Scimitar: Chasing 100mph

[Pieliker96]

Excuse my ignorance, but if the problem of a larger propeller generating more thrust is that it hits the ground, why not increase the number of blades and thus maintain the original shape of the propeller? It is true that a larger number of blades affects the efficiency but really it might be better than cutting the propellers. A 5-bladed propeller can decrease its diameter quite a bit compared to a 2-bladed equivalent.

The 9x9 for Power Pack C was maybe a tad too large for ground clearance. A 7x9x3 would work well if I could find any. You're right that multiple blades is the way to go for increased performance if diameter is limited.

The diameter for the cut-down props is lower but not due to ground clearance concerns. Because I have to have such a high pitch to get the pitch speed required for high flight speeds, the blade area (i.e. diameter) has to decrease to keep the motor within power limits. Essentially I'm trading off static thrust (less area and stalling the blades at zero airspeed) for dynamic thrust at high speeds.

 

[danskis]

What a great thread and plane. I started at the beginning and read it through again. Learned a lot too.

 

[L Edge]

You might want to think about a buddy to launch your plane(knows what he is doing) just like we did when doing pylon racing. Just watch a couple of videos so it's quite fast. This is the slow stuff.

 

[Pieliker96]

Well today's flight ended deadstick with a shorted motor - I guess my props may be way too much. On top of that I actually ended up slower at 70mph. The ESC held up well and runs like normal with the original FT PPC/9x9 combo back in the plane. I'm not sure if the smaller motor wasn't up to the task or my custom prop was just waaay too much. Hopefully I can get a motor/prop combo and maybe ESC down at the local hobby shop tomorrow that'll fit my needs better.

The wheel pants did reasonably well on grass, at least similar enough to no pants to be comparable. It needs a lot of up-elevator to taxi with the CG as fore as I have it at the moment.

Lightly toasted windings

 

[Tench745]

I bet the inefficiency of that cut-down prop is what drove your amp draw way up. Glad it was only the motor that toasted.

 

[Pieliker96]

I bet the inefficiency of that cut-down prop is what drove your amp draw way up. Glad it was only the motor that toasted.

I agree. You can get away with taking an inch or so off the prop tips, but it seems 4-5" was way too much.

There wasn't an appropriate combo at the hobby store today, though I did pick up some 8x8s.


I'd be well out of CG range if the motors got any heavier, so I punched a hole through a former an relocated a battery strap. The battery tray continues through the former so it's supported all the way back.

I've got a Cobra 2820/8 1450kv motor and 60A ESC on order, hopefully coming by the weekend. eCalc says 105mph on 770W with an 8x9, which seems realistic.

 

[Pieliker96]

Well it did it! With a Cobra 60A ESC and 2820 1450kv motor swinging an 8x9, the Scimitar averaged 107mph between upwind and downwind passes! Top ground speed was a blistering 113mph!

Without wheel pants it seemed to slow it down by a mph or five. Either that or one of my batteries is better than the other.

This motor is one bad mamma jama. Doing runups it pulled super hard. I have no doubt it would not hesitate to chop at least two of my fingers off at full beans, given the chance.

 

[Bo123]

Well it did it! With a Cobra 60A ESC and 2820 1450kv motor swinging an 8x9, the Scimitar averaged 107mph between upwind and downwind passes! Top ground speed was a blistering 113mph!

View attachment 233986 View attachment 233987 View attachment 233988

Without wheel pants it seemed to slow it down by a mph or five. Either that or one of my batteries is better than the other.

This motor is one bad mamma jama. Doing runups it pulled super hard. I have no doubt it would not hesitate to chop at least two of my fingers off at full beans, given the chance.

Seen the vid, looks like fun 👍

 

[L Edge]

Hey, you did it with ease. When you get the desire to have the "need for speed" again, think about a swept wing and "V" tail to reduce drag and with your expertise, bet your foam design could easily hit between 135-150MPH. It's all in the right size prop/pitch and motor setup.

Again, well done!!!!!!!!!!

 

[Mr Man]

Plans?

 

[Pieliker96]

Plans?

No formal plans, but I do have all of the CAD, untiled svg/pdf sheets and working files along with some build photos for guidance

Scimitar (Plans Release).zip

drive.google.com

 

[Mr Man]

Hey, thanks!