250lb@50kts for 15 nautical miles? POSSIBLE?

[djjones69]

Hey guys and gals,

Over the last couple of weeks i've been trying to get my head around how complex electric propulsion can be!

Basically, i'm trying to design a backup propulsion system that could provide enough power to glide an aircraft back to a runway up to 15 nautical miles away. This equates to 18 minutes flying time at our minimum drag speed of 50kts.

Here are some of my very, very, very basic calculations;

Assumed values:
Weight W = 1140.5 N (256lb)
Clcruise = 0.5
Wing area S = 7.747 m2
Density = 1.225 kg/m3
Vcruise = 25.7m/s or 50 knots
Cdo = 0.0275
Oswald efficiency factor = 0.861
Aspect ratio = 6.25

Drag coefficient=0.042

Drag of the aircraft=132.64 N

The power required by the propeller
P = DV
= 3411.8 W

Assume at this stage the propeller efficiency ≈ 0.9
Therefore electric motor needs to provide
Pmotor = Pprop/0.9 = 3790.9 W

Assume that the motor efficiency ≈ 0.9
Therefore batteries need to provide
Pbat = Pmotor/0.9 = 4212.1 W


BIG questions;

Is there off-the-shelf components out there that could provide this sort of power?

Although I haven't done any prop sizing yet I would like to be able to throw a larger prop, maybe 23"?

So,

MOTOR:

I'm thinking XTRM 7035/08, though if there is a bigger, more reliable motor that would be better!

4800W (190KV), therefore the 4200W requirement for 18mins seems way too high. Would the motor burn out? Are there any motors out there that could provide this performance reliably?

BATTERIES:

FP660062 "EVO 20" 6 Cell 22.2V 6600 mAh 6S2P LiPoly Pack

Could I use two of these in series and have 5 in parallel, i.e. 2S5P?

This would equate to 21 minutes flying time, 44.4V and 95A.

One issue I can't seem to get my head around is how the battery arrangement can allow bigger props. As i'm looking for relatively low speed cruise it would be nice to throw a larger prop at low rpm? is this correct? Also, does having a lower voltage, i.e. 1S2P rather than 2S1P provide better torque and therefore would this be better?

Would the heat from a battery pack as large as this be a massive issue? Would cooling air being vented from freestream be sufficient to cool them? and the other components for that matter?

Any suggestions for alternative cooling methods or battery types? Although cost is an issue if there were more capable disposable batteries could they provide better performance?

ESC:

Due to the big current and voltages that will be required the ESC is a big problem as most are limited to 50V which is pushing the current way up;

If we use the above battery arrangement (2S5P) the following speed controllers seem to do the job;

Pheonix HV-110: 110A, 50V
and
Jeti spin 200: 200A, 50V

Are these suitable?

Clearly I am very new to all this so any other suggestions or sources of info would be great.

------------------------------------------------------------------

Sorry for the million questions but if I don’t come to a conclusion on the potential of electric by this weekend I’m going to have to go for an IC engine as a backup which I don’t personally think will be as reliable.

Cheers for all the help guys and gals. I really do appreciate it. The plan is to be flying this a/c by this summer-I would love to be able to post a video of it flying on electric!!

Thanks again,

Dan

[blucor basher]

Your numbers seem reasonable. The main issue is, as a backup, lipoly batteries are a very poor choice. They don't like to be stored at full charge, they like about 1/2 charge for storage, and take significant time to charge, and the chargers are a significant expense and power draw.

[djjones69]

cheers for the info.

Are there any alternatives you would suggest?

It could be possible to design in a process where by the lipos are fully discharged after each flight, stored and then recharged before each flight. Each flight, without need for the backup, will last no longer than 8 hrs.

Does this seem reasonable or does it open up more issues?

[blucor basher]

You run into a new problem - any lipo has a life span measured in cycles...you do get more cycles if you charge and discharge gently, but not enough to make it workable in practice.

War story - At one point, we worked w/ a robotic UAV team involved in a university competition. They powered their craft w/ lipolies, and they were not getting any life at all, only a few sessions per (very expensive) pack. Everything seemed to be in order, until we found out that the nav light system was always lit, they used it as a quick check that power was still hooked up when the plane was on the umbilical. As it turned out, the software was set to discharge the system through the LEDs to a voltage floor, then slow charge up to a ceiling, then discharge again. The packs were going through several cycles per day without anyone knowing. After a few months, the packs were toast, no matter how gently they were used in the air.

I suspect you'd have similar issues.

[djjones69]

okay....

So would you say that the lipos would last longer;
a) being stored at full charge at all times
or
b) being discharged to a suitable level and then recharged for each flight?

Do you have any idea as to how long you would expect them to last?

Any suggestions as to alternatives?

Cheers for the info so far....learning more by the hour!


P.s I'm a bit worried about the motor. It'll be running at nearly 90% power for 18 minutes....is this possible?

