Propeller and motor combinations-What's not in the fine print.

You’ve found the home of broken foam.

Sooner or later you’ll want to go off script. You’ll want a faster motor, or a lighter motor so that you can lighten the planeload. You’ll want a larger prop, a folding prop, or a 3 bladed prop. You’ll stray from what you know and start going to the wrong side of town to talk to those people your mom warned you about for truth and wisdom. Near all the trash and rats and junkyard dogs chasing the rats out of the trash you find my office. Don’t judge me, this is all I can afford after buying so many RC planes. Welcome to the home of broken foam!

This is  your disorientation

Thank you for entrusting me to be your personal Yoda in the search for truth when it comes to matching up motors to props.  It’s a search for the truth and you’re going to use it to make your own decisions.  It’s time to forget all you know.  You are going off script.

Truth number 1

Your car can go faster than the speed limit. The point is that when you get to the maximum speed of the car it starts shaking and you’re consuming fuel like crazy while the engine is working very hard. It’s also getting hot. Top speed of a car is far greater than the maximum legal speed limit in most countries. Your car is designed to work efficiently at speeds it’s likely to achieve. This is what to strive for when selecting a motor for your plane. If you’re at full throttle to constantly keep the plane afloat you’re underpowered. You want to make sure you have more than enough power

Truth number 2

When riding a bicycle the front derailleur controls three gear rings on most modern bikes. These are the three rings next to your pedals. The smallest one is best for hills. Since it’s smaller its easier to make each revolution when you pedal your bike. When you’re going uphill this is the one you select because the other two cog rings are too hard to pedal. But, when you’re on flatland the small ring is too easy to pedal. You are spinning like crazy and not going anywhere. You move up to the middle cog ring and start moving at a pace you feel comfortable with. You find that the middle ring is more efficient for flatland riding. Then, lucky you, you come up to a slight downhill and you have the wind at your back. You switch to the largest cog ring and really start to move. You’ve found even more efficiency.

Keep this in mind when it’s time to talk about prop size. Larger props can be more efficient in the right conditions. They can also work your motor too hard in the wrong conditions. But, just like gear rings larger props take more work to spin each revolution. Typically, higher KV motors (more on this later) spin smaller props and lower KV motors are better suited for larger props.

Truth #3

The documentation about motors quite often leaves you with more questions than answers. Just because it lists a 4S motor doesn’t mean that the highest thrust listed was made at 14.8 volts. Another thing that screws up things is that propellers from one manufacturer to another can deviate in statistically significant terms when measuring thrust. One will likely produce significantly more thrust than the other. The specs are given to help give you an idea of what the motor can do under normal conditions.

Now it’s time for plane design.

KV isn’t kilovolts

The “KV” after each motor name keeps people guessing. I’ve seen a few people on Youtube refer to it as “Kilovolts” or 10,000 volts. Most of the motors I work with take less than 15 volts and thus putting 10,000 volts through the motor will turn it to, well, spontaneous combustion. The V in “KV” stands for volts. The K is the constant. KV is not an abbreviation but a mathematical formula. K is the number of RPM’s your motor is capable of doing without a propeller. This is ideal world condition, there is some resistance in the motor wiring for example that keeps it from achieving perfection. That being said, the number is usually very close to accurate. Thus, you simply multiply the number of volts times the K and get the ideal RPM of your motor at various power levels.

The propeller is a resistor

Once you put a propeller on the motor KV goes out the window. The propeller acts as a resistor. Typically, the larger the propeller the more resistance it puts on the motor. That’s why we go back to truth number #2 about chainrings on bikes. The larger they are, the more work to make them spin each revolution. The only thing that confuses this issue is the pitch of the propeller. It’s possible to have a larger propeller draw fewer amps (less work) than a smaller propeller if it has a significantly reduced pitch.
The thing to keep in mind is that your propeller is putting resistance on your motor.
Grams of thrust is what we’re concerned about.

Turnigy Thrust Stands are cleaner ways to test thrust.

There are many videos on Youtube with crazy people hooking up motors and propeller combinations to scales on yelling out numbers like 820 grams. The first time you see this it's kinda strange. They are talking about grams of thrust. If your plane was 800 grams for example (28.22 ounces) when ready to fly this means this motor should be able to make this plane fly. The problem is that it may not be the most efficient flight. Just as your car starts to shake at top speed and consume gas like crazy you may find a plane that can produce 1000 grams of thrust may give a more efficient flight without getting your motor hot. Again, it depends on your plane. If you have a powered glider for example even though you have a heavy plane it may be able to take off and climb with less than full power depending on conditions. Once in the air the plane only needs about a quarter of the power it needs when it takes off to keep afloat.

Not all propellers are created equal.

