Hey Everybody!

I've got a question that has been bugging me for 20 years now and I've got to get some opinions. I love to spend time creating as close to scale and detail in my airplane only to put stupid props on the thing to fly it. For example, I had an F8F Bearcat, stand off scale, approx 1/5 scale, 72" wing span, After working for weeks on the bearcat complicated landing gear system, I put on a 2 bladed 12x6 prop instead of the scale, 4 bladed 18x4. WHY!!!!! When the plane comes to rest after a great flight it loses all scale when that stupid 2 bladed prop stops turning and is screaming out on the front of the plane 'HEY LOOK AT ME I'M NOT SCALE!!!!!!!' I know I can change the prop,......but why should I have to? What's the deal with our hobby that we can't have fliable scale props? OK off my soap box. Well nope I'm not, A P-51 Mustange has a 4 bladed prop!!!!!! So Does a Corsair!!!!!!!, So does the Douglas Skyraider, Ok now I'm done. Sorry for the tyrate. Go ahead and hit me with all your opinions even if you think I'm dumb.

Calm Weather and Great Flying

Troy

I don't think you're dumb. I just completed my first warbird, a Top Flite 1/7 scale Spitfire. It was fortunate that I selected this scale because I was able to find a solution. I am powering my Spit with an RCV 90SP. Being a 2:1 geared engine, it will turn the APC 15.5 x 12, 4 blade prop. I took the plane down to the field for the first time this last weekend, to run the engine. Here's a link to a video clip with it running with the cowl off, and a photo of the plane with it's flying prop.


http://www.youtube.com/watch?v=ZpwJ0XYTeIU

Scott

Simple physics. The average model engine spins too fast to make a 4 blade prop work efficiently. Most any motor will turn some sort of 4-blade, but not a flyable prop. On the other hand, if you have the room in the model to put a monster power plant it might work. The other solution is a gear reduction of some sort to slow the prop down enough to use lots of pitch.

You can easily use a 4-blade prop, just not a very big one. Lets use a G-62 as an example. This will spin a 2-blade 24x8 MenzS at around 6400rpm, and a 4-blade 19x10 MenzS about the same. The 4-blade may have the correct number of blades, but will look ridicously small compared to the bigger 2-blade. And when in the air you will most likely only see the prop disc, and not the individual blades.
So your only option would be to make some sort of reduction drive to get the rpm way down, and thereby get the power to use a bigger prop.

WOW! Wish I had said that.

Hey Again,

I've thought of some of those solutions, but my question still stands, why can't some of the prop companies create the 4 bladed versions with out so much bite, thus they would spin up to the needed RPM for our motors. Any one,..Buhler, Buhler?

Troy

If you have no "bite" ie: pitch, you have no forward speed. If you have no forward speed, you cannot develop lift. If you cannot develop lift, you cannot fly. Physics.

The two biggest reasons are propeller efficiency and cost. No matter how good you are and what you fly sooner or later a prop gets broken.

I still have one 1/5th scale Spitfire with a working 4 blade but I find it much more practical to fly on two and display with three or four.

Take a look in my gallery and you can see why. I hate chipping $75 to $100 props much less something that can cost 2 1/2 to 3 times that much.

One of the biggest reasons is the matter of demand for the product. Everyone (well, almost) accepts the performance limitations vs scale appearance of multi-blade props, and uses two blades, for the most part. They just wouldn't sell enough to justify their manufacture. Byron used to make a reduction drive for their giant scale planes, and had a big 4-bladed paddle blade prop. It worked okay, but added unecessary complexity and required the engine to be mounted about a foot aft of the "normal" spot. I've seen a Mustang with this setup, it was fairly slow, scale appearance of the prop was offset by the required engine placement. The only time you could tell that the prop was scale was when it wasn't running. I also agree with the other posts regarding the cost vs. breakage plus the physics lesson.

This doesn't take into account using a 3 or 4 blade to get clearance between the prop and fuse. Been looking for high pitch 9, 10, and 11 inch electric props and not having much luck for a scale project that will be powered by electric and I sorta have to start with the prop to get the mounts, motors, batteries and such (a twin).

Best selection so far is Graupner. Links anyone???????

