Notices
Brushed/Brushless motors, speed controls, gear drives Discuss all aspects of brushless motors, brushed motors, speed controls (ESC's), gear drives and propellers in this forum.

Need More Thrust! How?

Old 08-13-2003, 02:33 PM
  #1  
rdeis
Member
Thread Starter
My Feedback: (1)
 
Join Date: Aug 2003
Location: Colorado Springs, CO
Posts: 90
Likes: 0
Received 0 Likes on 0 Posts
Default Need More Thrust! How?

I'm an experienced Sailplane modeler, just moving into the park flyer/RC indoor sceen.

I've got a Firebird Outlaw as an introduction, but at this altitude (6000ft, Colo Spgs) it appears way underpowered. And since thrust is the only source of control on this little beastie, there's very little control authority.

My question is, what can I do to get more thrust out of this system? Since I'm using it as a learning experience and springboard, I'm not averse to spending some time and money experimenting- but I'm not sure where to start.

More cells, more motor, more prop (where do I find small counterrotaing props?) something else?

How should I expect each of these to affect the system/flight?

Primary objective for the aircraft is reasonably stable, controllable flight without burning up the speed controls or frying the radio gear.
Old 08-13-2003, 03:20 PM
  #2  
Greg Covey
Senior Member
 
Greg Covey's Avatar
 
Join Date: May 2003
Location: Rochester, NY
Posts: 5,710
Likes: 0
Received 2 Likes on 2 Posts
Default Need More Thrust! How?

The "all-in-one" RTF packages like the Firebird Outlaw are difficult to modify. They come with battery packs that just fit in the plane and make it perfectly balanced.

In general, at your altitude, you are beating thinner air so you need to compsensate by increasing prop size or reducing the gear ratio (on geared systems). Alternatively, adding a cell may be effective if the design tollerances allow it.

I see that the Firebird Outlaw comes with a 5-cell battery pack. It mat be possible to replace it with a 6-cell pack of equivalent NiCd or NiMH or a 2-cell Lithium Polymer pack of similar size that can handle the current delivery of the NiCd (or NiMH) cells.

You may want to just ditch this model and try something that you can modify, if needed, like a GWS Slow Stick. When using an 8-cell, 800mAh, 5/4AAA NiMH pack on the stock power system, there is plenty of power.

Good luck!
Old 08-13-2003, 05:00 PM
  #3  
rdeis
Member
Thread Starter
My Feedback: (1)
 
Join Date: Aug 2003
Location: Colorado Springs, CO
Posts: 90
Likes: 0
Received 0 Likes on 0 Posts
Default Need More Thrust! How?

Slow stick comes later (or more likely a Sig Bristol).

This project has a specific objective of learning about differential thrust control in tiny twins, which makes the outlaw a useful platform for it. I'm hoping to learn enough to adapt a Guillow's B-25 to this type of control. (Much to learn about Guillows and warbirds, too, but that's in other threads..)

Modifying isn't easy, but I've been able to do the following so far:

* Reinforce motor mount joiner tube with brass (at the spot where it always breaks..)
* Extend motor pods to make room for 5" props (4" are stock)
* Added standard fighterbird wing (flies much slower now, may be suitable for gym use if I can get enough control.)

I haven't been able to find the pitch of the stock props for comparison to others. Is there a way I can measure it?

I've found a set of 5x3 props that might work, and I've got a 6-cell battery.

The bigger prop increases current draw through the motor at a given RPM, right? Which means more current through the speed control and possible overheat issues with either the motor or speed control?

Doesn't moving to a 6 cell pack do essentially the same thing?

Why would you pick one over the other? (I suppose batteries are heavier than props- maybe that's it?)
Old 08-14-2003, 12:56 AM
  #4  
MTEXX
Member
 
Join Date: Apr 2003
Location: Charlotte, NC
Posts: 67
Likes: 0
Received 0 Likes on 0 Posts
Default Need More Thrust! How?

rdeis-

I can't remember where I read this, but it makes good sense.

At high altitude, you want to increase the prop diameter by an inch" and increase the pitch by 2". There was a rule-of-thumb ratio for altitude, but I forget.

The batt, ESC and motor will work the same at sea level as it would in a vaccuum. The prop is your interface with the atmosphere.

Bottom line: go with a more agressive prop.

Good luck in finding counter-rotating props : )
Old 08-14-2003, 11:45 AM
  #5  
rdeis
Member
Thread Starter
My Feedback: (1)
 
Join Date: Aug 2003
Location: Colorado Springs, CO
Posts: 90
Likes: 0
Received 0 Likes on 0 Posts
Default Need More Thrust! How?

