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Oct 03, 2016, 09:33 PM
Aeronautical Engineer
New Product

New Props from Racekraft Info Teaser


Introducing Racekraft 5051 Triblade Propeller. Designed from the bottom up to maximize performance and control.

This triblade was designed to meet the Betz condition for maximum efficiency at 60mph. This isn't efficiency in terms of flight time, however how efficient power is delivered to the flow. 3D aerodynamic models were built using potential flow theories and verified using CFD software. Blade flex was also considered and utilized to maintain a linear throttle response through most of the range of your stick.

We slightly changed the number convention to be used with our props. The last 2 digits of 5051 does NOT mean pitch as were with previous props. Pitch is a number poorly describing the performance of a prop. 2 props with the same diameter and pitch will not mean they will perform the same. Its missing key factors such as chord length of the blade and how that changes with radius. Instead the last to digits will stand for Advance Ratio. Specifically this prop has an advance ratio of .51, therefore 5051 for the 5 inch triblade. For those who still want to know the pitch for this prop, it is technically a 4.4 inch pitch.

Advance ratio is a number describing the ratio of the intended speed of the aircraft and the speed at the tip of the prop. Here is a great explanation: https://en.wikipedia.org/wiki/Advance_ratio
Advance ratio strongly dictates prop performance and is a major design point for its use case. A prop with a higher advance ratio means it is designed to operate efficiently at higher aircraft speeds whereas a smaller advance ratio means a design for slower aircraft speeds. It is easier to know the designers intent for the prop knowing the advance ratio and you can more accurately compare 2 props of the same diameter knowing this.

More technical info and wind tunnel data soon to come!

-Sage
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Oct 04, 2016, 12:25 AM
What is landing :O
Looks amazing ! What websites will sell these ?
Oct 04, 2016, 10:38 AM
Quad Enthusiast
These look very interesting, I'm really curious on some first hand experiences
Oct 04, 2016, 05:33 PM
novuh @ propwashed
neonbjb's Avatar
What equation are you using for advance ratio? From the wiki page:

J=V/(nD)
J = Advance ratio
V = True airspeed of the aircraft
n = RPM (power system-dependent)
D = Blade diameter

I don't understand how these variables can describe the performance of a prop. It seems like every 5" prop would have the same advance ratio assuming the aircraft and power system was identical.

I do like the idea of digging further into what pitch means and how to compare props with each other.
Oct 04, 2016, 07:59 PM
Registered User
MaTT GRaNT's Avatar
1st post on RCGroups and Sage revolutionizes the quadcopter prop world!!! Can't wait to try a set!
Oct 05, 2016, 10:23 AM
Aeronautical Engineer
Quote:
Originally Posted by Nitro123
Looks amazing ! What websites will sell these ?
Here is a list of our current vendors: https://www.racekraft.com/team/

We are hoping they will be able for purchase sometime next week!
Oct 05, 2016, 11:05 AM
Registered User
I'm no expert in prop design but I'm guessing you will want to keep your quad's weight low, hence the use of the Shrike as an example in photo. It seems like heavier quads would suffer on drop catch situations with the tips of these props, as well as limited low end response.
Oct 05, 2016, 11:06 AM
Aeronautical Engineer
Quote:
Originally Posted by neonbjb
What equation are you using for advance ratio? From the wiki page:

J=V/(nD)
J = Advance ratio
V = True airspeed of the aircraft
n = RPM (power system-dependent)
D = Blade diameter

I don't understand how these variables can describe the performance of a prop. It seems like every 5" prop would have the same advance ratio assuming the aircraft and power system was identical.

I do like the idea of digging further into what pitch means and how to compare props with each other.
Thats exactly the equation. Advance ratio will tell you the intended design speed of the prop. Lets compare a 5 inch prop with advance ratio of .51 to a 5 inch with an advance ratio of .25 (maybe soon to come ). RPM is a mainly a function of motor KV and battery voltage so for one quad set up a higher advance ratio (.51 in this case) will perform and is intended to fly at higher rates of speed compared to the .25.

