View Full Version : speed kings

10-04-2005, 02:01 AM
Hi All,

This year we had an opportunity to havea go at doing the math for some competitors in the National Electric Flight Rally here in Australia and were amazed at how it went.

The new class had a limitation of lipo power of no greater than 100g, and a higher weight (cant remember what it was for nimhs).

AFAIK that was it.

We found that almost the entire field were using setups at 7.4V or low capacity 11.1V and 4.5 x4.1 props or so and going for 20-23k rpm.

upon crunching numbers we thought that the pitch speeds seemed very low, and we found that it was possible to make a 11.1V, 1570 3s pack at about 97grams weight, so amp draw could be as high as 20A on 12C for bursts.

We adopted a high rpm philosophy to maximise pitch speed and arrived at a 3 x 3.5 (we could only get a 3x3 prop though) at over 40k rpm.

This gave a huge advantage over the other setups and drew 20A static.

So the idea was to conserve the burst current draw by limiting it on the Tx and using only when necessary.

the first flight had our plane travelling with the pack for the entire race and then it dropped the throttle limiter to finish the race more than a lap in front (in the last 3 laps or so).

after several races we monitored battery temp and decided to use WOT for the entire last race. Needless to say it was a big win.

I am wondering if others use this approach (i.e. maximise pitch speed), as i just went over to the E-Hornet and noticed a mega 16154 and 5.5x4.5 prop setup at 21A and i thought that surely this isnt the fastest it can go.

Our plane covered the 260m round course in 8 seconds, which includes the turns- which is 117km/h, or 74mph average speed.

allowing for at least 1s turnarounds, the straights mustve been done in around 3 seconds- or close to 100mph.

frighteningly so, these speeds coincided beautifully with the math done.

i.e. 40,000 rpm at 3 inch pitch is approx 120mph.

whereas 21000rpm on a 4.5 inch prop is under 100mph.

is this an approach used globally?


10-05-2005, 01:04 AM
hmmm..ill reply to myself then...

i thought id go through this scenario and calc load, power, thrust, speed etc and plot them.

its interesting as you can see the tradeoff for pitch/diameter/rpm/speed and adjust according to whether you want to use your amps for thrust/acceleration, or outright speed.

i havent finished it yet- have plotted through to 6 inch diam by 8 inch pitch.

here it is as WIP.


Jason T
10-05-2005, 04:34 AM

Cool stuff. What airframes were you guys flying?

I was went to an F5B competition this past weekend, had a blast, learned alot and broke a go-fast plane.


10-31-2005, 07:34 AM
have to get back to you on this one...saw some more last weekend....even faster!

they are custom made here in Australia by Mani Reiderich. Fully molded.

ill ask him.


11-02-2005, 07:48 PM

Interesting choice of prop and logic.
I once asked the the 'Hot-Liners' on another forum,-

'why do you use such big diameter/pitch props, when pylon racers use small diam/pitch for speed, and all the old free flight power flyers used small diam/pitch for fast climb'

No one could come up with a really sound answer.
Perhaps we will see some screaming hot-liners in the near future.

11-02-2005, 08:03 PM
well, once the prop hits an airspeed that correlates to the pitch it is like a glow engine coming onto the pipe...i could only presume that that is the logic as they would provide poor acceleration up to that point...it really depends on what that ratio is. one thing is for sure and that is that a large diameter pitch accelerates less as airspeed goes up, the same is true for any unstalled prop, but the point at which acceleration occurs is different.


12-05-2005, 10:12 PM

Interesting choice of prop and logic.
I once asked the the 'Hot-Liners' on another forum,-

'why do you use such big diameter/pitch props, when pylon racers use small diam/pitch for speed, and all the old free flight power flyers used small diam/pitch for fast climb'

No one could come up with a really sound answer.
Perhaps we will see some screaming hot-liners in the near future.

ok heres the answer...the acceleration for a higher pitched prop lasts for longer and produces a relatively higher difference between when it comes on versus the large diam from static condition.

it is purely an equation of motion argument, but with a higher v1 for the larger pitch props than the velocity of the lower pitched props.

in essence you can accelerate (once the prop is at a certain airspeed) to a higher top end, which translates to better climb (as the relative velocity is higher). It doesnt climb as well at first, but more than makes up for that in the ensuing seconds.