> Good luck finding counter-rotating props

No kidding.. They're nearly non-existant in 5" diameter. I have yet to find a a matching set by the same mfr. Some guys on teh 1/2A board suggested buying 6" or 7" ones and cutting them down.

I'll try heating the ones I have and warping them to increase pitch first.

I'd still like to learn more about how motor, voltage, or prop changes work together. I can see that the three work together as a system, and I sort of see how each affects thrust, but I don't understand how they affect each other or what would make you choose to increase one instead of the others.
Old 08-15-2003, 12:57 AM
  #6  
MTEXX
Member
 
Join Date: Apr 2003
Location: Charlotte, NC
Posts: 67
Likes: 0
Received 0 Likes on 0 Posts
Default Need More Thrust! How?

hmmmm
cutting them down is an option, but i'd not dare try to warp them.

you can throw off the center of pressure (CP) of the prop. i can also see major turbulence in the prop wash which would hinder lift.

i'd rather deal with the torque of non-counter rotating props than warp.
Old 08-15-2003, 11:05 AM
  #7  
Greg Covey
Senior Member
 
Greg Covey's Avatar
 
Join Date: May 2003
Location: Rochester, NY
Posts: 5,710
Likes: 0
Received 2 Likes on 2 Posts
Default Need More Thrust! How?

At high altitude, the thinner air results in less power. This means that you are not pulling even the stock current through the motor and ESC. By increasing the prop size, increasing cell count, or decreasing the reduction ratio, you are simply bringing the current draw back up to par to compensate for the thinner air. Don't go overboard and increase things until they break!

On those little plastic props, you can twist extra pitch into them as needed. Sometimes heating them first with a heat gun or hair dryer works well.

I'm not certain what your objective is but from your first post it seems that you have had issues with ESC failures.

One way to get a better R/C experience is to use a model that does allow for easy modification. Typically, these all-in-one RTF designs simply don't allow it.

Good luck!
Old 08-15-2003, 12:49 PM
  #8  
rdeis
Member
Thread Starter
My Feedback: (1)
 
Join Date: Aug 2003
Location: Colorado Springs, CO
Posts: 90
Likes: 0
Received 0 Likes on 0 Posts
Default Need More Thrust! How?

I do want to get a "real" airplane, but I'm trying to put off buying the radio gear for a while longer. Even my HLG gear is *way* overweight by current standards.

Tried the 6-cell battery last night. A lot more power, but still not enough to fly well. Someone suggested 2-cell LiPo. 8.4V seems like a lot when it's designed to run 6.0, but the weight reduction would certainly help.

Got a lot of reccomendations from indoor fliers here to cut out weight through better batteries, motors, and wing. Seems like good advice, and I expect batteries and motors are a good investment that would outlast the relatively disposable plane.

Question on LiPo: Discharge voltage is 3.0 per cell, right? so I'd need to be careful not to let a 2-cell pack drop below 6.0V lest I destroy the battery. (And that could be very dangerous as well as expensive from what I've read, right?)

Since the design battery is 5cell NiMH, I expect the motor cutout is at 5V.. too low. (Not that it flies at that voltage..)
Old 08-15-2003, 01:01 PM
  #9  
rdeis
Member
Thread Starter
My Feedback: (1)
 
Join Date: Aug 2003
Location: Colorado Springs, CO
Posts: 90
Likes: 0
Received 0 Likes on 0 Posts
Default Need More Thrust! How?

At high altitude, the thinner air results in less power. This means that you are not pulling even the stock current through the motor and ESC.
So a given prop and given voltage will spin at the same RPM in thinner air, but draw less current and produce less thrust?

By increasing the prop size,
And thereby increasing the amount of air moved and therefore increasing thrust...

increasing cell count, or decreasing the reduction ratio
Each of which increase RPM

You are simply bringing the current draw back up to par to compensate for the thinner air.
I see, I think. I guess I'm confused about how voltage relates to RPM-- I would expect thrust to be directly proportional to RPM, which is directly proportional to torque on the prop.

I would also expect torque to be proportional to voltage, but it seems this isn't the case. (Searching through physics memories...) Wait- torque comes from the field in the coil, which comes from current, which comes from voltage-- but this isn't a simple V=IR case, is it, so there's a more complex relationship between voltage and RPM than that?
Old 08-17-2003, 02:08 PM
  #10  
grtdane
Junior Member
 
Join Date: Aug 2003
Location: LA, CA
Posts: 2
Likes: 0
Received 0 Likes on 0 Posts
Default Need More Thrust! How?

I don't think that you will be able to have any control authority without having control of at least 2 of the control surfaces
This plane was designed to get up in the air and fly in circles and not much else.

Harold

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell or Share My Personal Information -

Copyright © 2023 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.