Model Aircraft Prop Calculations

Look up Dr. Martin Hepperle's site (search on his name). He's got a prop calculator on there. It's mostly made as a tool for designing props, but with a bit of effort you may be able to enter in the information for your prop and get enough data to construct the curves you're looking for.

Most model airplane designers aren't up to doing the detailed calculations you need to do -- we either don't have the background or don't have the time. So we just put in a honkin' big motor and a honkin' big battery pack, and let any inefficiencies be absorbed by the joy of accelerating straight up.

On Mon, 22 Oct 2007 07:29:16 -0700, snipped-for-privacy@hotmail.com wrote in :

Not all 12x7 props are the same. There are probably a half-dozen commercial designs in that range. Some may have quite complex designs so that calling them "7-pitch" is a ballpark category and not a description of the shape of the prop from hub to tip.

If you really have to know this, you probably have to create your own test stand to get some information about static thrust from particular propellors.

With APC props, you can heat them and re-pitch them yourself. By "you" I mean "not me." I've only read about the technique. There's some fellow who does this for pattern pilots.

Reasoning from static tests to flight performance is a hard task, too. On the bench, you get to control a lot of variables; in the air, stuff happens. But I would guess that a prop that produces more thrust-per-watt on the bench is going to produce more satisfaction in the air, all things being equal. If you bench-test four props, then go fly them, that may help you figure out the right kind of model to use for predicting efficiency in advance.

Good luck with your project. Let us know how it turns out!

Marty

Goedendag Vincent ;-)

You could also ask around on

Drive Calculator:

Vriendelijke groeten ;-) Ron van Sommeren near Nijmegen, the Netherlands int. electrci fly-in

Cheers Tim ill have a look. However its cheating if i just use a calculator. A final report on this will require calculations from square 1!

But ill have a look

thanks

Vince

also

AXI 2820/10, 10v (3s LIPO)(4000mA/h) and 10x7 pulling around 30-35A

A 3Kg model won't be super sporty, but it will fly.

I do not understand what you mean by "required takeoff acceleration of ~ 7m/s." The units on acceleration are not m/s. Those are velocity units. I think what you really mean is enough thrust to get to a takeoff velocity of 7m/s. If so just size the prop and motor to give adequate performance flying and you will generally be fine on the ground. If your take off surface is grass or rough dirt you may have to use bigger tires to cut rolling resistance and bounce. If it is an old corn field or a beach with 5 cm diameter rocks on it you may need enough thrust to nearly hover to get off the ground and 20 cm diameter tires to boot. On a hard smooth surface almost any tire will work. At any rate there is no practical way to calculate these kinds of drag. There is even a big drag difference between 3 cm grass and 5 cm grass, particularly for small tires.

Are you going to build it and fly it or is this just to see how close you can get to a certain criteria? mk

Goedendag ;-) Vince,

You can use calculators to check your results or guide you. More calculators

Vriendelijke groeten ;-) Ron van Sommeren near Nijmegen, the Netherlands int. electric fly-in

motorcalc will supply basic info and program is free for first 30 day

so with two computers you can get 60 days??

at 5000 feet air ill be ~20 thinner and prop will need to be 20% large to do same work

ground speed will be 20 % faster and stall speed will be 20% greater however airspeed indicator operated from moving air will remain th same speed making calculations from this instument difficult

GPS will show true ground speed

best rule is to make static test speed of air leaving prop at WOT t be three times stall speed of plane

best to chose KV with low KV and slow turning motor for better global effiency of motor and props

best to opt for larger shallow pitch props for best grip at lowwe speeds and better fuel economy at cruising speeds

keep us posted on the plane and show some pics or suitable sites

Ral

-- treeho

----------------------------------------------------------------------- treehog's Profile:

That bit you got wrong. Large very COARSE pitched props are the most efficient. You want the pitch*RPM to be a shade above cruising speed.

If you have too low a pitch, you have the drag of having to spin the prop much faster to 'keep up' with the airspeed.

