Scimitar: Chasing 100mph

Pieliker96

Elite member
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The goal for this project is to go fast: 100 mph.

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The airframe and decal scheme are inspired by formula one air racing designs, particularly the Seguin Wasabi Special, shown below.

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It managed 70mph on 3s and 80mph on 4s using Power Pack C and a 9x9, 107mph on 4s with a Cobra 2820/8 1450kv and an 8x9!

It'll start out running Power Pack C with a 9x9 on a 2250 4s. If I don't hit 100 I've made CG allowance for a bigger ESC and motor. Speed will be determined by video recording, using doppler effect and time between two points. Projected weight is 900g, which with a wing area of 211 in^2 yields a high cube loading of 17.9. The wingspan is 40", supported by two thin carbon fiber rods.

I'm finishing up the interior structure at the moment. I'm planning to make the wing quick-detachable for easy transport: the goal is no screws to turn or servo connectors to fiddle with. I may make plans available after it's flown and I'm happy with the result.

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telnar1236

Elite member
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The goal for this project is to go fast - with the goal being 100mph. The airframe and decal scheme are inspired by formula one air racing designs, particularly the Seguin Wasabi Special, shown below.

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It'll start out running Power Pack C with a 9x9 on a 2250 4s, though that may be a bit too much prop for that setup. If I don't hit 100 I've made CG allowance for a bigger ESC and motor. Speed will be determined by video recording, using doppler effect and time between two points. Projected weight is 900g, which with a wing area of 211 in^2 yields a high cube loading of 17.9. The wingspan is 40", supported by two thin carbon fiber rods.

I'm finishing up the interior structure at the moment. I'm planning to make the wing quick-detachable for easy transport: the goal is no screws to turn or servo connectors to fiddle with. I may make plans available after it's flown and I'm happy with the result.
It's a great looking plane and it looks fast, though I would be worried about drag from the landing gear. On the other hand, any amount of drag can be overcome with enough power. Are you planning on using foam board or some other material?
 

Pieliker96

Elite member
It's a great looking plane and it looks fast, though I would be worried about drag from the landing gear. On the other hand, any amount of drag can be overcome with enough power. Are you planning on using foam board or some other material?
The plan is foamboard everything except for carbon fiber wing spars and 1/8" plywood for wing attachment.
The gear was as small as I could reasonably get them while still having acceptable grass-runway capability (2" mains). The wheel pants and strut fairings should help with drag. Like you said if it ends up being too much drag I can always throw more power at the problem, likely in the form of a beefier motor and ESC. The battery can do 650W at just 20C (rated for 35) so it's unlikely to be the bottleneck.

Pardon my ignorance. When you say your goal is 100 mph, do you mean actual or scale mph?
Real. Assuming 1/8 scale that would put it just over scale "mach 1" - far greater than the low to mid 200's of the full-scale.
 

danskis

Master member
Nerdnick's Spit isn't a stock swappable Flite Test Spit. Its got a lower wingspan and the next size up "can" for a motor (35XX) than @Pieliker96 is going to start off with. I haven't looked at the plans but I would assume Nerdnick put his famous "speed wing" on it. I would think that to reach 100mph with that small of a motor in the Scimitar he will have to reduce the wingspan or put in a 3536 or similar motor - or both. We'll see just how slippery that ship is.

I don't know anything about it but I've read on the forum that drag is exponential to speed - so that landing gear may not be an option.

Nerdnick here:

100mph 800mm FT Mustang - FREE PLANS | Flite Test
 
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Tench745

Master member
View attachment 232893
View attachment 232894

The goal for this project is to go fast - with the goal being 100mph. The airframe and decal scheme are inspired by formula one air racing designs, particularly the Seguin Wasabi Special, shown below.

It'll start out running Power Pack C with a 9x9 on a 2250 4s, though that may be a bit too much prop for that setup. If I don't hit 100 I've made CG allowance for a bigger ESC and motor. Speed will be determined by video recording, using doppler effect and time between two points. Projected weight is 900g, which with a wing area of 211 in^2 yields a high cube loading of 17.9. The wingspan is 40", supported by two thin carbon fiber rods.

I'm finishing up the interior structure at the moment. I'm planning to make the wing quick-detachable for easy transport: the goal is no screws to turn or servo connectors to fiddle with. I may make plans available after it's flown and I'm happy with the result.

