Prop choice and thrust vs top speed vs acceleration

[Andy.T.]

As I am about ready to maiden another one, this time an FT 3D, I've yet again been pondering what prop to put on this thing. Earlier this year I setup a rig with a scale to test thrust and amp draw with various motors and prop combos. But this last round of testing, I had some results that just made me question the relevance of something like raw thrust. I've been thinking about it harder this time because, I assume, with a 3d plane, there is more reason to care about maximizing certain performance aspects. But I don't know what conclusions to draw.
Maybe I'll start with one example of something that surprised me that can be seen in below charts (difference between those two charts was just the battery; one was 35c, and the other was 50c). 10x6E and 10x7 props. I was not surprised the amp draw was higher on the 10x7, but I was surprised it didn't come with any gains in thrust. In fact, not only was there no gain, it made a fair bit LESS thrust.
So this is where my real questions started. I know lower pitch prop is less aggressive, easier on current draw, better for acceleration, a bit worse for top speed. Kinda like low gear vs high gear on bicycle. But, do these thrust numbers for those two rows (bottom row and 3rd from bottom in particular...comparing 10x6E and 10x7) not really tell the full story? Is there a world in which the 10x7 is actually a better choice for the FT-3D than the 10x6E even tho the amount of thrust it made measured less on the scale? Basically, I'm trying to figure out if there's more to making the best prop choice than simply raw thrust.

 

[Foamforce]

Here is my very basic and probably incorrect understanding. This is a “you get what you pay for and this is free” advice. 😂

The prop is like a couple of wings swinging around. The higher the pitch, the greater the angle of attack. A high pitch prop has a very high angle of attack, so when the airspeed is low, such as from a standstill, the prop is stalling. It’s just making a bunch of turbulence and not corkscrewing through the air like it should. Once you get up to a high enough speed, then the high pitch props can shine because the relative angle of attack is lower, so it’s no longer stalling.

A friend of mine and I were playing with various props on the same plane and saw this very clearly when we put an 8x4.5 prop on a Bloody Wonder followed by an 8x8. The 8x8 had a higher top speed, but when we hand launch, it sagged toward the ground and took a bit longer to get up to speed.

Alright, somebody step in and correct me here.

 

[Merv]

...Kinda like low gear vs high gear on bicycle....

Yes, when considering the pitch, diameter & Kv, think transmition. Consider 2 motors with the same watt rating. One a low Kv motor with a large diameter & low pitch is going other to have a lot of thrust on the test stand but limited top speed. Similar to low gear on a transmission, lots on lugging ability but limited top speed. The other motor, a high Kv motor with a smaller diameter high pitch prop will have less thrust on the test stand but higher top speed. Similar to high gear on a transmission. A tractor and race car may have the same Hp. They will operate at different speeds and lugging abilities.

The question is at what speed do you want the power.

The prop type also has an effect. A prop ment for glow fuel engines will pull harder than the same size Eprop.

 

[L Edge]

Welcome to the world of 3D. When you finish finding the best prop, you will have a bagfull of props with different diameters, pitches to match your motor that don't work.

1) You will need to have a thrust/weight ratio at least 2 or better.
2) You will need to set an aft CG and set the throws to cover large amounts of aileron, elevator, and rudder.
3) Next pick the maneuvers you will like to do. (Hovering is usually the hardest to do)
4) You will find out that between 1/2 throttle to 3/4 throttle tends to give you consistency with the right prop for most of the 3D stuff.
5) Now that is where you start exploring thrust, pitch, dia. come in. You will find that determines how well you perform that maneuver.

For instance, maker of motor says it covers a range of props from 8 to 10". So cover the range of pitches, doing each maneuver and see what happens. Many will be eliminated, then it's time to hone in on the few that work. In general, the closer you do it to the ground, the lower the pitch, provides more reaction time and adds a safety factor between crash/no crash.

Other factors, some props have a wide blade made in certain dia, which can take a 3D plane in a hover and made it much easy. Also, if you really get into 3D, buy digital servos which will improve your reaction time. And again, build your plane exceptionally light.

You think 3D is hard, try 4D.

 

[Andy.T.]