Goedendag ;-)

A compilation of motor/prop/battery calculators:

-> propellor calculator (bottom of page)

Vriendelijke groeten ;-) Ron van Sommeren near Nijmegen, the Netherlands int. electric fly-in

Welllll, Hmmmmmmmm. Lets say you have a system that is matched for good performance at sea level. Such a system will have a prop pitch such that at max rpm the pitch speed is just a bit above the air speed as stated by TNP. Now when you take that same plane to 5000 feet there is less air.

The net result of less air is that prop that works great at sea level has too little air to provide needed thrust. In order for the prop to absorb the energy from the motor the rpms will go up. But this increase in rpm mainly results in stirring the air a lot generating drag due to the poor match between pitch and air speed. Without thrust the plane flys slower. If it happens to be a draggy plane it flys a lot slower. So now that the prop pitch speed is very poorly matched with what is happening in the air as pitch speed is much too high. The prop is slipping like crazy in the air.

The solution is to hold the prop pitch constant and increase the prop diameter and see if that will help performance. This, at altitude, will increase thrust, and therefore air speed, resulting in less prop slippage in the air and less wasted energy turning into prop drag. The next step, if that is insufficient, is to drop prop pitch and increase D more just like Ron suggested. The increase in diameter will dramatically increase the thrust generated while the small reduction in pitch will minimally reduce max air speed. At increasing altitude you simply have to accept a reduction in air speed as a consequence of reduced air pressure if you intend to continue flying at a constant power input to the prop. Of course as you go up and reduce air pressure stall speed also increases so the plane needs to fly at faster air speeds to stay above stall speed. At some altitude max air speed will drop to the stall speed regardless of how well the prop is matched to conditions and you can not fly higher. At that point the only solution is more power. ie bigger motor, bigger prop D and bigger pitch.

Welllll, Hmmmmmmmm. Lets say you have a system that is matched for good performance at sea level. Such a system will have a prop pitch such that at max rpm the pitch speed is just a bit above the air speed as stated by TNP. Now when you take that same plane to 5000 feet there is less air.

The net result of less air is that prop that works great at sea level has too little air to provide needed thrust. In order for the prop to absorb the energy from the motor the rpms will go up. But this increase in rpm mainly results in stirring the air a lot generating drag due to the poor match between pitch and air speed. Without thrust the plane flys slower. If it happens to be a draggy plane it flys a lot slower. So now that the prop pitch speed is very poorly matched with what is happening in the air as pitch speed is much too high. The prop is slipping like crazy in the air.

The solution is to hold the prop pitch constant and increase the prop diameter and see if that will help performance. This, at altitude, will increase thrust, and therefore air speed, resulting in less prop slippage in the air and less wasted energy turning into prop drag. The next step, if that is insufficient, is to drop prop pitch and increase D more just like Ron suggested. The increase in diameter will dramatically increase the thrust generated while the small reduction in pitch will minimally reduce max air speed. At increasing altitude you simply have to accept a reduction in air speed as a consequence of reduced air pressure if you intend to continue flying at a constant power input to the prop. Of course as you go up and reduce air pressure stall speed also increases so the plane needs to fly at faster air speeds to stay above stall speed. At some altitude max air speed will drop to the stall speed regardless of how well the prop is matched to conditions and you can not fly higher. At that point the only solution is more power. ie bigger motor, bigger prop D and bigger pitch.

Explain this: Goldberg Electra with brushed 500 motor that came with the kit and using recommended 8X4 prop and 7 cell NiCad pack. There was very little difference in performance at 8000 ft amsl versus 3500 ft amsl except for a noticeable difference in groundspeed at the higher elevation. It was handlaunched in both cases. I was surprised that it didn't immediately stall when launched up in the NM mountains. Theory is nice, but actually trying it is better. I have always used a chart like this as a starting point in prop selection and change it if I don't like the results.

