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Sep 01, 2020, 12:40 PM
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Build Log

Microblitz: a F3S plane under 250g


This will be my newest model: the microblitz, a under 250g F3S speedplane.
I was inspired by the work of powercroco, MikeRX and luke attubato (planemadman), after my micro F5B model, a speed plane should seem like a logical next step. My goal was to build a plane that could compete in the FDD u250g speed class, even though I have no intentions of competing. My observation was that the german fdd models were extremely aerodynamic and sturdy with their molded carbon airframes, but these airframes weighed quite a lot so they were restricted to relatively light power systems because of weight. Luke’s 250g fpv speed plane had a much lighter construction and could run a lipo twice as big as the fdd planes, but his plane was much less aerodynamic. The top speeds of all these models are quite close, all of them are faster than 250km/h even though their designs are vastly different.
Last edited by basgerritsen; Sep 01, 2020 at 12:54 PM.
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Sep 01, 2020, 12:41 PM
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The design, inspiration and some aerodynamic concepts.


All these previous builds seem to have one thing in common; a (realtively low) aspect ratio and fairly bulky fuselages. The idea behind the relatively low aspect ratio is probably that it causes less parasitic drag because of reduced frontal area with an equal wing area. The need for bulky fuselages is caused by the lipos and servos that are used, which are relatively wide. All the small speed models seem to have either a really blunt, wide wingtip (Lukes fpv plane) or a really sharply peaked wingtip. The blunt wingtips will cause a lot of induced drag, especially since a RC model capable of landing at a reasonable speed creates a lot of excess lift at high speed. The sharp wingtip of the FDD models should be slightly more efficient but the sharp increase in wing taper near the tips and pointed wingtips will probably cause a more turbulent airflow around the tips. I decided to build my model somewhere in between the FDD models and the models by Luke and MikeRX and utilize design concepts from them both and add some new ones myself.

I do not like models with a ludicrously high wing loading and thus my speed design needed a slightly bigger wing. Ultimately I chose to use a wing area of 5.5sqdm since that results in a wing loading I’m comfortable with. With my glass fiber over depron technique, I am able to construct very thin wings and I use very small 1.7g servos in the wings. Since I do not need to house fpv gear or servos which are more than 8mm thick, I can use a 6mm thick wing. This reduces the frontal area of my wing and allows me to utilize a higher aspect ratio wing, whilst keeping frontal area constant.

By keeping the wings thin and using really thin 450mah lipos in series inside the thin fuselage, the frontal area and parasitic drag should be reduced. I want to further reduce parasitic drag by using smooth curves all over my model and making the surfaces smooth by glassing them. I want to reduce the most drag in the induced drag department, since I will be running a slightly larger area wing than most, a lot of performance can be won right here.
Most of the small speed models have an aspect ratio of about 4-6, but my design has an aspect ratio of 7.44. Higher aspect ratio should reduce induced drag by losing less energy to wingtip vortices at equal wing area, they should also generate slightly more lift because of this. With the help of some of my fellow physics students at university I calculated that the ideal aspect ratio for a 250g model with 5.5sqdm of wing area should be around 7, so I am in the ballpark. The wing of my model is more elliptical in shape and should have a more efficient lift distribution than models with square wings. I will also crank the wintips up for extra stability and most importantly, even more drag reduction because they will act as winglets.
After taking all of this into account, I went into cad and drew up a design optimized around a 2208 motor and two 3s 450mah stick lipos behind each other. The fuselage has no more room than absolutely necessary and will house all my electronics. The wings have flaperons that are relatively wide but span less than ⅔ of the wing, this will result in a high roll rate (which I am accustomed to since I used to fly incredibly agile 3d models and pylon racers with full span ailerons). This will also allow me to utilize flaperons with a high deflection angle in order to greatly increase lift and drag at low speeds, resulting in a reasonable landing speed.