Great info, thanks guys! I was thinking that the lower thrust on the 10x7 must be indicating that there's more going on than just thrust ability. I can't wait to try a few different props in the air. I already have at least 2-3 pitches per diameter between 7 and 11 inches.
So, the disturbed airflow causing less thrust to read on the scale when testing on the bench, I bet that applies to nominal thrust gain noted going from 9x6 2 vane to 9x6 3-vane? I think I remember reading before that the air disturbance with more than 2 vanes was an important factor in choosing 2 vane over 3 vane for a lot of RC plane applications. But all of this indicating that raw thrust on the bench doesn't tell much of the full story, makes me SUER interested to try flying w/ some of the 3-vanes I have. I have a few 9x6 3vanes that look like super nice props; well built, sturdy, and just cool looking. Also not quite as thick of blades as a lot of the (uxcell I think is the brand) grey ones I've gotten cheap off Amazon.
So it sounds like, the main thing I'm proving on the bench is that a particular prop isn't going to be too much for motor & ESC. The thrust is in some cases enough to tell that a prop will at least be an option to try, but the final determination is going to come from actually flying with the props, since thrust measured on the bench isn't always indicative of performance in the air.

Also makes sense, Merv, about just deciding at what speed you need the power. There's not really a general starting point of something like "3D, you typically start with high pitch" or something? The main reason I just got some 10x7's was I thought I had noticed at least some of the competition 3D planes being set up w/ higher pitched props. But, L Edge, I hear your statement about lower to the ground, lower pitch for more of a safety net; why is that? Is it because better acceleration, so you can punch up and out of a bad situation easier?
Just thinking about flying in an almost constant stalled configuration, it seems like acceleration would matter more than top speed, which I would think would mean lower pitch.

I know my original attachment was showing stats on a Park 480, but I think I'm probably going to start with a Flashhobby D2836 1120Kv. That or the Turbo of that size, are what flies most of my FT planes so far, and is like half the weight of the Park 480. I think I saw an old post somewhere where someone mentioned that old motor, and I realized I had like, 3 or 4 of them mostly not in a currently used airframe so I wanted to try it out. OH! This actually was a whole other line of ponderings I've had after those bench tests. In general, I wasn't super impressed w/ the raw thrust numbers the Park 480 put up in comparison to the faster 1120Kv 2836's I've been using the last year or so. Is there perhaps more to a motor as well than not burning up w/ a given prop, and it's speed? Like, could it be that the Park 480 is just better delivering the low end acceleration than a D2836?
Sorry, so many questions. But it's SO interesting and exciting!!! The only thing more fun than gathering and charting data is analyzing and understanding it afterwards!

 

[Andy.T.]

Other factors, some props have a wide blade made in certain dia, which can take a 3D plane in a hover and made it much easy. Also, if you really get into 3D, buy digital servos which will improve your reaction time. And again, build your plane exceptionally light.

You think 3D is hard, try 4D.

Several more questions specifically to some of these statements. I built my FT 3D as light as I could I think, but not sure if it really was light enough. I think my AUW is going to be between 550 and 600g. It IS lighter than my FT P-40, so maybe light enough...?
I've also wondered about better servos than the cheapy 2-4$ per servo 9g MG ones that seem to have a DOA rate of ~10%. I haven't found what I think might be better quality substitutes. I'm hoping the 3D goes as well and fun as it does in the simulator, because I want to build a FT Edge sometime. that's where I was thinking maybe better servos would be well suited.

3d vs 4d....I presume that's a joke. Unless, 4D, the 4th dimension still being time, and you're saying that as you get older, that 4th dimension becomes more of a factor

 

[L Edge]

Several more questions specifically to some of these statements. I built my FT 3D as light as I could I think, but not sure if it really was light enough. I think my AUW is going to be between 550 and 600g. It IS lighter than my FT P-40, so maybe light enough...?
I've also wondered about better servos than the cheapy 2-4$ per servo 9g MG ones that seem to have a DOA rate of ~10%. I haven't found what I think might be better quality substitutes. I'm hoping the 3D goes as well and fun as it does in the simulator, because I want to build a FT Edge sometime. that's where I was thinking maybe better servos would be well suited.

3d vs 4d....I presume that's a joke. Unless, 4D, the 4th dimension still being time, and you're saying that as you get older, that 4th dimension becomes more of a factor

If you felt your FT 3D is built light, just now go out and fly it. Try what you did on the sim, and explore that with the plane. If you like, try some different props Take video and take the time to evaluate what your doing wrong. Make sure you are a couple of crash heights so you can recover.
As far as servos, you can go along with analog for now. Yes, a $12 servo(in general) is better than a $2-$4 one. See how you like it.

