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Virgil Kee
12-14-2005, 06:54 AM
How does THRUST relate to ALL UP WEIGHT?
What flight capabilities can be achieved with a given thrust rating? Are there any reliable guidelines? :confused:

Virg.

savydad
12-14-2005, 09:48 AM
Basically, you can get by with 1:2 thrust to weight, for normal/partially aerobatic planes. The only real significance is if you want to hover or have unlimited vertical performance, then 1:1 or greater is desired. Sailplanes and highly efficient wing designs can get by with far less thrust than others. Not sure of where there might be a guideline sheet or something, but I generally shoot for close to 1:1 as much as possible.

Todd

qban_flyer
12-14-2005, 12:29 PM
I read on some other thread here that for 3D one needs 150% to 1 thrust to weight ratio, with a bit more being highly desirable.
On the other hand, a Boeing 747 has a thrust to weight ratio of just 37% and so far they keep on flying. :)

Rule of thumb as used in the early days of electric flight was that one needs 50 watts per pound of plane weight in order to ROG and fly the model normally. 75 to 100 watts per pound for a mildly aerobatic plane and over 100 watts per pound for wilder flying things.

I believe that old formula still applies to our models today. :)

Matt Kirsch
12-14-2005, 02:30 PM
Still does, and it works even better because modern brushless motors are signifigantly more efficient than the cobalt motors the original rules were based on.

Thing is, Watts is not thrust, and that is confusing to many people. How can you not pay attention to thrust when powering an airplane? The simple answer is, it takes a certain amount of power to generate a certain amount of thrust. Watts is a measure of power that is easily calculated. Yes it's electrical Watts, but electrical and mechanical Watts are the same, and our Watts-per-pound rules take into account the energy lost converting electrical to mechanical in a motor. Thrust on the other hand, is difficult to calculate.

Basically, if you give a plane enough Watts, and use those Watts to turn an appropriately-sized prop, thrust comes along for the ride.

cyclops2
12-14-2005, 04:02 PM
Years ago on the E-Zone site they had extensive downloads of every factor that would be needed to fly from minimum to maximum Thrust/ speed on any type of model. I t also gave the power increases if you changed prop or battery counts. Very accurate then and now.
Only thing that has changed is the insane need to go faster at any cost.:)

qban_flyer
12-14-2005, 04:40 PM
Still does, and it works even better because modern brushless motors are signifigantly more efficient than the cobalt motors the original rules were based on.


Glad to see that power specs. wise things still hold true. :)

timocharis
12-14-2005, 08:38 PM
Virgil,

I don't think there are any reliable guidelines, golden age notwithstanding. For example, you'd think a 1:1 thrust/weight would hold a vertical upline, but it won't. You also have to contend with drag, so the plane will eventually slow down to a hover.

Airframe characteristics vary widely, and the requirements for thrust vary accordingly. For 3D, 1.5:1 will sort of do the job -- at least you can comfortably practice hovering. But some of the more spectacular maneuvers require more power than that to really be cool on a small plane. On bigger planes the rules shift down a bit.

The current vogue of using watts as a single-point generality is not particularly useful, but it does sell batteries and motors.


Dave North

cyclops2
12-14-2005, 09:02 PM
Go to www.ezonemag.com (http://www.ezonemag.com) Then the left side has a space-- FAQ--click on it. Then scroll down to section # 8 and click on, A 805. After that read . Go down to sect.# 9 and click, A 904.
Between them there are few mysteries about electric planes in the air.:)

hul
12-14-2005, 10:57 PM
thrust is misleading in many cases.

It makes you pick large diameter, fine pitch props because they have more thrust for a given power input than smaller diameter, coarser pitch props. But the fine pitch prop has low pitch speed and at flying speed (20+mph for most planes) fine pitch props have little or no thrust advantage over coarser pitch props, many actually have less thrust at a reasonable flying speed.

I find power/weight and pitch speed much more useful than thrust for conventional (non-3D) planes.
Power/weight follows the old rules, I use roughly 50W/lb for mild trainers, 80-100W/lb for warbirds.
Pitch speed at least 2.5 (better 3) times stall speed.

Don't want to go into the maths, but I find large diameter, square or oversquare props driven at very low rpm make for better flying conventional planes. An example would be my 1/8 scale FW190, 390W input, 81W/lb, 13.9x14.5 3-blade prop, about 58mph pitch speed, about 4200rpm, don't know thrust. I use a very efficient motor, it would need more input power with an average motor.

3D planes obviously need more thrust than weight, 1.5:1 or better.

