Author Topic: Electric Speed again (Read 3018 times)

Scott Jenkins · « **on:** March 28, 2007, 02:36:44 PM »

Guys,
Since we are talking about small CL electric how about the gear necessary for events 606, and 607 the rules set follows.
Really would like to see something in the rpm range of 40k + without melting down a set of batteries per flight. We talking a maximum of 15 laps at most.

3. Control Line.
3.1. General Specifications. The use of gear boxes and multiple motors is permitted.
Folding props are not permitted. No electrical power may be transmitted through the control lines.
The use of radio control is not permitted. All applicable sections of Control Line General and
Control Line Speed shall govern model and line construction and the timing of flights. There shall
be no loading requirements for Electric Speed models.
● 3.2. Class A Control Line Speed
For event 606.
Any motor(s) allowed but the battery pack is limited to a maximum of 8.4 volts nominal of any
battery type.
The model must successfully pass a 25G pull test prior to flight.
The flight speed will be calculated based on the ten (10) laps of the circuit following three
complete laps in the pylon.
Two (2) control lines, solid steel, shall be used and they shall be 42 feet long. Multistrand
lines are not permitted. The minimum diameter of each line shall be .012 inches.
The CD will determine whether hand launches are permitted.
● 3.3. Class B Control Line Speed
For event 607.
Any motor(s) allowed with the battery pack limited to a maximum of 42 volts nominal of any battery
type.
The model must successfully pass a 25G pull test prior to flight.
The pair of solid steel control lines must be at least .018 inches in diameter and not shorter
than 60 feet in length. Multistrand lines are not permitted. Maximum weight shall be 60 oz.
Flight speed will be calculated based on seven (7) laps of the circuit following three (3)
complete laps in the pylon.
The CD will determine whether hand launches are permitted.

Scott

dynasoar1948 · « **Reply #1 on:** March 29, 2007, 06:18:02 PM »

Scott,

Are there still maximum weight requirements in the AMA rules? A had a max of 30 oz and B 60 oz.

You really don't want 40Krpm in either class, and definitely not in B. Props would clearly have increasing transonic flow over the outer diameter portion. This is already a limiting factor in F2A Speed

A generalization that holds true over the full range of prop application (in gasses) is that efficiency peaks where diameter and advance/rev ("pitch") are the same. (an 8/8 at 20krpm is better than an 8/4 at 40krpm) My 120 MPH B Electric uses an 8/8.

Will Stewart AMA 1947

Mike Palko · « **Reply #2 on:** March 29, 2007, 09:59:08 PM »

Hi guys,
As far as I know there is no weight limit for class A speed and a 60oz weight limit for class B.

Scott, your information about prop pitch vs. RPM is intersting. I am not saying you are wrong, but the electric guys can't seem to get enough RPM. Thats where they get their best performance.

The only advantage I see to the lower pitch and higher RPM is that there may be less chance for "slip" in the three corners of the RC pylon course. I would imagine in the 30+ G turns if you lost airspeed you would recover slower with a higher pitch (not good in pylon racing)? However, to support your data, C/L speed has no corners (although I have seen some attempted!!!), so the speed gains with higher pitch and lower RPM may out weigh the "traction" gains seen in higher RPM?

As far as loosing efficiency, I didn't think you lost efficiency until you went supersonic? Do you know if efficiency drops off with RPM (is it linear or exponential)? I have read that using a prop that is over square (8x8 being square 8x9 being over square) you will loose efficiency.

Turns out a good friend of mine Bob Wilkie and I are putting together a class A speed ship. Bob offered to do the design work and building, if I put together a power system. If all goes well we are going to attempt a record flight at the NATS flying a profile test bed (They will have the officials and timing equipment). We are planning on a full fuse version for 08 and trying for a second attempt at the record. BTW, it will be spinning high RPM

.