"Anyolmouse" wrote in message news:1199029762 snipped-for-privacy@sp12lax.superfeed.net... | | wrote in message | news: snipped-for-privacy@c4g2000hsg.googlegroups.com... | On 28 Dec, 17:06, The Natural Philosopher wrote: | > treehog wrote: | > > motorcalc will supply basic info and program is free for first 30 | days | > > so with two computers you can get 60 days?? | >

| > > at 5000 feet air ill be ~20 thinner and prop will need to be 20% | larger | > > to do same work | >

| > > ground speed will be 20 % faster and stall speed will be 20% greater | > > however airspeed indicator operated from moving air will remain the | > > same speed making calculations from this instument difficult | >

| > > GPS will show true ground speed | >

| > > best rule is to make static test speed of air leaving prop at WOT to | > > be three times stall speed of plane | >

| > > best to chose KV with low KV and slow turning motor for better | global | > > effiency of motor and props | >

| > > best to opt for larger shallow pitch props | >

| > That bit you got wrong. Large very COARSE pitched props are the most | > efficient. You want the pitch*RPM to be a shade above cruising speed. | >

| > If you have too low a pitch, you have the drag of having to spin the | > prop much faster to 'keep up' with the airspeed. | >

| >

| | Welllll, Hmmmmmmmm. Lets say you have a system that is matched for | good performance at sea level. Such a system will have a prop pitch | such that at max rpm the pitch speed is just a bit above the air speed | as stated by TNP. Now when you take that same plane to 5000 feet | there is less air. | | The net result of less air is that prop that works great at sea level | has too little air to provide needed thrust. In order for the prop to | absorb the energy from the motor the rpms will go up. But this | increase in rpm mainly results in stirring the air a lot generating | drag due to the poor match between pitch and air speed. Without | thrust the plane flys slower. If it happens to be a draggy plane it | flys a lot slower. So now that the prop pitch speed is very poorly | matched with what is happening in the air as pitch speed is much too | high. The prop is slipping like crazy in the air. | | The solution is to hold the prop pitch constant and increase the prop | diameter and see if that will help performance. This, at altitude, | will increase thrust, and therefore air speed, resulting in less prop | slippage in the air and less wasted energy turning into prop drag. | The next step, if that is insufficient, is to drop prop pitch and | increase D more just like Ron suggested. The increase in diameter | will dramatically increase the thrust generated while the small | reduction in pitch will minimally reduce max air speed. At increasing | altitude you simply have to accept a reduction in air speed as a | consequence of reduced air pressure if you intend to continue flying | at a constant power input to the prop. Of course as you go up and | reduce air pressure stall speed also increases so the plane needs to | fly at faster air speeds to stay above stall speed. At some altitude | max air speed will drop to the stall speed regardless of how well the | prop is matched to conditions and you can not fly higher. At that | point the only solution is more power. ie bigger motor, bigger prop D | and bigger pitch. | | Explain this: Goldberg Electra with brushed 500 motor that came with the | kit and using recommended 8X4 prop and 7 cell NiCad pack. There was very | little difference in performance at 8000 ft amsl versus 3500 ft amsl | except for a noticeable difference in groundspeed at the higher | elevation. It was handlaunched in both cases. I was surprised that it | didn't immediately stall when launched up in the NM mountains. Theory is | nice, but actually trying it is better. I have always used a chart like | this as a starting point in prop selection and change it if I don't like | the results. | | -- | Anyolmouse

Here is the link I forgot to post:

Provider ----

I do not see anything at all unreasonable with what you observed. I am assuming by ground speed you mean the speed of landing. As you have gone up enough in altitude to raise stall speed about 20% ( flat out guess - I did not calculate the increase ) the landing speed should have gone up. It also is obvious that at 3500 ft amsl you are really over powered with the motor and prop you are using so when you go to 8000 ft the combination still has more then enough thrust to fly the plane just fine. Flying electric also has the advantage that the motor does not derate with increasing altitude. Altitude effects are often more pronounced with internal combustion then electrics for this reason.

