Hi everyone,
In May this year, I had posted a request to Anwar sir for a Tech Topics board. This was seconded by none other than rcpilotacro. However, this may have missed getting the attention of Admin.

Be that as it may, I thought I'll start such a topic anyway.
The first one is 'How much power is needed?

From the posts it seems to me that many many members consider a motor's kv (rpm per volt) as the indicator of its power. This is a totally incorrect notion.

1. A motor's power is indicated by its max watts rating
2. Its ability to convert its power into thrust depends on the prop, primarily the prop dia
3. Its ability to fly the model at appropriate speeds depends on the prop, primarily the prop pitch

So the next questions are:
- How much thrust do you need?
- How many watts needed to provide the required thrust?
- What speed is required for the model to fly properly?
- What is the required prop pitch to deliver the required speed, given the rpm/kv of the motor?

We'll look at these questions in the next post...

How much thrust do you need?

Long accepted thumb rules, and personal experience (supporting data available) indicate that:
1. A trainer type model needs only 30-35% thrust to weight ratio to maintain level flight
2. 50% thrust to weight is enough for safe take off and climb out
3. 100% thrust to weight is enough for most aerobatics
4. 150% thrust to weight will give unlimited vertical
5. More than that may be required for 3D, but I have (so far) no experience to comment from

Contd from post #1
How many watts to produce the required thrust?

One thumb rule is 5gms thrust per watt. (Static thrust)

Small props / high pitch / high kv motors may be in 3-4 gms/watt range
Large props / low pitch / low kv motors may be in 6 gms/watt range

Take the example of a 40-48" span trainer weighing about 600 gms.
300 gms thrust should be enough to fly it safely
At 5 gms/watt, that needs 60watts.

This is a reasonable starting point for selecting a suitable motor. For the above example, a 100watt motor would have sufficient reserve.

But can it fly the model fast enough to avoid stalling?

Following thread sir
A tech section should surely be started. Many one off tips/techniques/thumb rules are lost over time.Similar to ancestral  knowledge being lost over time
keeping them stored in an active section where all the experts can share their thorough know how would be awesome. Then the repetitive questions will be reduced.

Sorry Sir for intruding and posting in between.

Thank you very much sir for starting such a thread.

that seem to me interesting  

go on sir

Contd from reply #2
How fast does it need to fly?

The answer is - sufficiently faster than its stalling speed
Thumb rule: Should be capable of 2.5-3 times the stall speed.

How to find the stall speed?
Thumb rule: 4-5 times the square root of the wing loading. (Haha, my favourite subject!)

Take a 40-48" span trainer. Wing chord say 6-8". Wing area say 2-2.5 sqft. All up weight say 550-650gms.
That will give a wing loading of about 9oz/sqft (say 250gms per sqft)
Stall speed estimate in mph is: 4-5 times square root of wing loading in oz/sqft
In this case 12-15 mph (18-22 ft/sec, 6-7 m/s, 20-25kmph)

Cruise capabilty required is therefore (2.5-3 times stall speed):
30-45 mph, or 45-60 ft/s, or 15-20 m/s, or 50-70 kmph.

Contd from reply #5

What pitch is required to get say 60 ft/sec speed?

Staying with our 40-48" span, 2-2.5 sqft, 550-650gm, 9oz/sqft model,
Let us consider 4" and 6" pitch (fairly typical)

A 6" pitch prop (ie, 0.5ft) needs to turn 120 times to go 60 ft.
So for 60 ft/sec speed, it has to turn at 120 rps or 7200 rpm.
A 4" pitch prop needs to turn 180 times to go 60 ft.
So for 60 ft/sec (about 65kmph) it has to run at 180 rps or 10,800rpm

Subscribed.!! keep it coming Iyer sir. 
It would be great if you could summarize all those concepts that you already talked about in this thread. like prop selection etc.

Ashok.P

Subscribed.
This is a superb start for all the beginners and others too. Very well explained.

Very nice method to calculate the speed. 

Contd from reply #6

Quote from: K K Iyer on September 11, 2014, 11:34:57 PM
A 6" pitch prop (ie, 0.5ft) needs to turn 120 times to go 60 ft.
So for 60 ft/sec speed, it has to turn at 120 rps or 7200 rpm.
A 4" pitch prop needs to turn 180 times to go 60 ft.
So for 60 ft/sec (about 65kmph) it has to run at 180 rps or 10,800rpm

On 2s, say 8v, 6" pitch needs 900kv (rpm per volt), 4" pitch needs 1350kv
On 3s, say 12v, 6" pitch needs 600kv, 4" pitch needs 900kv.

