So I have a 3-inch Acrobrat (with Mamba 1404 3650 kv motors), Caddx Tarsier, 170 g dry weight and some modifications that allowed me to put a baro on a Mamba FC. So far it flies pretty well with 10-11 min flight time on 3S 1300 LiPo and even 15-16 min on a diy 3S LiIon pack.

So I have an idea to try to convert it into a 4-incher, by turning the motors upside down under the frame. I will just need to add some landing gear for take off and landing, so it's doable with a penalty of additional 3-6g.

I know that with larger props I will have more flight time, maybe additional 25-30% more, but will there be any drawbacks? I mean probably the flight characteristics will change, but in which way? As a general rule ,what exactly changes in drone flight performance if we just swap the props to a bigger size, all the rest being the same?

rotor authority is decreased, it has a decreased response - if the motor could provide its full abilities as a 3inch, if there was still sparepower you could get even better performance everywhere.... not that easy to tell, 3600kv on 3s and 4inch sounds pretty good, you should get a little bit decreased responsivness/rotor authority in comparison.

So it will be like decreasing rates?

its not truly the same, somehow yes. the response/authorithy does limit the speedchanges, next to deg/s around the own axies, rates can -feel- different on a more responsive craft. a lot pilots feel the need to decrease the rates on very different throttle authority compared to their usual quads with lower authority.
one thing is rc rates and the otherthing is rare performance, both com together, both affect to eachother.

it might change the rates truly if you could increase the rate endless, somewhere the performance of two motors would limit the deg/s but just in theory, it would be unflyable high rates anyway than.

you -could- be more more limited in fast speed changes, like to a tree 180turn and back. a low throttle athority quad struggles and drifts, while a high throttle authority quad seems on rails.

low throttle authority can smooth out the flight byitself, its a pleasure if you just lime to cruise.

Ok thanks, I definitely need much more flight experience to truly understand the difference 

Do it.

"authority" / response is issue if you are racing tight gates and/or big motors on long arms (14") or if quad is underpowered (whoop).

1404 is very good motor for 4".. ofcourse only light 4025 biblade. I tried 3 mfcr 1404 models and Xing is best most powerful. I dont know if Mamba is good

170g is a lot for 3" props. Should fly BETTER more stable and efficient with 4" props, but dont expect super acrobatic.

My Xing1404 4" was zippy but originally 81g dry (with canopy, LEDs, strap, etc).

Your build with 80g 650mah is.. 7:1 thrust.. best case.

Hi Major Tom,

Well, let's start with the type of flying you want to do and what you expect from your quad. From the perspective of "easy cruising" {maybe a little proximity flying, but NO Acro Stunts or Racing}, I don't find an appreciable difference in my quads. In fact, I pretty much leave all of my quads on the tune that comes on the FC; just as it comes. I like to see how it behaves in the default tune. So far, I am still running on stock tunes. So, how much difference you see is sort of subjective. I will say, more aggressive pilots seem to notice differences more than I do.

Those 1404 3650 Kv motors will work fine on a 4 inch quad, however, you might loose some aggressiveness for Acro Stunts. On the other hand, they will be fine for cruising or cinematic flights. 

My Phantom-4X is running Flywoo NIN-1404 3750kv Dave_C edition V2 ultralight  motors with 4 inch props and it flies just fine. I am getting 13-14 minutes from a GNB 3S 1100 mAh battery; AUW {with battery} is about 225 grams.

My FrankenFly-2-4 is a "Pusher" built on a 4 inch Armattan Gecko bottom frame plate mounted with Flywoo NIN 1404 4850 kv Dave_C FPV edition motors.  In the original build, I had Taycan ducts and 2.5 inch props. The ducts are heavy and the craft was over 250 grams with a battery. Although it did fly pretty good, it just seemed to take more throttle for I was getting out it. So... I removed the ducts and swapped the 2.5 inch props for 4 inch props. Now, we are talking. The AUW with a GNB 3S 1100 mAh battery is now down to 216 grams and I am getting  16-17 minutes of Fly Time. An advantage of the "Pusher" is that you don't have any props in the view. 