So, why doesn’t someone just measure every combination of motors and propeller and come out with specs once and for all? Two reasons I can think of. First, not all propellers are equal, even when they have the same specs. An 8040 propeller (8”) can have remarkably different thrust from one manufacturer to the other. Enough to be statistically significant. The other reason is we are living in an era of 3D printers. The cost to design a propeller and have a prototype ready to test is dirt cheap. There are companies like GETFPV.com that design, print and test new props all the time. And, the props can be turned out fairly cheap from China. Thus, there are new propellers on the market all the time. The combinations of motors and propellers are endless.

Those friendly little specs that are listed with a motor.

Motor specs can drive you mad after a while. If a motor is listed as ranging from 2S-4S for example, the top thrust list may be at 4S. But not always. You need to take total watts, divided by amps and see how many volts it was tested with.

And, this is the big problem

If all that mattered was grams of thrust then they would figure out what the peak grams of thrust for each motor would be. If they were testing for maximum wattage they would figure that. If they were testing for efficiency they would figure out a battery, propeller combination that maximizes it. Since they are out to sell motors I imagine that if they posted the true peak specs people would assume that this is how you use it all the time and would burn out motors quickly and leave nasty reviews. Thus, what you are likely to see are some results that are watered down. The suggested prop and motor combination will work fine in most flying conditions. It’s like saying your car can achieve 70mph when it’s top speed is really 100 MPH. If you were to drive at 100MPH all the time it would wear out the motor faster.
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Here’s what’s important.

There are volts, amps and watts, which one is the most important? If you go over the required voltage suggested for your motor then you shouldn’t cry when your motor dies. Watts are good for reference, but amps are what you need to pay attention to. If you run too many amps through your motor for too long it will melt the insulation on your motor wiring and then the motor will short and die. Heat is a sign that your motor is working too hard. If your motor is blistering hot when you land your plane it’s time to think about a new prop and motor combination.

Different flying characteristics

I heard one guy describe his 4-minute flights as the best four minutes of the day. He’s looking for maximum thrust and battery life isn’t important. If you’re flying a glider you’re looking for motor efficiency. You want to find the right combination that keeps you in the air longer. It doesn’t take long to become a motor connoisseur. You can start pairing the right motor to your plane. You will prioritize what’s important when dealing with a motor. Another factor not yet discussed is motor weight. The heavier your motor is the disproportionately more weight you’ll need to put on the other end to counterbalance it. Heavier motors start putting on the weight of your plane rather quickly Heavier planes fly differently than lighter ones.

Useful data to keep in mind.

Gliders are looking for efficiency. You may even be able to get away with an underpowered motor if you’re able to give your glider a good, brisk shove into the air at takeoff and keep off of full throttle in flight. You may be able to get away with a larger prop than is suggested for your motor as long as you’re not drawing above the suggested amperage.

Here are some numbers to keep in mind.

Efficiency-

(Grams of thrust)/(Maximum wattage) This is a comparison number. It’s used to compare two motors. The one with the larger number is more efficient.


Watts per pound


(Maximum wattage)/(Grams of thrust)=Watts per gram of thrust. The result is almost always less than 1. The higher the number the less efficient the motor is. However, it’s best that the ratio is greater than .23 If you multiple .23* 453.592(this is the number of grams in a pound) it equals roughly 100 watts per pound. 100 watts per pound should make anything fly. It’s the general rule of thumb.



Battery life matters

The reason why your car doesn’t reach top speed at the speed limit is that it would be inefficient. Though your car can do 100 MPH it will do most of it’s driving at under 70 MPH. It’s this overhead you want in a motor and propeller combination. Battery consumption of LIPO batteries drops considerably when you are at 20% from full throttle as opposed to being at full throttle.

Real world tests

Thankfully we live in the world of cheap motors and cheap propellers. All you need is a fairly inexpensive voltmeter and a digital scale and you can set up your own lab to test your own motor and propeller combinations. The thing to do is to take measurements of grams of thrust at full thrust and backed down to 10 amps, 5 amps and 3 amps. Change out propellers and keep records of each thrust measurement. You will want to stay within the manufacturer's suggestion for maximum amperage and make certain that your motor isn’t getting hot. If you’re pulling excessive amperage or your motor is getting too hot you’ll need to back down the propeller size and/or pitch.

Take it up

Instead of one pure winner, you’ll want to take your favorite 2 or three propellers out for a flight. Figure out which ones give the best flight characteristics while keeping you within the zone of flying time. Once you’re in the air your results are no longer scientific because flight characteristics change from the flight. If, for example, one flight lasted 18 minutes and the next one lasted 21 minutes the way you fly and weather conditions may have caused the variation. By this point, all you’re looking at is the flight characteristics.

3 bladed propellers

A very common problem is that you are landlocked on your plane. You want a 9” propeller for example but can only fit an 8” propeller at the largest. If you simply switch to a 3 bladed propeller you can get the what you’re looking for. The idea is that if you fly with an 8x4 (8040) 3 bladed propeller it’s around the same as a 9x5 (9050) propeller.



