LiftBuster's blog View Details
Posted by LiftBuster | Jan 06, 2012 @ 05:58 PM | 5,523 Views
Fascinating contemporary article about the 'Hiller' of 'Bell Hiller'. Mechanix Illustrated, December 1944. Definite inspiration for tinkerers and engineers.
Posted by LiftBuster | Sep 05, 2011 @ 07:52 PM | 5,553 Views
RC drives LiPo development.....LiPo's in turn drive manned flight developments. Sweet!
Posted by LiftBuster | Mar 13, 2011 @ 01:48 PM | 7,415 Views

Name: Test_Stand_Sensor_Area_ANNOT_rev1.jpg
Views: 349
Size: 93.9 KB
Description: Test Stand in 2009. It looks exactly the same in 2011.

note to readers: This is the third of a three part entry. It's likely to get edited for typo's, errors, and additional data , as were the previously posted parts I & II. When all three have stabilized I will knit them together into one blog entry.

last edited: Mar 26 2011, worked example blade-motor match added


This test stand was born out of the frustration of trying to find a set of rotor blades that was a good match to a semi-scratch fixed pitch RC heli. The LHS was no help at all on anything that wasn't on the shelf, or even where to go looking. And while the online offerings were numerous, about the only specification available for rotor blades was their diameter.

The message was: Buy a Brand XYZ Heli, Buy Brand XYZ replacement blades. Or third party replacements for Brand XYZ. Maybe you shave a little here and there and see if it works better (or worse).

Matching a rotor to a heli is a puzzle, and a very large piece of that puzzle is the rotor blade characteristics of lift vs. rpm, torque vs. rpm, and shaft power vs. rpm. Most plastic FP blades have curl, taper, and twist (i.e. shift in angle of attack as you go out toward the tip) making their characteristics very difficult to estimate even when you can get the dimensional specs for them. Who wants to do all that math anyway? So I built the test stand to simply measure the characteristics of a whole mounted...Continue Reading
Posted by LiftBuster | Mar 04, 2011 @ 11:35 PM | 12,401 Views
This topic has been covered a *few* times, see for instance....

...but for basics like this another worked example never hurts. Plus my use is non-standard and maybe yours is too.

Plenty of Kv/RPM meters that make accurate RPM measurements on BLDC's using just an electrical connection to the power leads can be purchased (e.g. Eagle). But as far as I can tell, most of them have a caution that RPM readings taken at partial throttle may not be accurate. At partial throttle ESC waveforms are full of high frequency components that end up inflating the RPM count. At full throttle ESC waveforms are crisp square waves with no high frequency components.

The rotor blade test stand detailed in this blog uses a direct drive BLDC motor to spin the rotors under test, and consequently it's operating speed will typically be under 25% of full throttle - very partial!

The Elenco Model M-1750 DVM ($25 street) when used per the scheme in the diagram below provides an extremely accurate measurment of the partial throttle RPM for the Park 480 1020 Kv motor on the test stand.

For a Park 480 speed of 2000 rpm the pulse rate = 2000 rev/min x 7 pulses/rev x (1/60 min/sec) = 233 pulse/sec. The meter has a resolution of 1 Hz and is accurate to 0.5%. Therefore the RPM resolution will be (1/233)*2000 = 8.58 rpm, and the absolute accuracy will be .5% of 2000 = 10 rpm.

In practice I have found that while the the motor is spinning it's frequency readout is actually stable to within 1 Hz, so the whole system makes good on the DVM's promise.
Posted by LiftBuster | Feb 21, 2011 @ 11:59 PM | 6,482 Views
If you are customizing swashplates, flybar rings, servo arms, or pushrod linkages to them or between them you might find the following useful.

Orienting yourself to the axis of the main rotor shaft, when the radius of the attachment point for the swash end of the link is different than the radius for the flybar ring end you gain the ability 'tune' the response of the flybar ring to the servo'd swashplate angle.

You can make the action either more agressive or more docile as desired. Or you might choose to better match the force & motion to servos that are lighter duty, faster, or just plain available.

The down side of using different mounting radii like this is that the angle transfer is an inherently trigonometric nonlinear function.

Worked out the following formula which answers all the questions related to the nonlinearity for any design dimensions. The graph was computed using measurments from a Walker 4#3B stock head with the dogbones seated on the inner set of swash sockets (i.e. the least nonlinear of choices).