RandySmith · « **Reply #3 on:** March 29, 2007, 11:09:43 PM »

""As far as loosing efficiency, I didn't think you lost efficiency until you went supersonic? Do you know if efficiency drops off with RPM (is it linear or exponential)? I have read that using a prop that is over square (8x8 being square 8x9 being over square) you will loose efficiency""

Hi Mike

What you read there is not correct, many planes go faster with a 8x9 than they do an 8x8 or 8 x 10 than they do a 9x9 or 10 x 10,9x8 etc So whomever made that as a blanket statement is incorrect.

Regards
Randy

Paul Smith · « **Reply #4 on:** March 30, 2007, 06:02:46 AM »

We have achieved supersonic tip speeds on F2d and Fast combat jobs.

The first clue is when the blades leave the hub, followed by general disintegration of the aircraft.

Not a matter of efficiency, but survival. The props look stubby, but stay together.

F1c solved this issue with geared Nelsons, as did the Army Air Corps.

Scott Jenkins · « **Reply #5 on:** March 30, 2007, 07:01:54 AM »

Guys,
The rules, complete, stated in my thread are the latest from AMA.
What I was looking for was some information on which electric equipment to use ie. motors, speed controls and batteries and stay within the rules. I am a mechanical type and electrically challenged when it comes to such specifications. My thoughts on the RPM range was that the increase in prop pitch would keep the rpm down and add more load to the batteries hence the reason to have more available RPM than necessary and only worry about controlling the amp draw. As far as props go a vision of a single blade folding prop came to mind. AKA F2A type, but folding to keep from breaking a prop per flight. Mechanically a challenge but not a big one.

On edit: Heres an outlandish thought perhaps a variable pitch prop system controlled through the speed control using the amperage draw curve to control the pitch. Next I will be wanting telemetery feedback for analysis of motor flight information.

More coffee please extra sugar too.

Scott

Paul Smith · « **Reply #6 on:** March 30, 2007, 10:56:34 AM »

Ways to handle prop breakage:

1. Pass a rules change to require landing gears.

2. Use a wire skid, won't slow you down much, take it off for important record attempts.

3. Consider the prop chump change compared to the motor, controls, and battery that you will smoke on a winning flight.

Scott Jenkins · « **Reply #7 on:** March 30, 2007, 11:51:31 AM »

Ya know speed and racing is alot like stunt we just tend apply our efforts in different areas. A competitive stunt model may take as long as 6 months or more to build and finish. A competitive speed model maybe 2 months and no kits here have not seen a commerical speed kit in decades. Usually an airframe is built based on other planes past performance with ones own features included after that it is all engine or motor and prop.

1. "Pass a rules change to require landing gears."
Most speed ships use a Dolly

2. "Use a wire skid, won't slow you down much, take it off for important record attempts."
Most all use a wire skid cheaper than new pans.

3. Consider the prop chump change compared to the motor, controls, and battery that you will smoke on a winning flight."
If you lay up your own props and molds, time, effort and materials $20.00 + ea. depending on how many different pitches one
goes thru to find the right prop. Most speed types are as fanatical about props as than they are about engines. Many hours go
into both

Smoking an engine won't happen if everything is right, loosing a piston and liner fit does happen. Blowing a winding on a motor
Ouch ! Coating the windings with epoxy and a rebalance could help prevent that. That use to work on slot car motors anyway,
these new brushless motors not sure.

All your points do help in breaking a record, except for the want of a landing gear

on a speed ship.
Remember all the options have pro's an cons deciding which ones to follow is up to the individual.

Scott

Paul Smith · « **Reply #8 on:** March 30, 2007, 02:40:18 PM »

I'm just a spectator on this one.

It seems to me that electric speed involves bombarding a motor with huge amperage in a very short period of time, much more heat than you can hope to discipate with air cooling.

As Dan Rutherford described in a bit about motorcycle drag racing, records are set by guys who are willing to burn down an engine (or motor in your case).

Certain classes of speed (proto, Formula 40, 21 proto) mandate landing gears, the same for everybody. Saves props. Also limits speed a bit so you don't have to go up on lines so often.

dynasoar1948 · « **Reply #9 on:** March 30, 2007, 07:10:53 PM »

Reply to Adm. Randy

Some comments on propeller characteristics: My statement that prop efficiency tends to peak where diameter equals pitch (in the same units) is an oversimplification dating from the days when most full scale aircraft had fixed pitch propellers. A discussion of the parameter "J" as applied to fixed pitch props might help; remember to convert pitch angle to distance forward per turn at about the 70% dia. station if you chose to work any examples. A good discussion of this is presented by von Mises in "Theory of Flight" and Dan Dommasch's "Elements of Propeller and Helicopter Aerodynamics".