The real downside to 3 bladed propellers is that they break on landings more frequently.

C1 Chaser Radio Controlled (RC) Airplane

Oversized Wing-Wing?

The Wing Wing is one of those planes that people just love. All EPO foam, a simple design, easy to build, easy to fly, durable, and a ton of fun to fly, The main directive of the Wing Wing is to make it a little speed demon. It’s loud and tears up the sky like nothing else. When the C1 was announced Matt Osborn put out a video saying that we’re all going to get divorced because we’re all going to buy one. He was crazy about it, and then another video came out where he said he hasn’t flown the plane in six months, why? The short answer may be that it’s not a Wing Wing. Yet, this is a plane that stands on its own merits.

The C1 Chaser-Overview

The C1 is a 1200mm EPO foam plane with a similar build to the Wing Wing. It looks like something that you may expect to see flying at Tomorrowland at Disneyland in the 1970s. It’s a Delta Wing with a serious spar going through the middle of it. The main bay is HUGE and can hold a lot of FPV gear if that’s the route you want to go. This plane is an excellent platform for FPV. It also demands larger batteries. Your 3S 2200mAh battery is probably too light for this plane and will need extra weight just to get it to fly. So, the downside to this plane is that you may need to buy some new batteries.

My FPV plane gripe

The radio controlled plane hobby is evolving. It started out from the need to simply fly planes line-of-sight. That means, as far as you could see it was as far as the plane could go. So, the planes were built for fun over short distances. But the newer FPV planes are designed with gear-loading in mind. They are designed to hold a lot of gear and travel much further distances than 2000 feet, so the fun factor for line-of-sight flying is forgotten in models designed for medium to long range flights. These flights are usually minimal on turns, going in fairly straight lines is the objective. Instead of being sports cars they are more like cargo trucks. If you try and fly them line of sight they do work but they may not be as fun and responsive as you would imagine them to be. The large Skywalker planes are not on my radar because I don’t plan on doing this type of flying. So, when I see planes like the C1 Chaser my first impulse is to think that it won’t perform. But, to my surprise, it does perform.

C1 Chaser - Normal Build.

What was suggested on the Banggood website.
11.1V 3S 3300mAh 20C Lipo Battey,




2212 2200KV Brushless Motor


6030 2-blade propeller


30A ESC






Hmm, the high KV motor and small blades means fast and loud. I put on an 8 inch prop with a triple bladed propeller instead. That provides lots of thrust and requires low amounts of power. It flies well even on 2S though hand-launching is more difficult with 2S. What I had to do was go through trial and error with batteries to figure out which one would work the best. I managed to make it fly with a 2700mA 2Sh battery with a little extra nose weight. The glide characteristics were off the charts! It caught thermals and increased in altitude with the motor off. I still didn’t have the right amount of weight in the nose which made it a bit hard to control. But it floated like a cloud. The lightweight characteristics meant that it didn’t have to move that fast in order to maintain flight. This is important if you plan on catching thermals.

The FPV Build

11.1V 3S 5200mAh 20C Lipo Battery


2217 1400KV Brushless Motor


8060 2-blade propeller


30-40A











This motor is heavier by around an ounce. That is counterbalanced by more than an ounce in the front The propeller is 8”, along with the motor it can definitely provide thrust. That battery is very heavy so it’s going to weigh the plane down. Yes, it will work with your FPV gear but it’s going to be a much heavier plane. It will require more energy to keep it moving and will need faster speeds to prevent it from stalling.
What do you want?


When you weigh this plane down it’s no longer a lazy glider. The extra gear is expensive, so perhaps you may want to work up to the heavier build of this plane. With more weight it won’t fly the same. It’s going to be harder to launch and is a lot less care-free in flight. My suggestion is to start off with the lighter batteries, figure out how to fly it then add your gear. When it’s built light, and you get the CG right it is very, very manageable in the air.

3 Thing I do and don’t like about this plane

#1. It’s a lot of great plane for the price!

Like the Wing-Wing, you get a lot for your money. For under $50 (USA) you get a plane that is actually very well built. The servos mount on the top which help prevent them from ripping off when you land on the grass. The lid snaps on and actually stays in place during fliight. The motor mount is not weak like the Wing Wing’s motor mount. It has different noses for FPV or non-FPV flying.

#2. It’s resilient

I’ve unfortunately given mine a good crash. Other than a broken propeller it was fine. I think I was more lucky than anything, but still. . . it can take a bit of abuse and hang in there

#3. It’s versatile

A lot of planes are one trick ponies. They are great for one setup only. What i like about this plane is that it can go many different directions. Do you want go full FPV? Sure, it can do it. Do you want a plane that you can fly with iNav? Yes, this is perfect. Do you want to make a glider? Why, sure, give it a try. Since it’s cheap it may be the model you use for more than one build.