12-05-2005, 10:13 PM
edit: not sure what the aim for hotliners is...that would be a clue though.

Elfi Flyer
12-06-2005, 01:23 AM
So, going from a 5.5x4.3 (direct drive on a Mega 16/15/3) to a 5.2x5.2, or perhaps a 5.2x6.25 should give me better top end speed but with less thrust? For that matter, a 5.0x5.0 would yield higher prop speed, less thrust, but more top end speed, at WOT once the plane was moving quickly?

The goal here is top end speed, not overall thrust in this given instance. The plane is a modified pylon racer, sort of a trainer pylon racer.

12-06-2005, 01:35 AM
there is some correlation between thrust and speed, but the pitch speed seems to be the most influential factor.

the simple answer to your question is ...yes, assuming that we keep the diameter either the same or within the ballpark.

12-06-2005, 01:35 AM
oh 1 thing to add...they tend not to unload as much though..be wary of that.

Barfing Pumpkin
12-06-2005, 03:40 AM
Is there a difference between "pitch speed" and "prop speed"? Where is prop speed measured? at the tip of the prop?


12-06-2005, 06:28 AM
pitch speed is the time it takes for the prop to move the distance of one complete revolution as if the prop was a corkscrew.

So the more rpm, the faster the pitch speed for any given pitch, AND also the higher the pitch, the more distance travelled per revolution...hence faster.

12-19-2005, 05:45 PM
Hey, I am terrible at math, I have a neu motor 1105/1.5YRPM/Volt 6800Volts 6-9Max Amps40*85+0.009n/a1.75with a 4.4 gearbox and run a thunder power 3cell 2100 Mha if this is not ejnough info just let me know oh and I am running a 35 e- flite speed control in a felip 400 plane weight is about 35 to 40 oz I think flying weight could be a little more. what should I be running on this I of course want the best of both worlds I like to climb fast I have a 12x8 I believe on it now I want it to still go vert. for climbs but would like a higher top end if possible. thank you for your time.

Jason T
12-19-2005, 07:17 PM
I had a Filip400 and my setup was a Mega 16/15/3 with 4.4 gearbox, 12x8 prop, CC45 esc with 10 cells of 1950FAUP nimh cells. This setup pulled about 38 amps on a hot battery.

What is the max amperage your TP2100 pack rated? If that is a 6800 Kv motor then 3 cells is probably too much voltage. But the gearbox will help prevent overspeeding the motor. That sounds more like a 7 or 8 cells of nimhs or 2s lipo pack motor. If your TP pack is not rated for at least 40amps then you will be pushing it very hard.

12-19-2005, 08:12 PM
My speed control is a 40 not a 35 and my battery is 15c cont. 20 burst

12-19-2005, 08:13 PM
3 cell 2100 Mhs

12-19-2005, 09:03 PM
11 x 7 prop will make it go slightly faster with slightly less thrust....but im with jason....big amps!..way over the ESC and motor spec. ..

have you run this setup? have you tested the current draw?

i get numbers over 50A for it...


12-20-2005, 06:22 AM
I don, know how to run the numbers I have a 1500 Mhp 7.4 2 cell lypo would that be better. I have been flying less than a year I don't really understand a lot of the tech. stuff. Can you point me in the right direction for study material. I learn very fast just have not been to this yet. To give you an idea of my learning ( hand eye coordination anyways) I was flying with a friend, flying a nitro trainer third time with a transmitter in my hands no buddy cord and a friend of his walks up and says to my friend you guys out here messing around with a trainer , my friend has some $6000 and $7000 dollar planes. He tells the guy ya this is his third time up. the guy to this day thinks that we lied " Know one flies inverted and does all what you were doing in that short of time.... You have been on a trainer." Anyways, enough about my obsessive nature. let me know if there is a place I can start to understand this. Thanks again everyone your help is priceless and I love this sight. sorry for the puffed up talk about myself everywhere I go everyone always asks how many years I have been flying .