Yes, there is 4D flying.

 

[Andy.T.]

If you felt your FT 3D is built light, just now go out and fly it. Try what you did on the sim, and explore that with the plane. If you like, try some different props Take video and take the time to evaluate what your doing wrong. Make sure you are a couple of crash heights so you can recover.
As far as servos, you can go along with analog for now. Yes, a $12 servo(in general) is better than a $2-$4 one. See how you like it.

Yes, there is 4D flying.

No way....that's SO awesome!!! Obviously I knew bi-directional ESC's were a thing, but...the prop makes enough thrust either direction?!?! 🤯

Uhg, I SO wish I had time to build and fly today. this whole weekend is slammed. I have LEDs built into the FT 3D tho and it glows beautifully. I had intended on flying it at the local HS's parkinglot, which is pretty well lit, and with winter coming and the days so short, I think the plane will be lit enough that it should be pretty awesome. Not sure if I'm brave enough to do a maiden after dark tho. I think tomorrow night is the earliest I could possibly have it air-worthy just cause my weekend is pretty well booked until tomorrow after supper :/

 

[Merv]

...There's not really a general starting point of something like "3D, you typically start with high pitch" or something?...

No, start with a large diameter, low pitch prop. It will produce the most thrust at low air speed, like in a hover.

On 3S, I like a 3536 900Kv motor with a 12x4 or 11x4 prop.

 

[Andy.T.]

No, start with a large diameter, low pitch prop. It will produce the most thrust at low air speed, like in a hover.

On 3S, I like a 3536 900Kv motor with a 12x4 or 11x4 prop.

So I had been planning on 2836 1120kv largely because the thrust numbers and weight said it either didn't matter or might as well save the weight. It looks like I could swing an 11x5 on either a park 480, or the D2836 @1120kv. Thrust numbers and watts came out just a bit in favor of the 2836 when I had tested those configurations. Also looks like the wheels do actually give enough clearance for the 11".
Think that sounds reasonable enough or is there a reason you'd go 3536 @ 900kv over the slightly smaller slightly faster?

 

[L Edge]

No way....that's SO awesome!!! Obviously I knew bi-directional ESC's were a thing, but...the prop makes enough thrust either direction?!?! 🤯

Uhg, I SO wish I had time to build and fly today. this whole weekend is slammed. I have LEDs built into the FT 3D tho and it glows beautifully. I had intended on flying it at the local HS's parkinglot, which is pretty well lit, and with winter coming and the days so short, I think the plane will be lit enough that it should be pretty awesome. Not sure if I'm brave enough to do a maiden after dark tho. I think tomorrow night is the earliest I could possibly have it air-worthy just cause my weekend is pretty well booked until tomorrow after supper :/

They don't use Esc's, motor is fixed rpm's, it flies like a heli, center throttle is zero pitch, up is increase pitch, lower than 50 it moves into reverse. Yes, props loses efficiency by design. But enough to do it.

 

[Merv]

...is there a reason you'd go 3536 @ 900kv over the slightly smaller slightly faster?

I like a 12 inch prop, more thrust. The 11 inch prop will hover it. But the 12 gives it unlimited vertical. A 3536 1200 Kv would swing the 12 inch, but I don't like to push my motors that hard. The 900 Kv runs a bit cooler.

 

[L Edge]

What maneuvers have you been practicing? Your talking about flying the FT 3D at night with the leds at the local school. Suggest you start in the daylight. Easy way to lose your plane and worst case, you don't want to deal with administration if you do damage to the school.

 

[Andy.T.]

What maneuvers have you been practicing? Your talking about flying the FT 3D at night with the leds at the local school. Suggest you start in the daylight. Easy way to lose your plane and worst case, you don't want to deal with administration if you do damage to the school.