Hi All

I think your right about the non scale prop look.

So this is what I was working on:

Of course I'm not going to get 2 blade performance from my scale blade, but I have plenty of two blade warbird models to fly.

The cost issue on a prop strike is a problem, thats why I'm fabricating my own carbon blades with my buddys input on what will work (sometimes one goes over board)
and will be practicle.

Horsepower and torque are the two main problems. My solution is to overpower the (*&^%$ out of the airframe is question.

At this point I am doing a prop for my 80" BF 109G and will start on a CMP 71" Zero ( note they are 3 balde models). I'm going to put my RCV SP 120 on the Zero with a nearly scale dia 3 blade with scale shaped blades to see what happens.

There are a number of dynamic concerns also, one can only learn by trying.



Steve

You can fly a scale prop.. If you choose not to, that's your choice. You could put an RCV 120 in your bearcat and probably fly a scale prop, or at least one that was closer to scale than the small 2 blade you are currently running. What you cannot do is put a scale prop on a normal model engine without some sort of reduction drive. The RCV is designed to turn the prop at 2:1 reduction. You could make your own reduction drive, in some cases purchase a commercial unit. Here are 2 pictures of my Ultra-rc Zero with a G62 and a Torquemaster 1.75:1 reduction drive with a scale prop.

Scalebirdman-- That's a really nice setup on your Zero. What I like best about the scale considerations is the fact that the engine mounts higher, and the cylinder head is thus hidden in the cowl. How do you route enough cooling air around the reduction assembly ?

Obviously I haven't flow it yet so I can't be sure, but I think I'll have enough air flow as it is; I have another g62 in a mustang that has considerably less air flow around it than this setup, and haven't had any overheating issues with it.

Vario-Prop has 3, 4 and 5(!) blade hubs and prop blades with diameters to 15 inches. Their "scale" blades provide a reasonable faxcimile of a scale prop. The "speed" props are more efficient but without some of the scale appearance.

The pitch is variable, so you can get the high pitch, small diameter arrangement.

www.ramoser.de

David

Helijack:

Try the "speed" props for electric. I am running 400w systems on my C-47 with 3 blade, 9.7" diameter speed blades.

With the blades pitched for 35a draw (about a 7.5 pitch), I am getting 40 oz of static thrust.

Hey Folks,

Checked out the vario prop site, really cool products. I'm going to try a 4 bladed 10x6 on my c-160 with O.S. Max .25FX power plants. They have more hp than the other 25s i've found. (.84hp each) While the 4 blades won't spin up to the 18,000 rpm that the motor advertises, I don't think thats neccesary for this aircraft. All the postes I've been reading are commenting that they only use 50 to 75% throttle for flites and take offs. And my dream project right now is a fully functional V-22 Osprey with interconnected tuborprop jet engines. I think that would be an awesome RC, I don't think AMA would certify it due to the weight though. Anyway, thanks for all the comments they all ment a great deal. Happy flying and landings to all!

Troy

I may be all wrong here, but I don't think your .25s will turn 10x6 2blade props, much less 10x6 4blades.

I not only like the look of a more scale prop, but also don't like seeing engine parts hanging out, or big holes in the cowl. In my case, I've eliminated the traditional air inlet hole cut into my cowl, instead, using the gap between the spinner and cowl for an air inlet. Of course, this is very limited, and doesn't really get much airflow from the prop. To address the issue, I added baffled to keep the airflow as close to the cylinder as possible and added an electric ducted fan to exhaust the air out of the cowl. This is still in the early stages, and I have not fully proved this setup, but last sunday, I ran my RCV 90 with the fan in operation on the ground for about 15 minutes with no obvious overheating. Perhaps a similar setup could be used in a situation where a reduction drive blocks some of the normal airflow.

Scott

I've questioned this set up all along during the build, but everybody swears these engines are way more than needed and they are the largest specified in the build plans. If .20s were installed then a 9x6 prop was called for, if a .25 or 30 4s then 10x6s. So I'm kindof with you on this but we'll have to see. Maiden flite very soon, engine run-up should be in a day or two, will know more then. Thanks for the post.

Calm winds, smooth landings

Troy