12-20-2005, 07:06 AM
the best way to do the math is from an already created software tool available either for purchase or for free on the net.

that'll give you an idea of where your numbers will be BEFORE you buy your setup.

unfortunately i think the motor youve got is too hot for your plane, as you wont get vertical and more speed for under 40A (your ESC limit) and your plane weight.

the 2 cell lipo will rob you of rpm therefore top speed and thrust...but even with the 4.4:1 box and 11 x 7 prop its going to draw all of the 40A....which means you'll need a battery pack of around 2500mah.

i'll hang in there and help you out, whichever way you go.


08-02-2006, 06:31 AM
I have a question for you watt_the,

I just bought a hacker motor and i am totally new to this hobby;

the hacker motor i got is a:

1440Kv (RPM/V)
70g (2.5oz)
16A continuous, 28A burst
2-3 cell LiPo
Shaft 4.0mm

using the thunderpower 11.1v 2100mah

estimating the plane is going to be some where around 30oz

I got a 9x6 prop, is that too much for this motor?

This is going in a parkzone fw190.

08-03-2006, 12:10 AM
on 3s that'll draw around 30A..

2s somewhere near 20A.

check this out also...


08-03-2006, 03:52 AM
Thanks for that link, so do you think that is a worthy setup for a 32 0z plane?

Also do you think i would be running at a safe rate?

Using that site, do you think the 8x6 prop would make more sense?

08-03-2006, 08:31 AM
what is the prop rpm for the stock plane?

i note it is a geared 480?

i actually flew the PZ P51 and it flew fine on the stock setup.

what you want to do is compare the stock fw190 setup with this new one...i.e. what rpm is it getting on stock with its 10x8 prop?

i'd say your setup would be insane on 30A (overkill) and more than enough improvement on 2s at 20A. The stock 480 motor doesnt like much over 12-14A or so....so youve got about 30% more efficiency plus 30% more power.


08-03-2006, 08:50 AM
sorry, i found the gear ratio...


so the rpm for stock on the 10x8 setup would be about 5000-5500.

with your 2s setup you can expect rpm to be around 8000.

so you'll get about 50% more thrust, about a minute more flight time and about 10% faster aircraft.


08-03-2006, 01:52 PM
Actually i al running a 3cell thunder power 2100mah

With the 3cell 11.1v using the 9x6 prop it calculates 44.8 oz of prop thrust @ 58mph drawing 32A = 276 watts.

Now i know the motor is rated 290 watts and i have a 40a ESC

44.8oz thrust on a 32 oz plane should give me more then enough vert. The question i have is:

Would the 8x6 prop be a better choice because it draws 25.93A but only gives me 39.3oz of prop thrust.

Thanks for answering some of these questions, i am very new at this hobby and these forums are great.

08-07-2006, 02:17 AM
It doesn't look as if the question regarding hotliners has yet been answered.

A "hotliner" is any high-performance electric sailplane. "High-performance" is an elastic term, with those sailplanes on the lower end of the performance spectrum often referred to as "warmliners".

Because it's a high-performance sailplane, a hotliner is expected to climb vertically for as long as necessary and glide at high speed for long periods of time with the motor off. While a very small prop spinning at tremendous RPM will give you good speed once you get on step, it won't give you the climbing ability to perform the "motor off" portions of an F5B course. Your goal in F5B is to inject as much energy as possible as quickly as possible in order to complete the course with the power off, and a vertical or near-vertical climb is the most efficient way to do that.

As a quick overview of traditional F5B rules, you are allowed to climb up to ten times to complete as many laps as possible within 200 seconds. However, those legs are not allowed to have any leg completed with the motor on! This explains the unlimited-vertical requirement of F5B, and the large-diameter props, which small-diameter props cannot satisfy.

Now, if you're just flying a hotliner as a sport flyer, why not spin a small prop at insane RPMs and use your hotliner more like a straight-up pylon racer? Why not (except for prop max RPM restrictions)? People entered hotliners using non-folding props at a recent exhibition electric pylon event here in Utah, and apparently did quite well. Even with some "flying wings"!

Hope that's an adequate answer as to why a "hotliner" uses a high-pitch, high-diameter prop. They usually are making their vertical uplines at really high speed from the bottom of a power-off lap or two, so those high-pitch props are "on step" for the climb.

08-07-2006, 02:26 AM
good post...