I plan to be using:
FS2A receiver, extremely light 4ch receiver
6-36v to 5v converter so I have the possibility to run 8s for absolute insanity
Iflight 2208 1700kV motor
4.7x14 folding prop by Marcus Koch (cut down from 6.5 inch)
30mm offset hyper spinner
HGLRC T-rex 60A 3-6s ESC
GNB 450mah 3s LiHV, two in series
1.7g, 2.5g or 4.3g digital servos, the 1.7s would be wing mounted and the 4.3s would be mounted in the fuselage in F3K style (I have accounted for the room needed for these servos in the fuselage)

Ecalc says it will do way more than 300km/h and usually those estimates are quite conservative, but who knows how well they work at such high speeds, we will see how fast it goes!
Last edited by basgerritsen; Sep 01, 2020 at 12:55 PM.
Sep 01, 2020, 12:44 PM
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The build plans


These are the build plans, entirely done in cad and all pdfs have vector images, so scale them up as much as you want.
The design was optimized by myself and some of my fellow university physics students and should perform very well
This model was designed around my power setup, so not every powerset will fit but you can surely experiment!

Feel free to build along with me to see how fast this thing can go!

Final parts list to come
Last edited by basgerritsen; Sep 01, 2020 at 12:51 PM.
Sep 01, 2020, 12:48 PM
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The build pt.1


I printed out my design and started working, the fuselage was cut out from 2mm and 3mm balsa (the bottom and top are 3mm, the sides 2mm), the wing was cut from 6mm depron and the tailfins were cut from 3mm depron.

I glued the sides parts of the fuselage to the side of the top part with CA and added 2x2mm balsa strips to the inside of the seam for extra strength. Next the fuselage sides were sanded back to the balsa strips so it became nice and rounded.

I added a 4mm tube wing spar to the wing in de location indicated in the plans, I glued the spar down with some foam safe CA and then I filled the rest of the 4mm deep spar slot with epoxy. After letting this cure for a day I started sanding the wing profile roughly to a TL54 profile using 60 grit sandpaper. I sanded the leading and trailing edge down using 120 grit sandpaper until they were so thin you could see through them. I finally sanded the entire wing down to 240 grit sandpaper and started glassing the top of the wing. I repeated the entire process for both stabilizers, except I did not add a spar and sanded them both to a symmetrical airfoil.

I bent the wintips up gradually until the tips had an angle of approcimately 30 degrees, this way they will somewhat act as winglets.

I glassed all the flying surfaces with 50g glass using my newspaper method. (see my F5B thread for instructions https://www.rcgroups.com/forums/show...tions%2A/page3)

This is where I am at in the build process now: the fuselage top and sides are done and the fuselage only needs the motor mount and the bottom, the wing and tail are glassed halfway and the only parts I am still waiting for are the 6.5x14 folding props and the spinner, they should arrive this week.
Sep 01, 2020, 12:49 PM
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The build pt2


I have now glassed all the surfaces and made a motor mount.
Since my f5b model, which is slower, exploded in midair I will add more reinforcements to the wing.
I will add an additional layer of glass near the root and near the tips, just like F3K models. The wing has a single spar running all the way through it so I am not that worried, it's already much stronger.

The motor mount is made from some thick though wall mounting plastic, with two layers of 0.5mm carbon plate on the front, it's really strong and mounted using CA and epoxy.
I have already given the motor some down and right thrust, but I might need to adjust later.

The motor fits the fuselage perfectly, the fuse was designed around this motor and this should really reduce drag.

*Edit* electronics are done now and mounted, wing is done expect smoothing out the bottom. I will probably mount the wing tomorrow.
Last edited by basgerritsen; Sep 08, 2020 at 01:06 PM.
Sep 01, 2020, 12:50 PM
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Flight footage *this post is reserved*


Here is the maiden flight!
Still need to dial in my rates and give it a bit more downtrim, the model still wants to climb a little bit.
Even though the speed passes weren't ideal and the model was climbing, I still got about 248km/h out of it according to my doppler software.
More flights to come!

microblitz: a 250g f3s model maiden and trimming flight. (1 min 36 sec)


