Scimitar: Chasing 100mph

Pieliker96

Elite member
Taxi test went pretty well. I was still getting used to holding back pressure for taildraggers so it prop striked a few times. The gear need some tweaking as well - I was relying on the gear pants themselves to hold the wheels laterally, they ended up rubbing so I'll drip some hot glue down around the axles to act as spacers.


Maiden flight is weather permitting.
 

Pieliker96

Elite member
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I had a successful maiden flight and got some good speed test data! It managed 70 mph on 3s and 80 mph on 4s - I'll be ordering a bigger motor and maybe ESC. Power Pack C handled it fine, though it did have some help from the 40F ambient temp. In any case I'm very happy with how it flies and will be working on making plans available over the coming weeks.

 

Tench745

Master member
250 below 10,000, 200 in airport traffic area - but those are for full sized. 100 max for UAS.
100 mph max when fly under the recreational exemption, right? I'm not positive, but I believe you can go faster when flying with a part 107 license.
 

CappyAmeric

Elite member
I'm flying under the Part 107 exception for recreational flyers. To my understanding, part 107 doesn't apply in this situation. The exception doesn't give any speed limits that I can see, and niether does the AMA to my knowledge.
Well the first rule for using the recreational exemption is “…Fly only for recreational purposes (personal enjoyment)” so if you don’t have a smile on your face while going faster than 100, the fascist (I mean feds), will violate you (I mean fine you).

The government hates us. 😝

I think the only time when people have gotten nasty grams from the feds for exceeding is when they put up documented YouTube videos that were monetized, which meant they fell under Part 107.
 

Pieliker96

Elite member
ECalc's predictions for my current setup (66mph 3s, 81mph 4s) match the speeds I measured within a handful of mph.
I've settled on my new motor, a Sunnysky 2212 V3 in the 1400kv flavor. Interestingly enough this motor is actually smaller and lighter (will help with the overly fore CG), and it claims higher power handling capability.

With a 7x10 prop, ECalc estimates 81mph on 3s and 102mph on 4s, while only pulling 37A.
With a 7x12, that estimate jumps to 89mph on 3s and 112mph on 4s, pulling 42A.

I couldn't find any props that size so I'll be cutting some bigger props down. I will be exceeding the current limit of the flitetest 35A ESC, though I think this is a calculated risk. According to ECalc, the PPC/9x9/4s setup was already pulling 36A without issue. I've added external heatsinking to the ESC and it has decent airflow.

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I'll probably stick to the 7x10 setup for "normal" flying and use the 7x12 for max speed testing only as the predicted static thrust starts to really fall off at higher pitches.
 
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Tench745

Master member
Over-square props (pitch greater than diameter) are kinda hard to find. I have some wooden props 8x10 and 8x12 that are decades old, and those are the only over-square props I've seen in person.
 

RossFPV

Well-known member
ECalc's predictions for my current setup (66mph 3s, 81mph 4s) match the speeds I measured within a handful of mph.
I've settled on my new motor, a Sunnysky 2212 V3 in the 1400kv flavor. Interestingly enough this motor is actually smaller and lighter (will help with the overly fore CG), and it claims higher power handling capability.

With a 7x10 prop, ECalc estimates 81mph on 3s and 102mph on 4s, while only pulling 37A.
With a 7x12, that estimate jumps to 89mph on 3s and 112mph on 4s, pulling 42A.

I couldn't find any props that size so I'll be cutting some bigger props down. I will be exceeding the current limit of the flitetest 35A ESC, though I think this is a calculated risk. According to ECalc, the PPC/9x9/4s setup was already pulling 36A without issue. I've added external heatsinking to the ESC and it has decent cooling.

View attachment 233466

I'll probably stick to the 7x10 setup for "normal" flying and use the 7x12 for max speed testing only as the predicted static thrust starts to really fall off at higher pitches.
The power pack c v2 and b v2 use 40A esc’s so it would probably be a good idea to get that esc (the only change with the v2 power packs is the esc).
 

Pieliker96

Elite member
I had a look at another FT 35A ESC I had lying around. It uses six Toshiba TPH1R403NL FETs, which are rated to 60A continuous at 25°C. The maximum R_DS_on (steady state on resistance) is 2.1 mOhms. With a constant load of 42A, the FET would dissipate (R*I^2) = 3.7W. My understanding of brushless ESCs is that they only fire one of three windings at a time, which takes two FETs - i.e. dissipating 3.7*2 = 7.4W on average (not including switching losses, though those are small at such low frequencies). 7.4W is a lot to deal with for some bare FETs on a PCB under some heatshrink, but that amount of heat should be more than manageable when transferred through a thermal pad to a finned heatsink that's exposed to a good amount of airflow.

In any case, the proof in the pudding is in the eating. If it lets the magic smoke out, just know I was wrong.
 

L Edge

Master member
Still suggest you go to the APC site and go to the performance link and take a look at the data for a props. With your knowledge and skills, you can work backwards with what electronic equip you have and determine the approximate speed.

In the old days, we had to cut dia props and shape the ends(wooden only) and quite often the success rate for speed was about 50 %. Along came APC,. then got consistency. Then got our 190+ mph. Pitch is important.
You need to spec out your motor and prop to the max.

Watch clearance between prop and ground. Still suggest 8-9" dia.

Sample of 8" prop. Since props range in the $3-$4 range, order direct from them a range of dia and pitch so the actual run is the final answer. You can get them mail to you very quickly.

https://www.apcprop.com/files/PER3_8x8.dat
 

Pieliker96

Elite member
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Here's the cut-down props: Top, 11x10 to a 7x10, and a 12x12 to a 7x12 on the bottom. These were the smallest I could find in stock with the required pitches, but I still had to remove a lot of diameter. The cut props are quite bullnosed. If these are too much for the powertrain I've been looking at gas pylon racing props which require less diameter reduction and have less blade chord.

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I also got the motor in today, it's a 2212 compared to the FT PPC 2218. Supposedly it can handle more power despite the smaller size. It has plenty of cooling.

I'm planning speed runs first with the 7x10 then the 7x12 over this coming week. I'll also get some experience with whether the wheel pants work on grass.
 

cdfigueredo

Elite member
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.