That is one sharp looking racer! I assume the goal is as much about scale-appearance as it is about speed.

When chasing high speed drag is going to be your enemy, so I would work to reduce it wherever you can. There is a point where throwing power at a problem won't always be enough to fix it.
To this end I would suggest ironing the LE of the tail surfaces and, if your scale preferences allow, eliminating the gear entirely.
I would also consider the interference drag those cowling cheeks will generate where it and the wing co-exist.

As far as performance is concerned, a 9" pitch on a 2250 motor on 4s has a pitch speed of 280mph, so you only need 36% efficiency to hit 100MPH. Can't wait to see it.
 

skymaster

Elite member
This kind of looks like AP RASCAL RACER. Is he still around.
 

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L Edge

Master member
A suggestion for you is to call APC props in Calif and talk to the engineer. Explain what you are trying to do and perhaps he can steer you in the right direction so you can accomplish your goal of 100 plus in the pylon area. With the right combo of motor/wattage/prop it could put you over. They have a lot of technical data at their site that you will understand.

Disagree with your wheel pants. One, pita due to grass which will rip them off. Second, if you use a very narrow wheel, it will do better. We attained speeds of around 190(shooting for 205mph) to compete with the big boys and I removed the pants and picked up 3 mph and by changing the compression ratio of the 65% nitro engine was able to kick it up to 203 mph. Used a radar gun to evaluate speed.

Are you going to use a nose cone?

Another way to better 100mph, is build a "V" tail for less drag.
 

Pieliker96

Elite member
I don't know anything about it but I've read on the forum that drag is exponential to speed - so that landing gear may not be an option.

Power required is proportional to velocity cubed, assuming induced drag is small. So theoretically, doubling motor input power results in just a 1- 2^(1/3) = ~26% increase in airspeed.

That is one sharp looking racer! I assume the goal is as much about scale-appearance as it is about speed.

When chasing high speed drag is going to be your enemy, so I would work to reduce it wherever you can. There is a point where throwing power at a problem won't always be enough to fix it.
To this end I would suggest ironing the LE of the tail surfaces and, if your scale preferences allow, eliminating the gear entirely.
I would also consider the interference drag those cowling cheeks will generate where it and the wing co-exist.

As far as performance is concerned, a 9" pitch on a 2250 motor on 4s has a pitch speed of 280mph, so you only need 36% efficiency to hit 100MPH. Can't wait to see it.

Looks are definitely a priority. If I just wanted to go fast I would've designed a super simple minimum-area fuselage and done hand-launching to get rid of the landing gear. There's being fast and there's looking fast, hopefully I can accomplish both.

I do plan to fold paper over bbq skewers on the exposed edges of the tail feathers and wing, both for drag reduction and durability/longevity.

Pitch speed doesn't mean much if the motor doesn't have enough oomph to drive the prop to that speed. Realistically I'm expecting at least 80mph from this initial setup: my combat wing with twice the wing area and a prop-in-slot managed upwards of 70mph on a 9x6.

A suggestion for you is to call APC props in Calif and talk to the engineer. Explain what you are trying to do and perhaps he can steer you in the right direction so you can accomplish your goal of 100 plus in the pylon area. With the right combo of motor/wattage/prop it could put you over. They have a lot of technical data at their site that you will understand.

Disagree with your wheel pants. One, pita due to grass which will rip them off. Second, if you use a very narrow wheel, it will do better. We attained speeds of around 190(shooting for 205mph) to compete with the big boys and I removed the pants and picked up 3 mph and by changing the compression ratio of the 65% nitro engine was able to kick it up to 203 mph. Used a radar gun to evaluate speed.

Are you going to use a nose cone?

Another way to better 100mph, is build a "V" tail for less drag.

Good point on the wheel pants. I've had pretty good luck with them on grass, though they do tend to get torn up over time. The wheels I'm using are 2"diam X 0.5" width - reasonably skinny . In any case the gear are swappable/removable so I can make a set without pants to test speed and grass handling.

Really I'm just guess and checking for motor power and props. I had some electronics and props laying around, this is the fastest config I could find within those. If it's not enough I'll step up the power and if that isn't enough then the prop.