What I don’t like

#1. It’s pure white with ugly decals.

White affects me more on some planes that others. This one just looks cheesy in white. It looks lIke a large flying styrofoam cooler. If you haven’t read my review of Krylon Foam Primer, this is a good time. This plane is begging for a good paint job and lamination. Since it’s in many pieces that are easy to paint separately it can make a very nice two tone paint scheme or even 3 tones.

#2. Cameras may be difficult to mount.

This ain’’t a Bixler. There are not many different mounting options for your FPV cameras. There is a hole in the nose that fits a camera lens. Which one, I don’t know, perhaps a Runcam? Other than that, it’s self-contained which makes it hard to mount cameras. Perhaps you can leave the lid off and make a base out of plywood? I’ve seen people mount cameras on the wing with Velcro.

#3. No spare parts

I don’t know what will happen if you break a motor mount. Supposedly, Smallpartscnc.com is making and aluminum motor mount for this plane. The owner tells me it’s on his wish list. If you break an elevon perhaps you can make new one out of foam or balsa wood. If you break a winglet and can’t glue it back together you may have to get creative with cardboard or foam. But, if worse comes to worse you simply buy a new plane and use the broken one for spare parts.

Overall

I fly this plane with a smile on my face. It’s a lot of fun. When properly trimmed this plane is very maneuverable. It can do flips and spins and various stunts though planes this size tend to not be as sporty as smaller planes. I am impressed how well it moves around a fairly small park, it’s very controllable. This is a plane you really need two versions of:

Version 1

This is the heavy build. It’s fun to fly this plane on 4S. For me a 2200mAh 4S LiPo worked perfectly. I had a 8x6 triple blade propeller. Yes, it tears up the sky. And yet it’s still fairly floaty. I’ve had larger batteries in it with longer run times. The extra weight helps the plane penetrate better on windy days. But, it flies like heavy planes do. This seems like a great plane for iNav or FPV. It also seems like it could do some good intermediate iNav flights. Perhaps out and back 4 or 5 miles. Play with different batteries, make it go for speed or distance, or perhaps tone it down a bit and have it float a bit more. Strange to say, but even with this build the plane is versatile. Just by changing batteries it can be a completely different plane. That’s what makes this plane so appealing in my book. It’s different flight characteristics on one build give it range.

Version 2

The goal of this is to make this plane as light as possible. I am going to power the plane with a D2208 1400KV motor and fly it with an 8” folding prop on a 2S battery. The idea will be for it to catch thermals. You can think of it as a low powered power assisted glider. The key will be a good launch. It may be underpowered for takeoff so the difference between success and failure may be a good shove into orbit. It may be a glider that you can take to the glider spots on windy days.

A word about launching

Matt Osborne talks about “The duck” of this plane, You launch it and the first thing it wants to do is fall back to earth on its belly. This can be reduced by tilting the nose up and giving the plane a good firm toss. Learn how to do it with your left hand. It needs to nearly fully power to take off but once airborne you should cut back on the throttle if the angle of attack is too steep.

Ideas for your first C1 Chaser

I’ve learned quite a bit about building planes and have learned a lot from my mistakes and less than stellar ideas. The motor mount can take huge motors and I know you may be tempted to start off with a D3536 as your first motor. I don’t know if this plane will balance with that heavy of a motor. Thus, to be safe, stick to 50g and 60g weight ranges of motors. There are a lot of great motors in this range.

Painting the plane

If you stick with white and land it on the grass over time the bottom will look very dirty as it rubs along the wet grass, time after time. This is a plane ripe for painting. If you don’t primer it first with Krylon Primer the EPO foam will melt a bit. It will give an alligator skin texture. It doesn’t look bad, but you may not like it. You can paint the wings one color and the fuselage another color. I painted mine red and black in honor of my college (San Diego State University). It looks so freaking sweet low to the ground. But at high altitudes the black is a bit difficult to see. My second C1 I painted the fuselage blue, the wings white with blue tips and red winglets. It’s a patriotic American red, white and blue. I am waiting on parts before I assemble that one. EPO foam is smooth and thus doesn’t need a lot of paint. Paint does add weight so try to get your painting done in as few coats as possible.

Lamination

Lamination reduces drag. It adds strength and rigidity. I go for a strong 5 mil lamination.


Battery- Start out with what you’ve got. Add weight onto the battery to get the plane to balance. For example, start out with a 3S 2200mAh battery if that’s what you’ve got. Go to hardware store, buy some lag bolts. They are long and heavy. Figure out which ones make your plane balance the way you like. Weigh your lagbots and battery combination. This weight combination is what you should look for in your next battery. It can be 3S or 4S if your motor supports 4S.