Hans

timocharis
12-14-2005, 11:57 PM
hul:

If one of my 3D planes gets up to 20mph, something is wrong!

But overall, I agree. The problem with all these silly single-point Rules Of Thumb In Your Eye is the difference in style, airframe, etc. What does thrust mean when your pitch speed is 120mph? 12mph? What do watts mean at 60% efficiency? Blah blah blah.


Dave

qban_flyer
12-15-2005, 02:20 AM
hul:

If one of my 3D planes gets up to 20mph, something is wrong!

Dave

Same here!!! :D :D :D

Dr Kiwi
12-15-2005, 03:03 AM
hul:

If one of my 3D planes gets up to 20mph, something is wrong!

But overall, I agree. The problem with all these silly single-point Rules Of Thumb In Your Eye is the difference in style, airframe, etc. What does thrust mean when your pitch speed is 120mph? 12mph? What do watts mean at 60% efficiency? Blah blah blah.


Dave
Yes, Dave - that might mean it is in a fatal vertical descent to terra firma!

timocharis
12-15-2005, 05:05 AM
Phil,

Oh man, does that remind me of a totally weird one yesterday! I did something remarkably stupid (I can't remember how I got into this) but my slofly 22 went straight into the grass almost 90 degrees -- and bounced off the prop -- and I kept flying!

The only problem was I laughed so hard I immediately planted it in some rushes nearby.


Dave

Virgil Kee
12-15-2005, 09:30 AM
Thanks to everyone who has contributed to this thread so far! And, thanks to these forums for attracting such experienced (and knowledgeable)
modelers! There must be at least a couple of ENGINEERS in the mix of contributors!

My question came about based on a possible project involving the Guillow's PBY-5a, electrified. I want to use 3-blade props for scale purposes --- the scale dia. being about 4.74", with the max. dia. being no more than 5" (due to the spacing of the two nacelles). AUW is projected at about 26 oz. The Cox 5x3 3-blader seems to be about the only ready-made possibility for proping.

Using performance specs for various (brushless) motors (on 2-blade) props, I would expect to see a thrust/weight ratio of ~54%. How might this sucker perform?

NOTE that my location is on the Outer Banks of NC where the wind would often range to at least 15 MPH. I can't build a "floater" as it would only be flyable about half the time, and in the terms of something I recently read on one of these sites, the model would become "trapped downwind"!

Virg.

qban_flyer
12-15-2005, 02:48 PM
Virg,

Under the conditions you describe I would feel safer with something more like @ least 75% - 100% thrust to weight ratio, just in case you have to force it back in because of the prevailing winds.

While this may appear to be a bit excessive, you always have the option of being able to throttle back for a more scale like flight, then have the extra reserve power available if it's needed. :)

Most of my models begin @ 75% to 1, with some of the wilder ones exceeding 205% to 1 thrust to weight ratio. :D

hul
12-15-2005, 09:14 PM
My question came about based on a possible project involving the Guillow's PBY-5a, electrified. I want to use 3-blade props for scale purposes --- the scale dia. being about 4.74", with the max. dia. being no more than 5" (due to the spacing of the two nacelles). AUW is projected at about 26 oz. The Cox 5x3 3-blader seems to be about the only ready-made possibility for proping.
this is a plane where I wouldn't worry about thrust and instead use W/lb and pitch speed.
50W into those 5x3 3-bladers give about 40mph pitch speed, which sounds ok, but to do it properly I would need wing area to estimate stall speed. What pack (mAh and C rating) did you plan to use? Any other info on motors ESCs etc?

This article by Keith Shaw explains the method: http://members.aol.com/kmyersefo/scale.pdf

Hans

timocharis
12-15-2005, 10:21 PM
I agree with hul. This is a case where you'll want to know pitch speed vs drag. Thrust will only really come into play when (a) you need to get out of trouble fast and (b) the drag from the wind can't be overcome by the system.

But!

That still means Qban is right to be wary. The drag can get enormous in a high wind, plus the tendency to have Things Suddenly Happen brings (a) into play. So I gotta agree -- I'd like to see high pitch speed and a bit more thrust. The Catalina didn't exactly present a slick aerodynamic profile.


