasi constant speed prop with a little tweaking of the pitch curve program.

You will need to check the rules to see what is allowed. If nothing else, a higher nitro content in your fuel may gain you the extra uumph you are looking for.

Just a few random thoughts for your consideration.

Carlos G.