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FT Goblin - Prop troubles

Duck

Active member
#1
My FT Goblin is by far the workhorse of my hanger and it almost always gets flight time when I go out. I've been on a slow quest to increase the speed ever since I saw someone else's FT Goblin at the field. We got to fly together which was amazing but they flew laps around me. I was running on 3S as that is what I had. I moved to 4S and it got quite a bit better. Initially I went down in prop size to a few different 4x3x3-ish props but those flew terrible. They had almost no thrust and a very low top speed. This includes a blunt nose prop. Afterwards I ran it on my amp meter I decided to move back to 5" props as the ESC had the headroom. I had a HQ 5x4x3 prop from my quad and these have worked the best so far. I tried a number of other 3 blade quad props with no success.

So I am on the HQ 5x4x3 which is OK but I know I can do better. At full throttle you can hear the flutter as the prop deforms due to the high tip speed. I am running an EMAX Red Bottom 2205 2300kv motor. HQ props are great for quads as they usually take a minor hit and keep going but I started to look for something stiffer and I went through my inventory of APC props. I found a number of better options for speed but I have a problem. None of them fit!

The problem I have run into is that APC props extend well past the prop nut very near the hole. There is a little forward peak on each blade. The red bottom motors have a flat surface and the prop must sit flush with it to fit. With my HQ quad props, this is fine! But to keep the numbers in the direction of flight, the APC prop runs into the motor and the nut can't be tightened down. I tried a range of APC props from 5.5x4.5EP, 6x4E, 5x5E, 4.1x4.1E and they all have the same problem. Is it possible to put a spacer in where I could still tighten the nut? Do I just need to swap motors? The goblin doesn't make that easy so I am trying to avoid it. And the motor is great!

The motor sits flush with the prop with my HQ 5x4x3.
1583386824093.png
The APC prop has parts that extend forward that will hit the motor and prevent it from being tightened. Numbers are on the face at the top of this image.
1583386854293.png

Any suggestions?
 

BATTLEAXE

Well-known member
#2
Does Master Airscrew make 6" props? I like them just as much as APC. A 6x4 would be a great prop on the Goblin. What size of ESC are you using?
 

evranch

Well-known member
#4
That looks like an APC thin electric. A couple options:

- APC pusher prop will not have to be mounted backwards
- APC sport props are a lot flatter in profile, much like a classic wood prop. They also handle ground strikes a lot better
- Why not a wood! Looks great, sounds great, flat on both sides.
 

Duck

Active member
#5
Does Master Airscrew make 6" props? I like them just as much as APC. A 6x4 would be a great prop on the Goblin. What size of ESC are you using?
A 4S 35 amp generic ESC. So I would assume 30 amps continuous would release a magic blue smoke. I would tell you the brand by the leads are really short and it is _hard_ to get it out of the fuse of the Goblin.

I would need to do a amp test for a 6x4x3 before I put it in the air as that would be likely above the amp limit for the esc if I had to guess. Ecalc gets mad at me if I put it in. I don't have the full version so I am using a 2204 2300 for the calc but it says 27A+ but 439W! It does say 80mph though :) That sounds fast enough!
 

Duck

Active member
#6
That looks like an APC thin electric. A couple options:

- APC pusher prop will not have to be mounted backwards
- APC sport props are a lot flatter in profile, much like a classic wood prop. They also handle ground strikes a lot better
- Why not a wood! Looks great, sounds great, flat on both sides.
evranch@ - I got very confused about APC's "pusher" props. I have a number of them, including the 5.5x4.5EP I mentioned above and they are identical in profile to normal props. The numbers are on the same face as the leading edge bumps. The only difference was the direction of the blade. I am assuming this is to avoid the prop unscrewing the prop nut on a regular CCW motor.

For sport props, I have never tried those. I have seen them once or twice in a few stores but APC is pretty bad about documenting how they expect customers to use their props and the applications. It may make sense when you already know what they say but is a poor introduction to the subject :( I can't find any side views of the propeller to confirm their flat face but I am happy to buy a few and see how they turn out. It looks like they are rated for higher RPM then regular 'E' props (https://www.apcprop.com/technical-information/rpm-limits/).

Although I just noticed:
Sport: Max RPM = 190,000 / 6" = 31,666 RPM
Electric: Max RPM = 150,000 / 6" = 25,000 RPM
Me: 2300kv * 14.6V = 33,580 RPM

I bet I don't hit that but apparently even the 6x4 is spinning too fast for 2300kv on 4S. I would happily use a 5x5 instead. ecalc gives an entirely different and less threatening set of warnings for that.

I have never flown wood. Can you recommend one for this use case?
 

Piotrsko

Well-known member
#10
A long time ago when I was into full size, there was a concept called making pitch, where if you are efficient, the distance the prop moves forward in one revolution is equal to the prop pitch. In the case of my pacer that was, iirc, 52 inches OR 125 mph. SO https://www.headsuphobby.com/calculate-propeller-pitch-speed/ could give an approximate airspeed, but small props are lousy for efficiency. As long as you don't exceed your esc amp rating in watts, that should make you go faster.
 

BATTLEAXE

Well-known member
#11
I do have this silly motor laying around.
https://innov8tivedesigns.com/images/specs/Cobra_2217-8_Specs.htm

I bought it as a replacement for my Alatus motor but never needed it. A 6x4 on 3S is listed as 76-94mph which is quite respectable. It would probably add 75g with it and the larger ESC needed. Maybe I can build a second goblin and race them?
It looks like anything in the 4s range will put you over the 30 amps range you are looking at. The sport props are meant for nitro motors so they are built a lot stronger then the electric props just because they need to be able to handle the pulsing from the compression and power strokes of a glow engine. Because electric motors don't have this pulse the electric only props don't need to be as strong. I have used electric only props and have seen a minimal amount of efficiency out of them compared to sport props but since my planes are belly landers for the most part, I use the sport props.