Also, can you suggest any ways of sizing a propeller for a particular application?

Cheers,

Dan

[blucor basher]

My personal knowledge of motor ends at 5KW in bursts...if I were you, I'd be talking to Hacker.

You'll have better luck charging the lipolies before each flight and 1/2 discharging them after...but unless you're planning on spending huge $, you'll be investing serious time in each charge cycle.

A better choice would be A123 cells, probably. They can be fast charged with simple equipment (like a wire, apparently), are about 20% heavier, are slightly cheaper, and are more tolerant of being stored in less-than-optimal conditions.

[GordonTarling]

There are many feasible solutions to your problem, but there's a few stumbling blocks on the way! The biggest is that every manufacturer of model motors and ESC's specifically prohibits their use in man carrying vehicles - the wording may vary between the manufacturers, but the intent is the same. Your best bet is to contact Plettenberg, who are very experienced in such matters and they should be able to advise on choice of equipment, they certainly make motors large enough! http://www.plettenberg-motoren.com/UK/index.htm

[diver don]

I would look at the A-123 chemistry.

Hold voltage well and can be beat to death.

The person you want to talk to is "Astro Bob" at Astroflight.com.

He just built a motor for an electric, man carrying, plane.

DD

[djjones69]

cheers for the replies lads,

The aircraft will actually be unmanned so shouldn't be a problem.

I'll have a look into your suggestions!

Dan

[Z06kal]

SO you need 30-45 pounds of thrust? Well her eis something that might fit the bill.

Neu Bam 2230/1Y 750kv Motor with a Neu 6.7:1 planetary gearbox. 14S Lipos using 5000mah packs with 6 in parallel. Jetti Spin 200 or Castlecreations SHV controller. (might be a bit too much voltage for the motor you'd have to ask steve Neu).

Something like a APC-E 26x15 prop.

This would give you about 5000 watts for 16 minutes. The calc predicts 44 pounds of thrust with a 74mph pitch speed. This gives you some head room for the batteries to run down and still have enough thrust and pitch to keep the plane flying. The entire package will weigh around 26 pounds.

You will find that the 2230 neu with a gear box will be the lightest possible motor you can use that is rated to handle 6000 watts continuous and 10,000 in burst. If you were to use A123 cells you can expect the same package to be 40 pounds or more. Not sure if 26 pounds is within your weight allocation or not but this is probably ball park what you are looking for.

Heating from these packs would not be an issue at all because you are only pulling 4C which would barely warm new chemistry lithium packs. The motor needs to be exposed to airflow for cooling and you will want the ESC exposed to some airflow. You want to run as high a voltage as possible as this lowers amperage requirements and increases effeciency throughout the system as less power is lost to heating. As you move to a bigger prop you will need more and more power to maintain the same prop pitch speed. Because you need both big thrust and at least 58mph of pitch speed your prop sizes are limited as you have about about 5000watts of practical power at your disposal before the package really goes up in weight.

[portablevcb]

Pick a motor mfg (Plettenberg is a good place to start, but, you want someone who does BIG motors, not model motors) and a battery mfg (I'd go to SAFT), tell them your issues and let them pick a package. There are a lot of variables to figure out and they are the best ones to do it. If you want the best it won't be cheap.

charlie

[djjones69]

cheers for the replies Z06kal and portablevcb,

really really useful.

My initial calcs give a system weight of 25.5 lbs; so really close to Z06kal's value.

I'll definitely look into the plettenburg motors and SAFT looks promising!

Cheers guys,

Do any of you have any opinions on reliability of electric and IC engines?

The main reason for not simply adding an extra IC engine to run as a backup is that the electric system will provide diversity (i.e. won't fail in the same way as IC) and that if designed properly I would expect it to run more reliably;

Does this sound correct?

If any of you have any idea where I can get reliability data on any systems or components that would be really really useful.

Thanks again,

Dan

[portablevcb]

I'd refer you back to the mfg's.

My experience with electric (besides models) is in the wire mfg industry. We used motors from 30hp up to 250hp, 24/7. AC and DC and various drive controls. Only had one motor fail in three years. Mostly it was encoder or drive component failures (which would sometimes fry a motor).

As for sitting around. We kept motor spares in storage for years before using one. You'd get a bit of that newly burned in smell, but, they fire up every time.

For battery systems the weak link (and probably the most expensive part) is the batteries. Charging and discharging cylces are very important to them. Talking to SAFT will help as they have experience with battery backup systems and how to automatically maintain them.

charlie

[Jocke]

If power is main consern, the most powerfull motors i know of today is the Pletti Predator 37 15kw+ and the Cyclon Big Boss that handles 15kw continuos 20kw burst, also low weight. For special motor its worth contacting Plett, Cyclon or Köhler directly for custom built motor.

[portablevcb]

...the most powerful motors made for model airplanes?