On the second flight after the aero improvements, I hit 270 +-7 km/h average speed on a 150m track...
Microblitz F3S second flight after aero improvements 270+-7 kmh average speed (1 min 53 sec)
Last edited by basgerritsen; Sep 15, 2020 at 02:19 PM.
Sep 01, 2020, 08:20 PM
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jwfinn's Avatar
This should be very cool. Your wingloading should be nice and low. Wish you were on this side of the pond so we could compare it to my 6s speeder.
Sep 02, 2020, 01:07 AM
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perttime's Avatar
Oh dear. Another one
One of these days, I need to build something at least inspired by your designs. Perhaps start with a warmliner.
Sep 02, 2020, 03:53 PM
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Quote:
Originally Posted by jwfinn
This should be very cool. Your wingloading should be nice and low. Wish you were on this side of the pond so we could compare it to my 6s speeder.
I will be measuring the speed using a doppler pass but a side by side comparison would be very cool. If I ever visit the states again I will certainly try to swing by.
Sep 02, 2020, 03:54 PM
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Quote:
Originally Posted by perttime
Oh dear. Another one
One of these days, I need to build something at least inspired by your designs. Perhaps start with a warmliner.
Most of the posts I have made are from builds I finished months ago, but I decided to start posting now. This and the micro f3f are my current projects.
And also rebuilding the f5b wing after that one blew up after a speed pass with the koch props
Sep 02, 2020, 10:37 PM
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Quote:
Originally Posted by basgerritsen
I will be measuring the speed using a doppler pass but a side by side comparison would be very cool. If I ever visit the states again I will certainly try to swing by.
What doppler tool are you using? I can try to use same so we can keep it a level playing field.

-Josh
Sep 02, 2020, 11:25 PM
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Is this the ESC, for example?
http://www.helipal.com/hglrc-t-rex-6...li_32-esc.html

I'm currently designing and I've found the Avian 30A ESC to be heavy and bulky.
Given your use of a 60A unit in a very slim plane, I'm intrigued.
I'm new here, so perhaps I'm intrigued by the ordinary or ignorant of the extraordinary.

Your aero design philosophy is sound, reduction of frontal area is free power to be applied as you see fit.
More speed or more lift or lower speed range it's all free!

I'm curious about this size and weight range at literally the opposite end of the envelope, but there's so much overlap.
I'm working with my interpretation of Tim Weisbach's Gurke wing twisting.
Sep 03, 2020, 12:17 AM
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What if we put on A Tmotor f80 pro
Sep 03, 2020, 12:20 AM
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....Fantastic....!!...
Sep 03, 2020, 01:23 AM
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Quote:
Originally Posted by jwfinn
What doppler tool are you using? I can try to use same so we can keep it a level playing field.

-Josh
I am just recording the passes with a stationary microphone and then I analyse the spectrograms in sonic visualiser (free audio analysis software from university in england). Then I just use the doppler shift equations to compute the difference in radial velocity based on the shift in audio frequency.

If you have done such a thing before you can just install the software and get Dopplering, if you don't know how to do it I can make a tutorial.
I also do error analysis on the speeds so you can get a range where the speed should be in.

Keep in mind that a doppler pass only measures the change in *radial* velocity, which means you would need to pass right trough your microphone in order to measure the speed perfectly. If you fly over your mic or to the left or right your values will be too low by a factor of Cos(Ω). Ω is the angle between the line from the mic to your plane at the end of its pass and the straight line trough your mic in the direction of travel of your plane.
In plain English this means that in order to measure the speed perfectly you will need to either fly TROUGH your mic or come flying in from infinity and continue your pass all the way to infinity.
If you just make passes very close to the mic and continue your pass for a long time, you won't have too much of an error in your measurement.

The deviation factor should be CosΩ = Pl/((Cd˛+Pl˛)^˝), where Pl is half the length of your speed pass and Cd is the smallest distance between the mic and the plane (distance between plane and mic when it flies over the mic)
If you can estimate these values, you can just devide your measured speed by this factor and you should get a more accurate speed.


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