Considering speed models, a good discussion of transonic propeller flow at high RPM is on the net in the Control Line Speed Forum, where it is recognized as the key limiting factor to further significant increase in FAI Speed.

In most prop driven speed classes, motor characteristics dictate the prop load which permits peak power. If prop diameter is reduced to permit pitch sufficiently high to obtain theoretic airplane design speed at peak HP engine RPM, mass flow through the prop disc may not be enough to overcome drag and terminal speed will be less than the potential maximum. Similarly too large a prop diameter will not allow high enough pitch at peak engine RPM to reach the target airspeed max. It is this matching of prop pitch and diameter to the power output characteristics of the motor and the drag of the airplane/control line system that is key to extracting the maximum .

In the earlier example, considering Class B Electric speed which I'm most familiar with, the combination airplane/line system drag matches well with an 8-inch prop diameter prop. If my motor could turn a 9-inch pitch prop at the same RPM as it now turns my 8-inch pitch prop, the airplane would obviously fly faster. The cost in motor power to accomplish this would have to take into account both the incremental drag of the higher blade element incidence and the reduction in overall prop efficiency accompanying the less than optimal 'J".

Taking the forgoing into account, I repeat that 20,000 RPM into an 8-inch pitch propeller of realistic diameter (say 8-inches) would produce higher airplane speed than a similar prop of 4-inch pitch turning 40,000 RPM for two reasons: Transonic drag rise would both greatly increase power required to churn at that speed, and flow breakdown would subtantially reduce thrust developed (with areas of reversal) over much of the prop disc. Very obviously, efficiency (which simplifies to thrust out/power in) would be reduced.

William Stewart
VP, Advanced Projects AirAnalytics Corp.

Mike Palko · « **Reply #10 on:** March 30, 2007, 08:25:34 PM »

Quote from: RandySmith on March 29, 2007, 11:09:43 PM

Hi Mike

What you read there is not correct, many planes go faster with a 8x9 than they do an 8x8 or 8 x 10 than they do a 9x9 or 10 x 10,9x8 etc So whomever made that as a blanket statement is incorrect.

Regards
Randy

Thanks for correcting me Randy. I know I have heard that statement more than once. I will question it if it comes up again.

Quote from: Scott Jenkins on March 30, 2007, 07:01:54 AM

Guys,
The rules, complete, stated in my thread are the latest from AMA.
What I was looking for was some information on which electric equipment to use ie. motors, speed controls and batteries and stay within the rules. I am a mechanical type and electrically challenged when it comes to such specifications. My thoughts on the RPM range was that the increase in prop pitch would keep the rpm down and add more load to the batteries hence the reason to have more available RPM than necessary and only worry about controlling the amp draw. As far as props go a vision of a single blade folding prop came to mind. AKA F2A type, but folding to keep from breaking a prop per flight. Mechanically a challenge but not a big one.

On edit: Heres an outlandish thought perhaps a variable pitch prop system controlled through the speed control using the amperage draw curve to control the pitch. Next I will be wanting telemetery feedback for analysis of motor flight information. More coffee please extra sugar too.

Scott

Scott,
To stay within the rules of class A speed you can use any motor, ESC or timer. The battery is limited to 8.4 volts which would be a 2S lipo or a 7 cell NiCd or NiMH pack.

For class B speed you can use any motor, ESC, or timer. The battery is limited to 42volts which would be a 10S lipo or a 35 cell NiCd or NiMH pack.

If you would like you can buy an Eagle tree in flight data recorder and download the data to your PC after the flight. I know it is kind of archaic

.

If you really want to pick a power system we can help. This is a great discussion. If electric speed catches on standards for both classes will soon surface.