We all know there is a wide altitude range for any given engine/motor and prop combination. I have flown glo models a mile over my head without tweeking a thing after the plane left the ground. I have seen others do the same thing. Easy enough to do and prove you have done it. I suggest a reclining lawn chair, a spotter with binoculars and put one of those casio watches in the plane that records max altitude. Sure helps if it is a big plane also. Stay out of clouds. If you fly into clouds put it in snap rolls and it will fall almost straight down.

----

So much baloney about props here.

Speeds to do NOT vary by 20% between sea level and 5000 feet. Far more altitude than that is required. My calculator shows a 20% difference between indicated and true airspeeds at 12,000 feet, and much of that difference is the much lower reading of the indicator due to the lowered density and is NOT a lower airspeed. Groundspeed will rise, since stall speed is still indicated speed, and since indicated is lower than true at altitude, you might notice an increase in landing speed if you're flying in Denver but you're from LA. If you learned to fly models in Denver you won't notice anything.

Propellers do not lose vast amounts of efficiency at altitude. As air density decreases, so does drag. Commuter airliners using turboprop engines fly with those props up beyond 30,000 feet. At those altitudes, the overall lower drag allows the aircraft to cruise faster.

Static thrust has little meaning. A prop with a very low pitch will generate a high static thrust for two reasons: (A) Most if it is unstalled, compared to a prop with much higher pitch, and is therefore generating more thrust, and (B) since the motor is turning faster, and HP is a funtion of torque times RPM, more power is delivered to the low-pitched faster-turning prop. A low-pitched prop will have a low top forward speed, and I have seen airplanes struggle to stay aloft with their motors screaming just because the prop is too fine. Beware the static readings.

Internal-combustion piston engines lose power as altitude increases, unless they're supercharged or turbocharged. That's where the losses are. Not in the props.

Dan (Commerical Pilot, Aircraft Maintenance Engineer)

Why spoil the fun with facts? :) Thanks for the post. mk

News Provider ----

Much misreading of what was said Dan. The equation for lift has, as one component, linear density. Air density drops by 20% on going up

Stall speed has absolutely nothing at all to do with how well the plane flys or does not fly as long as propulsion is able to keep the plane well above stall speed. So any plane which is able to fly at say four times or greater above stall speed at msl will fly just fine at 7000 feet with exactly the same prop. But it will take off and land at 20% faster speed at 7000 feet.

Can you agree with this?

By the way, I am not confusing static thrust at all with thrust at speed. But TNP is. His whole thought pattern is based on static thrust. That is why I asked him to do the static and dynamic experiments. Experiments that he will not do because he fears the answer.

Signed (The guy who trys to teach guys like Dan enough about aerodynmics that they do not kill others)

Provider ----

Actually you are 100% wrong, such a blatant misrepresntation leaves me breathless.

I never mentioned static thrust at all. It was you who cionused static thrust with thrust at speed.

Also no plane flies at 4 times stall speed unless its grossly overpowered. Not in real life. not in a model.

In reality a typical plane will cruise at part throttle at about twice stall speed, and maybe have a top speed about three times stall speed. That already shows you that having a pitch speed way above three times is wasteful, unless you want to have a real racer on your hands.

Flaps are deployed to get the planes stall speed DOWN for landing and takeoff purposes.

Secondly, since the conditions for economical cruise are met with the prop angle of attack such that its quite close TO the pitch speed, pitch needs to go up for economical cruise at altitude. Since the economical cruise speed and the stall speed will increase by similar amounts.

To maintain thrust,which will probably need to be similar AT THAT SPEED. the diameter will need to go up a little too. But wing (blade~) are is the square of diameter..all other things being equal, so a 20% increase in thrust only needs a 10% increase in diameter.

So +20% on diameter, +20% in pitch will give you the 20% sped increase needed to fly ion 20% less dense air.

Note that at mo point have I mentioned static thrust. YOU are the one who has constantly mentioned it.

Unless you want to hover, or fly a helicopter, static thrust is totally meaningless with one exceptiion - getting off the ground, or more particularly water, where there is super high drag before the plane gets airborne.

Gawd help the pupils is all I can say.