I have a 3D plane that weighs 6lbs with a MDS 78 in it. What is a good prop for this setup. A APC 13-4w or 14-4w. Not sure which would be best! Or what else would work. learning how to tork roll! Love this motor, runs well.

try the 14/4 you mite be able to use a 15/4 they tried to get it to turn big props at lower rpm most people run 14/6 on it i would say the 15/4 would work best

Ummm..... the 15x4W is the max for the Saito 91 4 STROKE, how can u use it on an mds 78 when an O.S. 91FX uses an 12x8?????????????????????????????????????????????? ???

i run a 15/6 on the os 91 fx i look for the best prop for power not rpm some time a engine likes to be loaded more than a jet or nelson turning 22 grand plus the apc 16/6 works good on the 91 anything over 10500 on a satio is wasted pull[8D]

ok and hope you never have to use the engines warnty on the os your under proped it tecks a 13/8[]

the 15/4 was just a thought it mite not work any better than under proping a engine but that engine is not for rpm it more for tork they run 14/6 props on them i dont see how a 15/4 would be so unreasnable


unless your at really high altitude that would be the reason your engines are not proped right[X(]

Ran a 14x4 on mine on a 6lb 110z cap plenty of vertical power and good throttle response. Was going to try a 15x4 but sold the plane.

A 14x6 prop has a prop factor of 34
A 15x4 prop has a prop factor of 34
12x8 prop factor is 32
props with the same prop factor take roughly the same amount of horsepower to turn the same rpm, ive been using this formula for a while now and its not uncommon for an engine to pull a slightly larger prop factor than it was designed for and pull it well.

prop factor = diameter X 2 + pitch

Your right about the mds design its made to turn bigger props with less rpm's this is partly done with a longer stroke on the crank.

Thanks for all the comments, I am glad that this motor can turn a large prop. A 14-4w seems like a good place to start. With my 12-6 I have unlimited vertical in a 6lbs 3D setup, and can pull out of a hover. So a 14-4w should perform even better! We will see.

Just to let you know OS mufflers, TT interchange with MDS

Thanks Kennyroy, I find the muffler on my MDS to be effective and quiet. Are OS any better and what do you mean by TT ? Also like the prop factor idea, however the width of a prop must also have a factor, also the make! APC props are very efficient and would be able to preform with higher prop factors than say a Zinger.

tt means thunder tiger model engines i love my mds engine have the 2.18 in my patty wish they made a 160/180 for my edgethe yneed to come out with a 200cc 250 cc found some half scales i want but the engines are 2500 dollars mds would be 1200 dollars

Prophanger1 is right TT stands for thunder tiger. Just a guess but i would say the factory mufflers are about the same on performance. I was just letting you know this in case you wanted to try an aftermarket like a MACS or something. Your absolutely right about the prop factor and different brands or styles. I seem to pick up a few RPM with an APC compared to some other props ive tried. Prop factor numbers will keep you in the ball park when trying new props.

Just be careful in lowering the prop, if you go too low the long stroke engine will be very unhappy at top rpm, it may exceed the designed limits of the engine and lose a rod. Plus if the pitch is too low it may hover well but be a nightmare on take-off and regular flight, meaning not enough air speed for foward flight and control of the surfaces.[]
That's my trial and error findings.[:-]