Mike

Scott Jenkins · « **Reply #11 on:** March 30, 2007, 08:35:34 PM »

Will,
Thank you, your point is well taken, I only had to read it three times for the info to sink in. So in essence your saying look for a motor with a 20,000 rpm maximum for a class b model event 607 with a high amperage rate specification and a controller to match the amperage flow plus batteries that will discharge at or near the specified amps for the motor and the controller and not exceed 42 volts for the number of batteries in series. This theory would also apply to a Class A model with a maximum of 8.4 volts total. Am I even close on this ?

Scott

dynasoar1948 · « **Reply #12 on:** March 30, 2007, 10:22:07 PM »

Scott-

You're closing in on it. First point is a prop diameter of 8-inches is a good starting dimension for a B Electric flying on present AMA lines. Next, RPM should be such that prop Mach number is less than say 0.8. This would be about 29,000 RPM, so determine pitch to permit this maximum. If your power setup is sufficient to allow 8-inch pitch, you're looking at nearly 200 MPH (and about 110Amp at 42Volts). This is a little beyond present capabilities, but does define a reasonable propulsion limit for a single motor model. Note that pull would be beyond both line strength and human pylon rotation rate.

Will

Scott Jenkins · « **Reply #13 on:** March 31, 2007, 08:22:22 PM »

Will,
Let's talk about Class A for this exercise here we have a maximum of 8.4 volts and probably a flight time of 30 sec.
max. I surfed around an found that a Lipo battery to big enough for a maximum amperage surge 60 amps for 30 sec. without blowing up weighes about 5+ ounces this with a motor of about the same weight plus airframe to withstand a 25G pulltest is going to weigh in at 15+ oz. which from my point of view is 5 oz. too much. Just for instance a motor with old 400 size motor power in a brushless type strong enough to swing a 4 to 6 in.pitch or less prop for ten laps insert amps here _____ at a rpm of 35,000 with a airframe and power source total weight of 10 oz. to achieve 100 mph or more with a 20 sec. amp surge. I have a notion that prop pitch will determine the load on the motor thus governing amperage draw. Tell you what somebody come up with a motor, battery, esc, package and I will cad up and build an airframe to carry it. All this electronic mumbo jumbo relative to the prop and aero dynamics is giving me a headache. I think I will go work on my F2A trainer.

Scott

dynasoar1948 · « **Reply #14 on:** April 01, 2007, 03:18:17 PM »

Scott,

Howard Doering's present A Electric record holding ship is powered by an AstroFlight FAi 05 with four turns in the armature. Battery complement is seven NiCad SCR 600 (I think) cells. There is a good picture of it in the CL- Speed Review website.

I would not go overboard with weight reduction, but build a robust low drag airplane.

I have not competed in this class yet, due to smoking my Aveox commutation module a couple of years ago, however if I do build something, it will pull at least 700 Watts to be worth fooling with.
(incidentally, if anyone has a good obsolete Aveox commutation module for my AVX1409/3 Delta motor, please contact me)

Will

Dean Pappas · « **Reply #15 on:** April 01, 2007, 09:20:26 PM »

Hi Will,
You know that you can run a sensor-type brushless motor with a sensorless controller like the Castle.
Just don't use the sensor wires.

Dean P.

dynasoar1948 · « **Reply #16 on:** April 01, 2007, 10:00:47 PM »

Dean,

Is this true for the older Delta winding configuration?

Will

phil c · « **Reply #17 on:** April 02, 2007, 09:04:06 AM »

Guys, go buy a copy of Electri-Calc,or find a similar prop/amps/motor calculator and start playing with numbers. It will get you close, with a combination of battery mah, motor, and prop. The physics of what goes on in an electric motor is a lot simpler(fewer variables) than an IC engine and is pretty well understood. The fine tuning for prop size, shape, pitch, and efficiency will have to start there.

Unfortunately, Class B is already poised to go way beyond any safe limits. 42 volts and Li-poly batteries allow up around 3360 watts, over 4 hp. You just will need 11 4000 mah hr. cells($500) and be willing to burn it up in 4-5 flights, like the RC guys do.