NOTE: If you are working on 45 degree flybar control designs that employ flybar ring mixing (like the MSR for instance) this is a good foundation but you have to append a second similar function onto the end of this one to cover the 'ring-to-grip linkage and account for the 45 degree angle between the flybar axis and the feather shaft axis.
Posted by LiftBuster | Oct 01, 2010 @ 07:38 AM | 6,224 Views
September issue of IEEE Spectrum mag has a great article on the science, engineering, trials, 'discoveries' that went into the record breaking Sikorsky X2 helicopter. Written by the guys who did it. Presently available online free:

If you tinker on RC heli's, you will see some 'familiar material'.
Posted by LiftBuster | Sep 29, 2010 @ 09:26 PM | 7,022 Views


A useful first approximation of performance of any particular mounted heli rotor blade at ALL head speeds can be determined from a SINGLE simultaneous measurement of the following three quantities:

rotor speed,
blade thrust,
blade torque.

Restated, take down just three numbers for a mounted blade and you can estimate hover rpm, rotor torque, power consumption for that blade at hover for any AUW.

For definiteness of example, the following symbol and units conventions are set (by me, arbitrailty):

Quantity Symbol Units

rotor speed RPM ( rpm’s)
blade thrust F ( gf , grams force)
blade torque T (gf*in, grams force x inch)


If you are testing FP blades you will have to make a flat mounting plate that can be secured with the prop mount that comes with the Park 480 BL motor. The distance from the motor axis to the blade mounting holes should be the same as it is on the actual heli rotor head you are considering using. If you change the blade mounting hole spacing, the blade characteristics change and you should take another set of data.

If you are testing CP blades you will have to incorporate some means of setting the pitch of the blades safely for the test. If you change the pitch of the blades, the characteristics change and you should take another set of data.

Posted by LiftBuster | Aug 30, 2010 @ 10:52 PM | 6,898 Views
Wanted to actually measure the effect of various rotor head/flybar mods on FFF speed of micro helis (mostly Walkera 4#3 class). Turns out it's not as easy as it might seem, so I pass this on for anyone who curiosity leads them down the same path.

This technique only uses a digital camera and a media player and could be used to measure straight line speed of any park flyer or car either outdoors or indoors in a gym.


I did some searching on radar guns and micro heli's speeds (some of which, like the E-flite mCX) fall at the extreme low end of their range - not good for measuring small improvements. There is the 'Hot Wheels' radar gun, however it is reported that the small moving subject has to be very close to the gun to get a good reading....requiring two people one of whom will probably get hit - again, not so good. There is the stopwatch and the basketball court technique but that requires a gym and a helper...not as much fun as flying, so help is hard to corner (accurate when done correctly over a 30 ft distance though). There is a "Doppler technique" that involves flying by a microphone and then using some software to sort out the flight speed....not sure about that one, but it appears that one can obtain an iPhone app that makes it easy.


Determine the time of flight over a measured distance.

Use digital camera in movie mode (mine is a Cannon PowerShot A1100 IS which does 30 frames per second)...Continue Reading
Posted by LiftBuster | Aug 15, 2010 @ 03:19 PM | 8,046 Views
(Later parts will deal with getting results and what to do with them)

A very bare bones FP micro heli kit that didn't work so well started me on a quest to answer the questions:

How do you pick a rotorblade-motor-ESC-battery combination that basically hovers (as opposed to basically burns up)? How do you get a setup that minimizes battery current draw at hover?

There are only so many motors and gears that can be fitted into a given heli. But blades.....blades can actually be cut down and/or pitched to exactly match a motor-ESC combination.

However, re-shaping blades, mounting them to your heli, test flying, measuring battery current draw....this is a very time consuming and frustrating process because the variations resulting from assembly and disassembly often make it difficult to assess improvements.

For hobby work there is no sense in trying to calculate the lift and drag characteristics for whatever weird blade (curled, tapered, swept, etc) you want to try out, so I built a helicopter rotor test stand on the cheap just to see what I could see and it turned out to be surprisingly useful and a very interesting tool. So I pass it along in case anyone else might get inspired.

This is not meant to be a step-by-step, just meant to show the way. In RCgroups and other forums you will find examples of better designs, but none of them is as quick or easy to build.

At the outset of my search I...Continue Reading
Posted by LiftBuster | Feb 04, 2010 @ 10:34 PM | 7,297 Views
re: Adjusting the flybar angle for the Walkera heli mod detailed in thread "The ULTIMATE, PHASING-CORRECT ,~45 flybar 4#3B Bell head mod (aka Bell/Hiller Mod)"

This mod uses stock parts, is not too involved, and offers a unique opportunity to study the effects of varying the flybar angle for an undamped 'Bell' type rotor head.

It is not necessary to do any calculating at all to complete the mod. If however you are interested in tradeoffs possible with the components in this linkage, in designing similar linkages, or in a foundation for determining tolerances in similar linkages you may find the following information useful.