Whats nice with advance ratio is that it is independent of the blade geometry and number of blades. So we can compare a quad blade to a tri blade, for example, using the advance ratio and know where the prop is designed to perform. Actually for an example, a tri blade and a quad blade with the same advance ratio will produce similar thrust and consume very similar power operating at that advance ratio (assuming design to meet the Betz condition which all of our props will).

A higher advance ratio would be preferred in a race since its meant to be operated at higher speeds whereas a lower advance ratio prop might be better for freestyle.

Advance ratio is used in the design process and it strongly dictates how to set up the geometry of the blade such as the pitch. Since pitch is not constant along the span of the blade for a properly designed prop it just doesnt make sense to describe a prop by it.
Remember that pitch is defined as the translation distance for one rotation in a solid medium. This make sense for a screw inside piece of wood. However for a prop in the air, air is not a solid medium and slippage occurs. Actually during hover your props are rotating quite fast and there is no translation... wouldn't that mean near 0 pitch? The point is there could be better ways to describe a prop and we think advance ratio is a more meaningful number.
Oct 05, 2016, 12:15 PM
Sticks and Nuts
JB_Jacobo's Avatar
Quote:
Originally Posted by Muninn_20
Thats exactly the equation. Advance ratio will tell you the intended design speed of the prop. Lets compare a 5 inch prop with advance ratio of .51 to a 5 inch with an advance ratio of .25 (maybe soon to come ). RPM is a mainly a function of motor KV and battery voltage so for one quad set up a higher advance ratio (.51 in this case) will perform and is intended to fly at higher rates of speed compared to the .25.

Whats nice with advance ratio is that it is independent of the blade geometry and number of blades. So we can compare a quad blade to a tri blade, for example, using the advance ratio and know where the prop is designed to perform. Actually for an example, a tri blade and a quad blade with the same advance ratio will produce similar thrust and consume very similar power operating at that advance ratio (assuming design to meet the Betz condition which all of our props will).

A higher advance ratio would be preferred in a race since its meant to be operated at higher speeds whereas a lower advance ratio prop might be better for freestyle.

Advance ratio is used in the design process and it strongly dictates how to set up the geometry of the blade such as the pitch. Since pitch is not constant along the span of the blade for a properly designed prop it just doesnt make sense to describe a prop by it.
Remember that pitch is defined as the translation distance for one rotation in a solid medium. This make sense for a screw inside piece of wood. However for a prop in the air, air is not a solid medium and slippage occurs. Actually during hover your props are rotating quite fast and there is no translation... wouldn't that mean near 0 pitch? The point is there could be better ways to describe a prop and we think advance ratio is a more meaningful number.

That's a really good point. By giving us this characteristic we can with some degree of accuracy select different props for different tracks.
Oct 05, 2016, 01:22 PM
☢ Self-Proclaimed n00b FPVer ☢
EngineerX's Avatar
Very interesting new prop. I like the fact that it's actually a new design and not just a duplicate of the already existing props, like the 5x4x3 that has already been duplicated by bunch of companies.
Props and much respect to RaceKraft for venturing into new propulsion designs and not following the herd. I'll be looking forward to checking these out for sure and hopefully RKraft makes these in 4" size and maybe even 3"..... though going to small scale applications the aerodynamics of propulsion don't play nice when we demand crazy amounts of thrust form tiny motors (1306 & 1407)/ high KV (3500 to 4000KV) / and relatively high (4S) voltage.
Oct 05, 2016, 05:07 PM
Registered User
Sweet. Gonna try these.
Oct 05, 2016, 11:48 PM
novuh @ propwashed
neonbjb's Avatar
Quote:
Originally Posted by Muninn_20
Thats exactly the equation. Advance ratio will tell you the intended design speed of the prop. Lets compare a 5 inch prop with advance ratio of .51 to a 5 inch with an advance ratio of .25 (maybe soon to come ). RPM is a mainly a function of motor KV and battery voltage so for one quad set up a higher advance ratio (.51 in this case) will perform and is intended to fly at higher rates of speed compared to the .25.