Dean Pappas · « **Reply #18 on:** April 02, 2007, 09:59:50 AM »

Quote from: dynasoar1948 on April 01, 2007, 10:00:47 PM

Dean,

Is this true for the older Delta winding configuration?

Will

Yup!
Dean

RandySmith · « **Reply #19 on:** April 02, 2007, 01:06:21 PM »

Quote from: dynasoar1948 on March 30, 2007, 07:10:53 PM

Reply to Adm. Randy

Some comments on propeller characteristics: My statement that prop efficiency tends to peak where diameter equals pitch (in the same units) is an oversimplification dating from the days when most full scale aircraft had fixed pitch propellers. A discussion of the parameter "J" as applied to fixed pitch props might help; remember to convert pitch angle to distance forward per turn at about the 70% dia. station if you chose to work any examples. A good discussion of this is presented by von Mises in "Theory of Flight" and Dan Dommasch's "Elements of Propeller and Helicopter Aerodynamics".

Considering speed models, a good discussion of transonic propeller flow at high RPM is on the net in the Control Line Speed Forum, where it is recognized as the key limiting factor to further significant increase in FAI Speed.

In most prop driven speed classes, motor characteristics dictate the prop load which permits peak power. If prop diameter is reduced to permit pitch sufficiently high to obtain theoretic airplane design speed at peak HP engine RPM, mass flow through the prop disc may not be enough to overcome drag and terminal speed will be less than the potential maximum. Similarly too large a prop diameter will not allow high enough pitch at peak engine RPM to reach the target airspeed max. It is this matching of prop pitch and diameter to the power output characteristics of the motor and the drag of the airplane/control line system that is key to extracting the maximum .

In the earlier example, considering Class B Electric speed which I'm most familiar with, the combination airplane/line system drag matches well with an 8-inch prop diameter prop. If my motor could turn a 9-inch pitch prop at the same RPM as it now turns my 8-inch pitch prop, the airplane would obviously fly faster. The cost in motor power to accomplish this would have to take into account both the incremental drag of the higher blade element incidence and the reduction in overall prop efficiency accompanying the less than optimal 'J".

Taking the forgoing into account, I repeat that 20,000 RPM into an 8-inch pitch propeller of realistic diameter (say 8-inches) would produce higher airplane speed than a similar prop of 4-inch pitch turning 40,000 RPM for two reasons: Transonic drag rise would both greatly increase power required to churn at that speed, and flow breakdown would subtantially reduce thrust developed (with areas of reversal) over much of the prop disc. Very obviously, efficiency (which simplifies to thrust out/power in) would be reduced.

William Stewart
VP, Advanced Projects AirAnalytics Corp.

Hi William

I agree with your statments from above , however my statement was not about using a half diameter prop at twice RPMs, or going supersonic. I simply replied to the statement that you would loose effiency by going to a ..example... 8x9 from a 8x8 , and that any prop with more pitch than diameter was less efficent
I simply said this as a blanket statement was not correct.

Regards
Randy

dynasoar1948 · « **Reply #20 on:** April 02, 2007, 08:30:51 PM »

Hi Randy,

I've gone back thru my postings about prop characteristics, and can't find any comment, by either of us, about "using a half diameter prop at twice rpms". Where did that come from? My example involved half pitch and twice RPM with diameter unchanged.

My generalization that peak propeller efficiency is often encountered where diameter and pitch (distance of advance) are equal, has its origin in comments in several American texts, and one prewar British technical report on propeller design, where curves of efficiency versus pitch or advance ratio are superimposed on the same plot. A line connecting efficiency maxima often peaks at or near the point where pitch and diameter (Depending on blade shape and solidity) are the same. This same relationship between efficiency, pitch and diameter can be found in the performance of low solidity fans.

I wish there was some closed form solution for determining prop efficiency, but to the best of my knowledge there is nothing (other than lengthy CFD programs) to prove or disprove either of our positions. Just for fun, I ran the numbers for the prop on my Grumman Tiger, which, I believe, is the fastest certificated airplane with a fixed pitch propeller. This Sensenitch prop, which has been noted for its installed efficiency, is 75" diameter by 63" pitch ( this narrow blade planform is square at 0.85 diameter).