Many other more powerful motors out there, including light weight ones. I wish I could remember the motor mfg we used on our last job as they did custom work for really good prices (they were outrunners that were mounted inside a vehicle wheel). But, any of the custom shops can make what you need.

charlie

[Jocke]

Yes ofcorse i ment for models;)

[portablevcb]

I kinda figured :D

[Z06kal]

I highly doubt you can readily find a lighter 6000 watt continuous motor thatn a Neu 2230 bam with gearbox.

[DaveKan]

Hey guys and gals,

Over the last couple of weeks i've been trying to get my head around how complex electric propulsion can be!

Basically, i'm trying to design a backup propulsion system that could provide enough power to glide an aircraft back to a runway up to 15 nautical miles away. This equates to 18 minutes flying time at our minimum drag speed of 50kts.

Here are some of my very, very, very basic calculations;

Assumed values:
Weight W = 1140.5 N (256lb)
Clcruise = 0.5
Wing area S = 7.747 m2
Density = 1.225 kg/m3
Vcruise = 25.7m/s or 50 knots
Cdo = 0.0275
Oswald efficiency factor = 0.861
Aspect ratio = 6.25

Drag coefficient=0.042

Drag of the aircraft=132.64 N

The power required by the propeller
P = DV
= 3411.8 W

Assume at this stage the propeller efficiency ≈ 0.9
Therefore electric motor needs to provide
Pmotor = Pprop/0.9 = 3790.9 W

Assume that the motor efficiency ≈ 0.9
Therefore batteries need to provide
Pbat = Pmotor/0.9 = 4212.1 W


BIG questions;

Is there off-the-shelf components out there that could provide this sort of power?

Although I haven't done any prop sizing yet I would like to be able to throw a larger prop, maybe 23"?

So,

MOTOR:

I'm thinking XTRM 7035/08, though if there is a bigger, more reliable motor that would be better!

4800W (190KV), therefore the 4200W requirement for 18mins seems way too high. Would the motor burn out? Are there any motors out there that could provide this performance reliably?

BATTERIES:

FP660062 "EVO 20" 6 Cell 22.2V 6600 mAh 6S2P LiPoly Pack

Could I use two of these in series and have 5 in parallel, i.e. 2S5P?

This would equate to 21 minutes flying time, 44.4V and 95A.

One issue I can't seem to get my head around is how the battery arrangement can allow bigger props. As i'm looking for relatively low speed cruise it would be nice to throw a larger prop at low rpm? is this correct? Also, does having a lower voltage, i.e. 1S2P rather than 2S1P provide better torque and therefore would this be better?

Would the heat from a battery pack as large as this be a massive issue? Would cooling air being vented from freestream be sufficient to cool them? and the other components for that matter?

Any suggestions for alternative cooling methods or battery types? Although cost is an issue if there were more capable disposable batteries could they provide better performance?

ESC:

Due to the big current and voltages that will be required the ESC is a big problem as most are limited to 50V which is pushing the current way up;

If we use the above battery arrangement (2S5P) the following speed controllers seem to do the job;

Pheonix HV-110: 110A, 50V
and
Jeti spin 200: 200A, 50V

Are these suitable?

Clearly I am very new to all this so any other suggestions or sources of info would be great.

------------------------------------------------------------------

Sorry for the million questions but if I don’t come to a conclusion on the potential of electric by this weekend I’m going to have to go for an IC engine as a backup which I don’t personally think will be as reliable.

Cheers for all the help guys and gals. I really do appreciate it. The plan is to be flying this a/c by this summer-I would love to be able to post a video of it flying on electric!!

Thanks again,

Dan


I think you would need a much bigger propeller, to get 90% efficiency

out of the prop. It would also need a pitch to diameter ratio around

1:1 .

Dave :)

[Ercoupe Ed]

Question!
256 pound aircraft?
This is 201 pounds over AMA legal limit. Who is going to insure and sanction the aircraft in flight? Where is it to be flown?
Will it be airworthy?

[DavidB.]

This is not a model after all, so the AMA does not apply..it's a UAV when you get to these weights.

http://www.uavm.com/uavregulatory/airworthinesscertification.html
http://www.rcgroups.com/forums/showthread.php?t=805550

It seems we're almost in the wild west when it comes to small UAV's and regulations. Try to enjoy the freedom while you still can. If you can find some wide open areas where chances of damaging private property are zero.. I would just go out and fly, why worry about it? I mean...pretty soon they'll start regulating large birds and throwing them in jail for not being flight worthiness tested....

[djjones69]

Thanks for the posts guys........really good stuff.

I've contacted most of the companies you have suggested and have got some really good feedback. I've got a meeting tonight with some American colleagues as to whether we should use an electric or IC propulsion system as a backup to the main IC.

The main reasoning for using an electric system will be the reliability and diversity it offers.

If you have any opinions on whether electric or IC is best, I would really appreciate hearing them.


Thanks again guys,

Dan

[djjones69]

p.s I've posted a simple poll on the reliability issue..