The cool thing is that is that if you use a lower kv motor you will draw less amps, meaning you can use more voltage to stay in the 30 amp range. Plus a lower kv motor will have more torque so you can also use a higher pitch prop. This also relates to more static thrust. But like anything else there is always a trade off some where.
 

Duck

Active member
#12
It looks like anything in the 4s range will put you over the 30 amps range you are looking at. The sport props are meant for nitro motors so they are built a lot stronger then the electric props just because they need to be able to handle the pulsing from the compression and power strokes of a glow engine. Because electric motors don't have this pulse the electric only props don't need to be as strong. I have used electric only props and have seen a minimal amount of efficiency out of them compared to sport props but since my planes are belly landers for the most part, I use the sport props.

The cool thing is that is that if you use a lower kv motor you will draw less amps, meaning you can use more voltage to stay in the 30 amp range. Plus a lower kv motor will have more torque so you can also use a higher pitch prop. This also relates to more static thrust. But like anything else there is always a trade off some where.
Yes, that motor is silly. I have a 60A ESC to go with it :) The original Alatus motor/esc combo was overpropping the motor. They have swapped out the motor on their new kits to a smaller/lighter one.
 

evranch

Well-known member
#13
APC's "pusher" props ...identical in profile to normal props. ...The only difference was the direction of the blade.
Yes, this is to avoid unscrewing the nut. If you are willing to run a regular prop backwards and risk losing it, they are the same. I usually take a regular prop and polish the numbers flat and mount it facing forwards, since pusher props are less common. This works on sports, but as you have noticed thin electrics have a lot of curvature that stands proud of the hub.

Sport props are made as a replacement for wood props on gas motors. They have a much thicker, flatter profile. They aren't as efficient as thin electrics, but they are a LOT tougher and they also sit flat like a wood prop would. They don't have any projections like the thin electrics do, they are more like a stick with some material removed.

Size them pretty much with the same twist and diameter as the electrics. They are just a bit less efficient.

Wood again is much the same as a sport prop. Pretty much use the same size. You can get thin electric woods, they have a beautiful scooped shape but again stand proud of the hub. A couple notes with wood, you usually need to balance them while you can usually get away without doing so on plastic. And while they are pretty tough for belly landings in the grass, they are much less resistant to chips from things like gravel rocks getting sucked up on a takeoff roll.

But just look at how nice they look.

IMG_20200305_171241.jpg

IMG_20200305_171323.jpg


Notice how the 12x8 sport sits perfectly on its side. It's a stick from which material has been removed. You can mount it with either face touching the motor. The 12x7 light electric will not sit on its side, it tips over because only the back is flat and the front is scooped. As you can imagine, the sport prop is significantly tougher, it's a classic prop for gas motors. The gentler torque delivery of electric motors allowed the development of the lighter, but more efficient thin electric profiles.
 

Duck

Active member
#15
I just used a nylon washer from the hardware store. This gives me enough clearance to run the APC 6x4 two blade prop on the Emax 2305
I'll try this first. The selection of sport props I can find is fairly limited and I already have nylon washers in my bucket of miscellaneous fasteners. Thanks.
 

Duck

Active member
#16
Yes, this is to avoid unscrewing the nut. If you are willing to run a regular prop backwards and risk losing it, they are the same. I usually take a regular prop and polish the numbers flat and mount it facing forwards, since pusher props are less common. This works on sports, but as you have noticed thin electrics have a lot of curvature that stands proud of the hub.

Sport props are made as a replacement for wood props on gas motors. They have a much thicker, flatter profile. They aren't as efficient as thin electrics, but they are a LOT tougher and they also sit flat like a wood prop would. They don't have any projections like the thin electrics do, they are more like a stick with some material removed.

Size them pretty much with the same twist and diameter as the electrics. They are just a bit less efficient.

Wood again is much the same as a sport prop. Pretty much use the same size. You can get thin electric woods, they have a beautiful scooped shape but again stand proud of the hub. A couple notes with wood, you usually need to balance them while you can usually get away without doing so on plastic. And while they are pretty tough for belly landings in the grass, they are much less resistant to chips from things like gravel rocks getting sucked up on a takeoff roll.

But just look at how nice they look.

View attachment 160371
View attachment 160370

Notice how the 12x8 sport sits perfectly on its side. It's a stick from which material has been removed. You can mount it with either face touching the motor. The 12x7 light electric will not sit on its side, it tips over because only the back is flat and the front is scooped. As you can imagine, the sport prop is significantly tougher, it's a classic prop for gas motors. The gentler torque delivery of electric motors allowed the development of the lighter, but more efficient thin electric profiles.
Thanks for the information! I really wish APC actually documented this themselves :( I found a bunch of conflicting information on sport props. Even if I don't pick up a wood prop for this I will give one a try.
 

Headbang

Well-known member
#17
Think dalprop. The T5045 blunt nose props are awesome for speed. They make a triblade props as well. I like to use 5045 props on 4s with a 2205 2300kv motor on my goblin. It is crazy fast, pulling 32amps. I use a 50amp esc.
 

Headbang

Well-known member
#19
Not as fast as a buddies strix goblin (240kph) but faster then any edf I have. If I had to guess I would say in the 160kph range. If you want to go crazy fast, look at the strix goblin setups. 2207 motors, 5x4x3 props, ect.
 

Headbang

Well-known member
#20
Now I think about it.... I have a 2207 2400kv motor laying around, I was saving it for a new northern pike. It would work well with a 5x5 carbon fiber prop on 4s and a 45amp esc. Might be a good excuse to try one of those new spectrum avian 45amp smart esc's.