Sorry for such a long time between posts. I was working some crazy hours, but I'm home now
Ok where were we? Oh yeah....the power section. As much as having your plane set up correctly, having the correct balance between motor/prop & battery is just as important. Since the motor is selected for us, battery and prop selection become more critical.
In most race setups, you want a small diameter prop for max rpm, and hefty pitch for speed. We give up some low speed acceleration for top end velocity. After all, it's racing right?
Lets talk a bit about the motor before we go too much further. The Park 450 motor has a Kv rating of 890. (rpm's per volt) Simple math will tell us that 11.1v X 890 = 9879 rpms. Thats pretty low for a racing application. This motor is about 75% efficient giving us a no load rpm of about 7400. It is not going to turn much faster than this no matter what. You cannot exceed the Kv rating. This means the normal small diameter, big pitch prop is not the way to go.
Now those of you that have tached your motor with say a 9x9 APC prop on it will note that it will turn about 71-7200 rpm. This means is that we are not working the motor very hard at all, as it it very close to the no load rpm even with a good size prop on it. Current in flight is in the 10-12 amp range. The motor won't turn faster than the Kv, so we need the most effecient prop to make use of the limited rpms we have available. At this rpm, the motor is very torquey, and will spin a pretty hefty prop. A 9" is about the right diameter, and 9" pitch gets the job done. But what else? Pitch is what gives us the speed we want, and since it won't turn any more rpm's, more pitch is the only answer. I have retwisted 9x9 to get another 1" pitch, picked up some speed and didn't drop any rpm. That means I'm headed in the right direction, and more pitch is in the future. There is more testing to be done here.
This is where motor timing may help. I hear a lot of people bragging that they are running "high" timing thinking that it will give them more rpm. Wrong! We are already up against the Kv limit of the motor, so high timing does nothing but create heat and pull more current. I go the other way, med or low timing and load the motor up with more pitch to get the current up. This gives me more torque to turn more pitch.
As far as batteries go, I want the least capacity (for the lightest weight) that will get the job done. This is where "C" rating comes into play. A battery with a higher C rating will give higher voltage for a longer period of time. Here is a simple example to illustrate the point:
A typical 2100 Thunder Power can have a 20C rating, meaning it can deliver 42 amps continous.
A more modern 1600 pack has a 35C rating, meaning it can deliver 56 amps! We are not pulling anywhere close to that many amps, so this smaller, lighter pack is more than adequate and may actually deliver better punch for the duration of the race. It's a win win all the way around.
Props are sort of a black art, and the subject is really too deep for me to go into really great detail. Prop design has always facinated me and is the simplest way to up the performance of any airplane. You would be surprised how a tweaked prop of the exact same size with make a huge difference. If you want to learn more specific information, drop me a line, ask a question here or give me a call and I will be glad to help. It takes a good deal of patience to work a prop, but the rewards are worth it.
Some basic prop theory might be in order. A prop is just a rotating wing. It has all the same advantages and suffers the same setbacks as any other wing, but it flies through the air much faster, so small improvements pay off big here. Airfoil shape, airfloil cleanliness, planform and tip design all come into play.
First above all else your prop must be balenced. An unbalenced prop can cost 2-300 hundred rpm, which is close to 20% of your thrust! Trust me here, it is huge. If you don't have a balencer, beg, borrow or steal one and learn how to use it! I won't go into much detail here, all the balencers come with good instructions.
You can not bolt on a prop out of the bag and expect to go fast. I spend on average at least an hour for each race prop I work. Here are the steps in order I use to get a prop ready to go. If this is your first time, start checking balence after step two to make sure it's not way out. Most APC props have the center hole drilled off center, so make sure you fit and balence them with the included bushings.
1. Take an Exacto knife and scrape the flashing off all around the perimeter of the prop.
2. If you are going to clip the prop, measure each blade carfully and cut with a razor saw or sand to length.
3. If you are going to change the planform, use heavy grit (80) sandpaper and a sanding block to make the new shape, including the tip.
4. Use 150 grit and a block to sand the LE, TE and tip till they are nice and thin. The same principal applies here as a wing airfoil. You want a sharp trailing edge, a smooth but not blunt leading edge and a thin tip.
5. Go over the areas you just sanded with 220 grit further smoothing the entire blade. At this point I'll usually throw it on the balencer to check progress.
6. Final sand it with 400 wet. Nake sure all edges are super smooth. It should feel like satin all over. Dry it off and check the balence.
7. Mark the heavy blade with a sharpie and sand the back side at the tip till it balences.
8. Wax it. Yes you heard me, wax it. Several coats, buffing in between.
I have some ideas that I'd like to try with props. If any of them work out, I'll be sure to share them here. One I have in mind is pretty radical, so if you see me flying some wierd looking prop....look out!
Well, thats about it. I'm sure there are some details along the way that I have missed. Please feel free to share some of your progress here, ask some questions or make a comment.
Go fast & turn left