The disadvantage of an open prop pusher is that the props are on the bottom and are subject to damage unless they are shrouded by ducts. 
If you don't have ducts, then it needs really decent and pretty tall landing gear. If the landing is not perfect, the props take one H*LL of a beating...as in they break. 

Ok, so I don't land very well. Thing is, the props are on the bottom and if the landing angle is off or the landing gear doesn't hit the ground first, then the props will. Depending on what they hit and how hard, there is very high probability that one will break. 

I will fly the FrankenFly-2-4 a little longer as a "Pusher", but I will likely "Flip" things back to a standard "props up" orientation. A Pusher really needs prop guards or some really super landing gear...Plus a pilot that lands a lot better than I do.  

Now for some other options.  

1) Use small arm extensions such as these ones (picture) that I have on my Phantom-X. 
    Making extensions is not difficult. I made these from Micro Alien 3 inch replacement arms.
    The Phantom-4X uses full Tomoquads Mosquito replacement arms; Picture 1,   Picture 2.
    You could also just cut a small plate from Carbon Fiber, Lexan, Fiberglass sheets, etc. and drill mount holes.
    Using arm extensions is actually extremely workable and you don't have to change the quad, 
    just move the motors out a bit and use Racewire to extend the motor wires.

    This Racewire even has LED lights. 
    Here is a tiny Racewire with LED.   <-- I would consider this one if I wanted an LED.             
    This Racewire is small, No LED, but has wires on one side.
    Here is the above Racewire without the wires.  <-- I would likely use this one, cut the wires, solder it in.

2) Mount the electronics and battery on the bottom and keep props up.
     This method takes a little more time and effort. Plus, you might need a method to keep the quad upright until take off
     since everything will be on the bottom. If your 3 inch has a stack, you might be able to "flatten" it by separating it into
     the individual components and mounting them on a single layer. If it is a single board AIO, then you just need to 
     consider where the VTX and Rx receiver will be mounted. Mounting everything on the bottom of the quad is
     a little trickier, but it is workable...just need to stabilize the quad in upright position for take off.

     If the props "touch", you can raise the diagonal pair with spacers which will allow the higher props to spin "over"
     the lower set. I don't know if I can find it, but I saw a video where this guy built a 5 inch quad on a 3 inch frame.
     It is workable, but also involves some work. Yes, he had to raise the two diagonal motors to clear the props of
     the other 2 diagonal motors, but it worked.

Personally, I would NOT make it a "Pusher", I would just extend the arms and keep everything else like it is.
If the motor wires are too short, I would suggest using "Racewire" rather than "extending" the wires; this way
you could just cut the wires and solder in the Racewire. 

Just a side NOTE:  I save and use all kinds of plastic; 
some plastics are strong enough for arm extensions. 
Just a thought 

Wow thanks IFly that's a lot of information! At least I'm not the only one to put the motors into the pusher position.

I was thinking about arm extension but could not find yet any suitable material. I could reverse the frame completely but in this case the camera when tilted up will get the props into view, and the whole concept of Tarsier and hd recording will disappear.

I was thinking also about new larger arms made from carbon tube which is light and rigid and would make a perfect H-style frame, but could not figure out how to attach motors on it.

So I will go with my initial plan of adding some standoffs under the frame. At both ends of the frame there is holes 6 mm each which I enlarged to 7 mm to accommodate a 7 mm cf tube. I will then use the tube and 2 mm cf rod to make a sort of "inverted T" to place it under the frame and glue altogether with epoxy. Should be enough for normal take off and landing, don't know however whether will survive any severe crashes... will put here some pics of the process...

(21-Apr-2021, 01:41 PM)Major Tom Wrote: Wow thanks IFly that's a lot of information! At least I'm not the only one to put the motors into the pusher position.

I was thinking about arm extension but could not find yet any suitable material. I could reverse the frame completely but in this case the camera when tilted up will get the props into view, and the whole concept of Tarsier and hd recording will disappear.

I was thinking also about new larger arms made from carbon tube which is light and rigid and would make a perfect H-style frame, but could not figure out how to attach motors on it.