If I can locate the British TR, there is some interesting data on the design and optimization of a wide blade, high (fixed) pitch wooden propeller for the early Spitfire, which also was nearly "square" at a little more than 100% dia.

Best, Will Stewart

Scott Jenkins · « **Reply #21 on:** April 03, 2007, 09:39:17 PM »

Will,
OK, its me again I have been surfing around looking at various motors and battery combinations. I looked at a Aveox motor very expensive $245 not ready for that kind of motor with my next to nill knowledge of electric motors. I did find out that a 700 maH battery will discharge at 42 amps for 1 minute. And I did find a possible combo of motor and battery. The basic inputs, motor and battery, are conjecture on my part.

Battery: Battery:
Cell Type: Sanyo 0800AR
# CELLS: 7
# Parallel: 1
Cell Capacity:800 mAh
Cell Weight : 1.165 OZ
V per cell: 1.25
V Cell Resistance: 0.006 ohms
Pack Weight :8.155 oz
Pack Voltage :8.75 V

Motor:Astro Cobalt 05 FAI 5T#18
RPM/V 3214
Kt (InOz/amp): 0.420659

Resistance:0.021
Current:5A
Weight: 5.5

Gearbox / Prop:
Gear Ratio: 1
Prop Diameter in Inches: 5.5
Prop Pitch in Inches: 6

Prop Blades: 1
Prop Type: FAI F2A

Speed Controller:
Speed Controller Resistance:0.015 ohms

Motor Calculated:
Motor Amps: 28.512
V to motor: 7.12
Motor RPM: 20974.77

Watts In : 203.14
Watts Out: 150.44
Efficiency: 74.1%

Max Efficiency: 74.4 %
Current @ M.E.: 39.11
V to motor @ M.E.: 6.42326

Watts In @ M.E.: 251.21
Watts Out @ M.E.: 186.98

Prop Calculated:
Prop Eff Diameter: 4.79
Prop RPM: 20975
Prop Static Thrust oz: 28.4

Prop In-flight Thrust oz: 10.4
Prop Pitchspeed: 119.2 mph
Approx Power System Weight: 15.021 oz Batteries + Motor + 10%
Full-throttle duration: 1.41 minutes

Yes, no, maybe ?

Scott

dynasoar1948 · « **Reply #22 on:** April 03, 2007, 10:26:35 PM »

Hi Scott,

Did you use Electricalc?

Power into the prop at 150W...1/5 HP does not appear too promising with respect to record setting potential. Howard Doering's Astro motor resistance is closer to 0.017 Ohm, with substantially higher current draw.

What type cells did you specify?

My unbuilt A, using an Aveox 1409/1Y, seven pushed N-1000SCR cells and a two blade 6.25/6.50 prop with about 400W input (at 80A) calculated 114MPH, using the same aero parameters that accurately predicted 120 for my B.

Will

Scott Jenkins · « **Reply #23 on:** April 04, 2007, 06:40:21 PM »

Will,
The cells I spec'ed out were Sanyo 0800AR not really knowing anything about them mainly just trying to keep the weight down and looking for a maximum run time of 2 min. the program only showed a efficiency of 74.1%. I was hoping to find something in the 80 to 85% range. The motor calculator I used was on the Diversity Model Aircraft site http://brantuas.com/ezcalc/dma1.asp mainly because it was there and looked as if would yeild some good info although it may be biased toward their products the motor was the closest I could find like Howard's. Still on dial-up here at home and do get tired of waiting for results. Is there another calucator on line that is better. Tried a few other combination but so far nothing that will give 400 watts at the motor. Best battery choice so far is Sanyo CP-2400SCR-Zapped but the weight increase is huge.

Scott

dynasoar1948 · « **Reply #24 on:** April 05, 2007, 01:40:46 PM »

Scott,

I've got an old version of ElectroCalc, which I still use. Post your characteristics, including motor constants if a new motor and I'll try to run some points for you using reasonable drag parameters..

Will