Whats nice with advance ratio is that it is independent of the blade geometry and number of blades. So we can compare a quad blade to a tri blade, for example, using the advance ratio and know where the prop is designed to perform. Actually for an example, a tri blade and a quad blade with the same advance ratio will produce similar thrust and consume very similar power operating at that advance ratio (assuming design to meet the Betz condition which all of our props will).

A higher advance ratio would be preferred in a race since its meant to be operated at higher speeds whereas a lower advance ratio prop might be better for freestyle.

Advance ratio is used in the design process and it strongly dictates how to set up the geometry of the blade such as the pitch. Since pitch is not constant along the span of the blade for a properly designed prop it just doesnt make sense to describe a prop by it.
Remember that pitch is defined as the translation distance for one rotation in a solid medium. This make sense for a screw inside piece of wood. However for a prop in the air, air is not a solid medium and slippage occurs. Actually during hover your props are rotating quite fast and there is no translation... wouldn't that mean near 0 pitch? The point is there could be better ways to describe a prop and we think advance ratio is a more meaningful number.
Ahhh I see, cool idea. Is there a way to measure this without a wind tunnel or fluid design app?
Oct 06, 2016, 02:26 AM
Registered User
urico's Avatar
Pre-ordered some from RDQ.
Oct 06, 2016, 08:57 AM
iFly SL
By convention, a propeller's nominal pitch is the distance that the 75% span station will travel forward in one revolution. The blade angle at that station is directly calculated from the advance ratio chosen. If the prop does not have washout then all the blade angles on all stations are typically calculated directly from the advance ratio. In fact the advance ratio is the very first thing that any prop designer first calculates when designing a prop to set the initial angles of all stations. Then he/she adds his/her own tweaks to those angles based on station chord, airfoil section for that station,...Soooo...what's the need for another convention to define the same thing some other way? Marketing?

Sportjet
Oct 06, 2016, 10:50 AM
Aeronautical Engineer
Quote:
Originally Posted by sportjet
By convention, a propeller's nominal pitch is the distance that the 75% span station will travel forward in one revolution. The blade angle at that station is directly calculated from the advance ratio chosen. If the prop does not have washout then all the blade angles on all stations are typically calculated directly from the advance ratio. In fact the advance ratio is the very first thing that any prop designer first calculates when designing a prop to set the initial angles of all stations. Then he/she adds his/her own tweaks to those angles based on station chord, airfoil section for that station,...Soooo...what's the need for another convention to define the same thing some other way? Marketing?

Sportjet
You bring up a good point. Your design approach also seems to follow blade element theory. Which is a great place to start, however you are not including spanwise flow effects caused from the circulation created at each blade element station to create lift. This circulation will induce different velocities at different stations along the blade. So blade angle is not purely a function of advance ratio; it also includes chord length, airfoil properties, and the chosen Cl per station.

The problem with describing a propeller based on its pitch is that it is hard to figure out where the prop is designed to perform at. If we give the design advance ratio it gives a spec to the prop that you can back out the designers operating choices for that prop such as free stream velocity and RPM target. This gives you a sense to when you should choose this prop for a certain flight case. It is hard to figure that out only knowing the pitch. Plus whats the point of giving a spec only true at one point along the blade? Its more meaningful to give a number that embodies 3 critical design choices: RPM, Speed, and diameter.

Also if you look at typical propeller performance charts (see an example attached), it is always plotted as efficiency(η) vs Advance Ratio (J). We are collecting wind tunnel data of our props so we can develop graphs like these so we can actually get a true performance comparison. So we want to start specing our props with the designed advance ratio now so we can have performance comparisons further down the line with more of our designs.

-Sage


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