So I will go with my initial plan of adding some standoffs under the frame. At both ends of the frame there is holes 6 mm each which I enlarged to 7 mm to accommodate a 7 mm cf tube. I will then use the tube and 2 mm cf rod to make a sort of "inverted T" to place it under the frame and glue altogether with epoxy. Should be enough for normal take off and landing, don't know however whether will survive any severe crashes... will put here some pics of the process...

Hi Major Tom,

The way that I have extended the arms is to use replacement Carbon Fiber arms and just attach where the motors mount. For me, the advantage is that I don't have to drill motor mount holes as they are already factory drilled. The Tomoquads Mosquito arms are wide enough that all 4 original motor mount holes can be used to secure it. I just cut the arms to size with a saw; no problem.

Now, I also keep several thicknesses of Carbon Fiber sheet which can be cut and drilled. This also works just fine especially if you have a drill press, but can be accomplished with hand drill or Moto Tool as well. I have recently discovered fiberglass sheets that are used in model rocketry which appear to be very strong and light weight. Now, I hate to venture too far out on a limb, but a heavy gauge plastic will also work. Perhaps, like the side of a blue plastic electrical box...just a thought. Anyway, the point is...there is a variety of suitable material.

Now, regarding the round carbon fiber tubes. They do make motor attachment parts, but I just don't know where to get them.

Send a picture of what you have going with the frame. I would like to see how it looks.  Thanks.

That's how it looks like now:

Hi Major Tom,

Very nice. 

About landing gear. You could mount them off the fuselage like you said. Are you sure that 4 inch props will clear them? If so, then I think that your idea should work, at least for take off. What it will not do is protect the props.

Here is a picture of the landing gear on my  FrankenFly-2-4 Pusher. I have extended the fuselage standoff columns down and mounted a piece of clear plastic as a bottom plate, then used "stick on" foam landing gear. It works, it just doesn't protect the props which I have broken several times upon landing...so this is not a drop from altitude, but a "try to be controlled" landing. So...buy a bunch of extra props.

By the way, Please start a BUILD thread for this endeavor so that we all might benefit from your efforts. I am interested to see how you proceed, the various stages of any tear down, the parts that you use or create, how it all goes together, and {of course} some flight footage.

If I was doing this 4" conversion... lots of cg problems:
you mount motors as pusher on opposite arm side.

And for "landing gear", just screw in 4aluminum stadoffs. Done.
But.. the battery is high up on top off cage farther from motors.

Yes Roman you're right about higher CG, but... a higher CG means the drone would have a more 'vertical' stand for the same force pushing forward, and more vertical means more effectively pushing down? I don't know, I will have to test it how it behaves...

(21-Apr-2021, 09:53 PM)Major Tom Wrote: Yes Roman you're right about higher CG, but... a higher CG means the drone would have a more 'vertical' stand for the same force pushing forward, and more vertical means more effectively pushing down? I don't know, I will have to test it how it behaves...

It depends on your objectives. If you want to do some ACRO stunts the CG might make some difference. On the other hand, if you want to do cruising or cinematic flights, the CG really won't make that much of a difference. Since I don't fly ACRO stunts, I can't tell the difference. I say GO FOR IT.  The mods will likely be much better than you expect. In fact, I have run a variety of quads with CG all over the place and can't really tell the difference. 

Major Tom, go for it...Flip the motors, build your landing gear, and give it a try. That is the way we learn. I have never been swayed by what some call standard conventions. I think out side the box and play where I want to. What I have have found is that the "standard conventions" are really just guidelines and you can work way beyond those restrictions. It all depends on what you want.  

If you are super ACRO stunt guy or a heavy into Racing, then you want certain characteristics from a quad. However, these criteria are specific to high performance, balls-to-the-wall flying. That is great if that is what you want to do, but it is certainly NOT the only way. It all depends on your perspective and how you want to fly. 

Major Tom, I say go for it. Do the pusher, Do a BUILD thread, Let's see how this thing comes out. In my opinion, it will be just as good or better than you expect...and...likely much better than others expect. Let's do this thing. I am with you All-The-Way. Let's go it.