Since the motors that are generally available run at about 75% efficiency (actual rpm/v compared to theoretical kv), the above figures have to be divided by 0.75.

So,
On 2s, say 8v, 6" pitch needs 1200kv (rpm per volt), 4" pitch needs 1800kv
On 3s, say 12v, 6" pitch needs 800kv, 4" pitch needs 1200kv.

Now we know the pitch, we know the kv. What about dia?

Contd from reply #10

For prop dia I guess we'll just have to make up a thumb rule!

How does 15-25% of wing span sound?
Many of our 36" span models probably have 8" dia props (22%)
48" span models probably have 10" dia props (21%)

So let us assume (using 3s lipo), that we need:
1. Prop dia of 9"
2. Prop pitch of 5"
3. Motor of 1000kv
4. Capable of 100watts (or say 8-10 amps)

Will post examples shortly!

Excellent info sir.. my vote also goes for a dedicated tech section 

Contd from reply #11

Example: my electric trainer
Span 52", chord 8", area 2.9sqft, weight 500g w/o battery, 590g with 3s 100mah, 690g with 2200mah.
Wing loading 7.25/8.25 oz/sqft with 1000/2200mah battery.
45 degree climbout on 5amp, 60watts.
Maintains height on 2.5 amps, 30 watts.
Prop 9x5 ordinary orange, not EMP or APC.
Motor Turnigy D2830/11, 1000kv, rated at 210watts, thrust 890gms as per manual!
This motor is far too much for this model, as i never need over 30% of the available power!

Contd from reply #13

Example: my Spitfire
Span 34", avg chord 7", area 1.65 sqft, weight 360gms
Wing loading 7.7oz/sqft with 3s 1000mah battery
Prop 8x4 ordinary orange.
Motor L2210 1650kv
Flies loops and rolls at half throttle (72w) thrust about 280gms
Vertical rolls out of sight on full throttle (210w) thrust over 650gms (far too much!)


Edit: In case anyone is wondering what I'm talking about, kindly see from first post. Thanks.

subscribed sir !!
Looking forward to learn something new every day

subscribed for some amazing gyan   

Iyer Sir

Quote from: K K Iyer on September 11, 2014, 10:43:25 PM

One thumb rule is 5gms thrust per watt. (Static thrust)

Small props / high pitch / high kv motors may be in 3-4 gms/watt range
Large props / low pitch / low kv motors may be in 6 gms/watt range

I'm sorry sir, but whereas this may apply frequently, it cannot be termed a rule. There are too many other factors involved. Some large diameter low KV combinations are frequently in excess of this

Great work Iyer sir....
Expecting a lot of good discussion here....  Subscribed!

Super Iyer sir! Just a suggestion that everything in a single post will be easy to read and digest than skim through all posts.

This is a very good idea sir. I have a suggestion to anwar sir though. I agree with Vishal ^, but have a better idea, if it can be implemented. Can we have an article section for stuff like this? It would keep all information posted by the Author in one place and then comments/discussion can be added at the bottom, rather than having comments mixed in between posts related to the topic. The author should also be allowed to add to his article any time he feels like.

Very Nice Iyer sir.
Sir are questions allowed.

Iyer sir one question if I have an 2836 1000kv and an 2822 1000kv motor both of them with a 10*4.7 prop. Basically both the motors should be producing the same thrust as they are of the same size and running the same prop but why does the bigger motor produce more thrust.

Good question Nitesh (although after asking 'can I ask a question' - (which was a question  ) you have without waiting for Iyer sir's reply asked a question, thus raising the question is it questionable to ask these questions  

What is good here is you have indirectly asked the 64 million $ question - how to predict the rpm (and therefore voltage)drop under load!

Quote from: rcrcnitesh on September 13, 2014, 11:29:44 AM
Iyer sir one question if I have an 2836 1000kv and an 2822 1000kv motor both of them with a 10*4.7 prop. Basically both the motors should be producing the same thrust as they are of the same size and running the same prop but why does the bigger motor produce more thrust.

@rcrcnitesh,
Same size, or is one bigger? 2836 vs 2822

"From the posts it seems to me that many many members consider a motor's kv (rpm per volt) as the indicator of its power. This is a totally incorrect notion."

Both motors being 1000kv simply means that if you connect a 12v battery, they will both run at 12,000rpm. But this is without any load, ie without prop.

If you run both motors with the same prop at the same rpm (verified by a tachometer), the thrust will be the same.
But as you increase the throttle to full power, you will find that the smaller motor will not be able to match the rpm of the bigger motor with the same prop.
Result: The maximum thrust from the bigger motor will be more.
(Remember that the power consumed will also differ)

Edit: 'Are questions allowed?' was answered in the affirmative via pm yesterday