Dave

cyclops2
12-16-2005, 04:39 PM
If you NEVER take off from water all the above will work fine.
PBY's taxing and trying to turn into the wind are VERY difficult. They settle deep in the water. Have the props so close you CAN NOT put big efficient props on them. You need both propspeed AND tons of TORQUE to DRAG that body fast enough to start climbing up to the step and break loose on a CALM day.
I have seen MANY just plowing the hulls around at full throttle. At some water meets the PBY's need the chase boat to go out and rough up the water for them to take off. Failing that, a lot of guys just give up and a GREAT hand launch person heaves it.
Landings are not that easy. The plane sucks the surface with a vengence and can easily nose over if speed bleed off AND ELEVATOR are not really well coordinated.
It is definately a plane that a GREAT water man should pre-fly.
Once in the air they are beautifull.
PRACTICE hand launches and landing in dense and 2' high farm field.
A good landing is Fast just above the tops of the weeds and gradually add up elevator to slow the plane to settle.
Never bring it in too slow. As you have nothing to work with when the water suddenly increases friction. FAST AND LEVEL keeps you in full control. You can always then start throttling back.
The only right solution is to move the engines farther apart so you can use the best props.. I did that on the China Clipper.
What good is a seaplane that takes off like a tugboat?

Virgil Kee
01-05-2006, 07:27 AM
this is a plane where I wouldn't worry about thrust and instead use W/lb and pitch speed.
50W into those 5x3 3-bladers give about 40mph pitch speed, which sounds ok, but to do it properly I would need wing area to estimate stall speed. What pack (mAh and C rating) did you plan to use? Any other info on motors ESCs etc? Hans

Here is the way it looks so far as to the specs you mentioned:

Wing area ........... 1.86 sq ft.
AUW .................. 26 oz.
Pack .................. 2s2p 1200 mAh LiPos (10C) <-------VERY tentative!
Motors ............... GWS 300H DD (if Brushed) (2 ea.)
OR: 300DF *Outrunners (if Brushless)
ESC(s) ............... One if brushed, two if brushless (I believe I've read)
Props ................. Cox 5x3 3-bladers

I need a lot more education on parallel vs. series on this twin!

*Do they fit the Guillows nacelles, does anybody know? There are more than one of these offered. They all have a dia. of just about 1".

Any help?
Virg.

hul
01-05-2006, 10:15 PM
your wing loading should give a stall speed of roughly 14mph (= 3.7 x sqrt (wing loading), according to Keith Shaw). So you're aiming for a pitch speed of 35 to 42mph (2.5 to 3 times stall speed).
Motocalc lets you pick motors that work with those props and packs.

I have no experience with water planes. Don't know how much power or thrust they need to lift off water. This website makes me believe it's not a lot though: http://www.geocities.com/ivansplans/
Probably depends a lot on hull design, which seems to be pretty poor for the PBY.

Brushed twins used to be wired in series in the old days, mostly because the cells at the time couldn't cope with high current (series draws the current of one motor out of the pack). Not much of an issue today.
Most are wired in parallel now, brushless doesn't leave you a choice (must be parallel). If you want to use a BEC, parallel is the way to go, series will usually go over the BECs' voltage limit. And high voltage ESCs are more expensive.
Some people successfully use one ESC with two brushless motor. There is another thread on this subject.

Hans

qban_flyer
01-05-2006, 10:51 PM
I have no experience with water planes. Don't know how much power or thrust they need to lift off water. This website makes me believe it's not a lot though: http://www.geocities.com/ivansplans/
Probably depends a lot on hull design, which seems to be pretty poor for the PBY.

Hans

The hull design on older flying boats used to be rather inefficient. The Widgeons and Catalinas were examples of that inefficient hull design, thus requiring more thrust than land planes and modern design flying boats to ROW properly. Those older designs were rather draggy.

Even early float equipped planes required a longer take off roll once they got "on the step" because of the added drag the water presented to the pontoons supporting the airframe. It is much easier to roll a plane on wheels on pavement than to drag it on water while on floats.

My Goldberg Skylane 62 was designed to be used with a 19 to 35 size glow engine as a land plane. I had to resort to a trusty Super Tigre 45 in order to get it "on the step" easier and to prevent it from stalling on ROWs.

That being said, I would apply the old formula of early electric flight to this particular case, though it is a very valid formula still.
Since 50W per pound is/was the recommended figure to use for ROG, I would have to assume that for ROW, 75W to 100W per pound is needed just to be on the safe side.

BTW, most modern 300 motors are around one inch in diameter or slightly over, so the Guillow's nacelles may have to be modified slightly to accomodate them.:)

hul
01-06-2006, 03:11 AM
Since 50W per pound is/was the recommended figure to use for ROG, I would have to assume that for ROW, 75W to 100W per pound is needed just to be on the safe side. 100W/lb would be about 160W total, 80W per motor. That would spin the 5x3 3-blader at about 14000rpm giving 40mph pitch speed (assuming 50% motor efficiency). 80W would be about 8amps on 3s Lipo (or 12amps on 2s). On 3s Kv would have to be around 2000rpm/V (motor Kv/gear ratio).
A speed 400 7.2V direct would probably work. Mega 16/7/10 should work too.
A good brushless would need fewer amps for the same output power and would be lighter than a brushed motor too.
I don't know small motors very well at all, can't really recommend anything.