For SURE will start in the daylight!! I think the night time will be when I'm comfortable enough with how the plane handles and I've settled in on the motor/prop I like. It's a huge parkinglot with open space around it on several sides; I'm super not worried about hurting anything.
What I've got down pretty well are knife edge, slow rolls (honestly probably my second favorite; they just look SO awesome), harrier, getting pretty passable with rolling harriers too (at least briefly ), inverted all day long, elevators, flat spins (working on inverted here too). I think that the school parkinglot will be fun to goof off in with some harrier'ing around when I'm too out of other options due to snow and lack of daylight, just to keep me in the air more thru the winter. Maybe pretend I'm pylon racing around the light poles
I haven't figured out what other lights I'll need/want. Pretty sure I'm going to want something on the tail and nose as well. At the moment just have the strips in each wing. I'll report back when I finish at least the top side paint. I'm pretty jazzed about the scheme

 

[Andy.T.]

Ready for a maiden. Still have to paint the bottom and add some more decals, but everything else is ready. Hopefully flies more than once. Initial CG check tho, as far back as I could get the battery without bumping up against the landing gear, CG was about top of the airfoil. Quite a bit further forward than what I was hoping for. Still pondering on that. Also on initial flight settings. I have 50% expo on all surfaces in high rates, and bumped back a bit for low rates, 60% expo with 75% travel.
I was goofing off a bit in the cul-de-sac taxiing around in the dark. Lights are definitely cool; would realistically hope for a couple spot lights, like a landing light and something on the tail, for actual night flying. BUT I think I need something different on the electronics side for that. Wile I was just taxiing around, I had a 30a RoHS, with a 3a BEC, and it was feeling and smelling hot. I'll do it again tomorrow without the lights, but I'm sure that's whats extra from a heat perspective. No idea what else to do tho; haven't found any ESC's with larger BECs in this amp range.

 

[Merv]

...CG was about top of the airfoil. Quite a bit further forward than what I was hoping for. Still pondering on that....

I would recommend setting the CG as the plans indicate, at 25% of the average mean cord.
When you get the plane trimmed out and a bit of flite time on it, then slowly move the CG aft.

 

[Andy.T.]

I would recommend setting the CG as the plans indicate, at 25% of the average mean cord.
When you get the plane trimmed out and a bit of flite time on it, then slowly move the CG aft.

Perfect, sounds like a good plan. Get it trimmed, then move it back.
When measuring the chord on an aerobatic plane like this which has such huge ailerons the entire length of the wing, does that distance also count in the mean chord calculation?

 

[Merv]

...When measuring the chord on an aerobatic plane like this which has such huge ailerons the entire length of the wing, does that distance also count in the mean chord calculation?

It's not the length of the aileron that matters but the shape. If the aileron has the shape of the airfoil, then include it.
On thin ailerons, like the FT3D, that don't continue the shape of the airfoil, don't include them in the calculation.

 

[Andy.T.]

I like a 12 inch prop, more thrust. The 11 inch prop will hover it. But the 12 gives it unlimited vertical. A 3536 1200 Kv would swing the 12 inch, but I don't like to push my motors that hard. The 900 Kv runs a bit cooler.

I got to fly my FT-3D a couple times over the holiday weekend. Before I got it trimmed, holy cow was is squirrely!!! Surprised me a bit how much left aileron trim it needed, but it's fine. I probably just need some stick time, maybe even when I'm not shivering in 10 degree weather, but even the 50% expo felt pretty twitchy at times to me.
As usual, I had a question though. With only trying on an 11x5 so far, I think I can see your point about a 12" being even better. But with the 11", I only have 2" of clearance between prop and flat surface of workbench. That already seems to be more than eaten up when transitioning between uneven patches of grass, as the prop seemed to dig in on all the landings. Granted, landings were first 2 out of 3 rather dicey. So I was curious how long your landing gear wire is and how much prop clearance you have. I used the same gear as I built for my FT Simple Cub with 2.75" wheels. That leaves the "height" part of the landing gear wire at about 4.25".
Also related to prop clearance, the aluminum motor mount kept getting bent when prop dug in, significantly altering line of thrust. You don't have issues w/ the motor being bolted directly to the firewall, and it always ripping the firewall off the powerpod if the prop ever hits the ground or something? The heavy duty packing tape and hot glue for motor mount seems like maybe a 3536 is getting a big for that. And I'm still only running on a 2836

 

[danskis]

My motor mounts on foamboard planes have evolved. Because all my FT planes are belly landers I've moved to reinforcing the front of the sides of the fuse with balsa wood. Peel the paper off the front of the fuse back to the wing and glue/laminate some 1/16 balsa to the fuse. Glue the firewall to the balsa. Go ahead and use filament tape too.

Great thread...thanks for bringing up the topic.