I do have a set of spinners that are around the right size, though I'm primarily concerned with motor cooling at the moment. If temperatures permit I'll consider it for drag reduction.
 

Pieliker96

Elite member
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Here's my solution for tailwheel steering and rudder off the same servo. I used three of the four arms of a control horn to anchor the tailwheel wire, then routed it down to the tailwheel. The fourth arm drives the rudder.

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Tail feathers glued up

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Empennage under construction and completed with decals
 
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L Edge

Master member
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Fuselage substructure: batt tray, motor mount, landing gear pocket, and fore wing attachment

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Some test-fits and mockups

By the way, it looks like it already doing 75 mph as it stands.

With a 9" prop, how much clearance to the ground when fuse is level? Thinking about your takeoff and landing with clumps of grass on strip especially if prop stops vertically on landing. Maybe review 8" size pitches as a start. Be prepared to use rudder if you hit a clump under the wheel or excessive torque on takeoff. That motor looks real powerful.

Also, start thinking about if you will have ground effects of low wing airplane on landing. You can ride it or start your flare to drop out of it to land. In old days, we pinched the fuel line(servo) to land so there was no go around. Watched $500 airplanes get trashed when they ran out of runway due to not flaring earlier.
 

Pieliker96

Elite member
By the way, it looks like it already doing 75 mph as it stands.
With a 9" prop, how much clearance to the ground when fuse is level? Thinking about your takeoff and landing with clumps of grass on strip especially if prop stops vertically on landing. Maybe review 8" size pitches as a start. Be prepared to use rudder if you hit a clump under the wheel or excessive torque on takeoff. That motor looks real powerful.

I have a tad over 3/4" of prop clearance nose-level, 2+3/4" in the three-point attitude. It's definitely less than I'd like for grass runways. I'm not planning on scale takeoffs though - the thrust-to-weight ratio is going to be up around 2 so the plane will probably leave the ground somewhere between a three-point and nose level. In any case I can cut down the 9" props I have if I end up prop-striking regularly.

Also, start thinking about if you will have ground effects of low wing airplane on landing. You can ride it or start your flare to drop out of it to land. In old days, we pinched the fuel line(servo) to land so there was no go around. Watched $500 airplanes get trashed when they ran out of runway due to not flaring earlier.

I've dealt with low-wings in the past and will be flying at fields that have many times the runway I expect to use, so I should be able to feel it out without consequence - especially with the capability of go-arounds. I'm more concerned about this being the first taildragger I've flown in a good while, and the most high-powered at that - so taxi and takeoff rudder control will be the name of the game. I've put in three degrees of right to help with the left-turning tendencies, hopefully that'll alleviate some control load.
 

Pieliker96

Elite member
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Fore wing attachment and MPX connector setup: the wing slots in to foam formers and some wood. This also holds the landing gear in

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For the aft wing attachment, I made a lil spring-loaded pin that drops into a plate on the rear former. The result is a completely tool-less and servo-connector-less wing attachment setup, quite convenient.

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Fuselage sheeted, along with the exhaust duct (underside of the wing)

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The main hatch is retained by two pairs of 1/2" diameter wafer magnets (second pic)

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Well she's done! The CG ended up far further forwards than I expected, so I added 5 nickels (~25g + glue) as tailweight to balance. I came in under my mass target of 900g at 842g including tail weight. Taxi tests and a maiden flight will be coming soon, pending weather - It's chilly up here in Chicago and we're due for a blizzard at the end of this week.
 

The Fopster

Master member
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Fore wing attachment and MPX connector setup: the wing slots in to foam formers and some wood. This also holds the landing gear in

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For the aft wing attachment, I made a lil spring-loaded pin that drops into a plate on the rear former. The result is a completely tool-less and servo-connector-less wing attachment setup, quite convenient.

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Fuselage sheeted, along with the exhaust duct (underside of the wing)

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The main hatch is retained by two pairs of 1/2" diameter wafer magnets (second pic)

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Well she's done! The CG ended up far further forwards than I expected, so I added 5 nickels (~25g + glue) as tailweight to balance. I came in under my mass target of 900g at 842g including tail weight. Taxi tests and a maiden flight will be coming soon, pending weather - It's chilly up here in Chicago and we're due for a blizzard at the end of this week.
Wow. That looks amazing - brilliant work!