Hans

qban_flyer
01-06-2006, 03:17 AM
A good brushless would need fewer amps for the same output power and would be lighter than a brushed motor too.

I agree 100%. A bit costlier in the beginning but more rewarding in the long run.:)

TeslaWinger
01-06-2006, 04:18 AM
Thrust is what you measure when you tie its tail to a scale and let the motor wail!

Plot the WattMeter parameters with the thrust numbers (a Thrust Measuring Event) and that is what I call an experiment that people might actually want to do.

A videotape showing the thrust scale readings and the WattMeter display simultaneously would help to graph it later. Thrust is the payoff for all this electrical stuff.

"Throw a tractor pull and they will come!"
They seldom hit 20 mph either! :D

General rule of thumb. If a gusty wind speed equals the flight speed of the model, the turbulence will be powerful enough to cause loss of control on ocassion with a small model- you will just spend most of your flight 'parked' in one spot and land straight down- or explore downwind and struggle to get back. What effect does the wind have on the water at these wind speeds? It would be like doing the Perfect Storm with a seaplane!

TW

PS: A Lipo/Brushless seaplane would have a huge advantage in power/weight/endurance/acceleration. I think 100 watts/lb is reasonable. Sticky stuff, that aqua- it takes power to get unstuck and take off with authority.

hilgert
01-06-2006, 07:11 AM
You should get a wattmeter to measure your power at WOT (wide open throttle) static (with the plane on the bench). That will give you an idea if you have mismatched your prop, motor, ESC or batteries. For example, you might have a proper prop/motor setup that draws 270 watts at 11.1 volts (roughly 25 amps), but if your ESC is a 25 amp ESC you are at the "edge" of it's ability to keep up, and it will shutoff the motor to protect itself. Also, if you are using 3S1P (3 serial cells) 2100mah (2.1 amps for one hour) 11.1V lipos that are rated at 10C (10 times the mah current rating), then you would be pulling too much power through the batteries and you *will* ruin them (some lipo batteries allow 15C in short bursts, however).

Also useful as a *guide*, but not a bible, is a program like MotoCalc. While it is not entirely accturate, it does give amazingly good directional information on various motor and prop combinations. http://www.motocalc.com

Virgil Kee
01-06-2006, 07:16 AM
hul, qban & TelsaWinger: Sorry, Guys, I thought I had mentioned one other project parameter - Model to be hand launched, over grass! Your comments on ROW power requirements were interesting, but wont be pertinent to the project.

SUMMARY OF RECOMMENDATIONS:

>100w/lb;
>Read Keith Shaw;
>Use Motocalc;
>Wire the motors parallel --- (on Y from Batt to 2-ESCs?), and,
>"Lotsa LUCK, Fella"!

THREAD STATUS:"Dead in the Water"? (Like an underpowered PBY?,Yuk,Yuk)

Unless the above sparks additional input, I'll just say, Au revoir, Ciao, Hasta la vista, Auf Wiedersehen, or Sayonara (your pick), and THANKS,

Virg.

qban_flyer
01-06-2006, 07:28 AM
hul, qban & TelsaWinger: Sorry, Guys, I thought I had mentioned one other project parameter - Model to be hand launched, over grass! Your comments on ROW power requirements were interesting, but wont be pertinent to the project.

SUMMARY OF RECOMMENDATIONS:

>100w/lb;
>Read Keith Shaw;
>Use Motocalc;
>Wire the motors parallel --- (on Y from Batt to 2-ESCs?), and,
>"Lotsa LUCK, Fella"!

THREAD STATUS:"Dead in the Water"? (Like an underpowered PBY?,Yuk,Yuk)

Unless the above sparks additional input, I'll just say, Au revoir, Ciao, Hasta la vista, Auf Wiedersehen, or Sayonara (your pick), and THANKS,

Virg.

Nope, Model to be hand launched, over grass! was left out of your posts pertaining the Guillow's PBY Catalina.:eek:

http://www.wattflyer.com/forums/showpost.php?p=29044&postcount=14

http://www.wattflyer.com/forums/showpost.php?p=34621&postcount=19

At any rate, all the advice given still applies if you are going to be building and flying that model on 15MPH winds.:rolleyes:

I am unsubscribing from this thread. Adiós, over and out!:p