Static Thrust to Weight

[Doug L]

Hello, I am looking at engine options to plane weight for scale/semi scale aircraft, and interested to understand what a good Thrust/Weight ratio would be. To explain more if it was 1:1 I would assume it would generally fly too fast and unrealistic without constantly backing the throttle off. So would a Thrust/Weight of say 0.75/1 give more realistic flying capabilities? I realise wing design loadings and props affect this but, so was thinking rule of thumb. 

 

Cheers.

[J D 8]

  1 to 1 plus some is what is needed for a model to be able to prop hang.  So I would say your .75 to 1 is about right for most types but something like a light weight motor glider will be happy on a heck of a lot less.

      My big motor glider 110 span 10lb fly's well on its old open rocker Enya 40 but need a good run to get airborne

Edited April 15, 2022 by J D 8

[Jon H]

You cant really calculate it this way as its very difficult to get accurate thrust data as we can only test on the ground. 

 

There are also so many variables in propeller choice it would be almost impossible to account for them all. 

 

As an example, my 63 inch Hurricane is 9.5lbs and has a laser 80 fitted. I have flown the model on 14x6 apc, 14x6 master, 14x7 apc, 14x7 3 blade master, and 15x6 powermaster wood. 

 

Each of these propellers gave a different performance from the same engine and in the end i decided the 14x7 apc 2 blade gave me the best performance but this was mostly down to the feel of the aircraft in flight rather than a quantifiable number. 

 

Beyond that, the type of aircraft, its wing etc are all far more important than its power/weight or wing loading figures. 

 

Which model are you planning to power? And what is its wingspan and weight? 

 

 

 

 

[Doug L]

Im thinking WW1 plane from a plan, or kit and plan. What i know is, if the weight specified says 4kg, after I've built it, the weight will probably turn out to 4.5. Also i don't want to over power it. Saito give kg of thrust so i have in the past matched engine to airframe successfully from this, rather than a 60 size or 90 size etc. hopefully that explains it.

Edited April 15, 2022 by Doug L

[Denis Watkins]

The numbers you distrust Doug, pan out beautifully.

A .60 will comfortably fly 4Kg.

Anything less would be wallowing close to stall most of the time.

[Outrunner]

If you want to power a WW1 aeroplane in a scale like manner then you need just enough power to take off and keep it just above the stall speed, nothing worse than a Sopwith Pup type flying around like a Merlin powered fighter.

[Shaun Walsh]

24 minutes ago, Outrunner said:

If you want to power a WW1 aeroplane in a scale like manner then you need just enough power to take off and keep it just above the stall speed, nothing worse than a Sopwith Pup type flying around like a Merlin powered fighter.

Just had a quick look at Wikipedia entry for the Sopwith Pup and power/weight ratio works out at about 50W/Lb so about 0.6hp for a 4Kg model

Edited April 15, 2022 by Shaun Walsh

[Andy48]

I generally fly with a static thrust about equal to or somewhat greater than the weight for every model I've built, even my Flair Sopwith Pup. Its a dead easy rule of thumb and works every time. All mine are electric, and its really very simple, you don't have to fly on full throttle!

[J D 8]

  Not having to fly at full throttle is fine most of the time, the problem comes with over powering an IC type like the pup and then having to fly at low power all the time. Engine cools and can start to miss and splutter and then be slow to respond to pushing the stick up or even cutting out.

 As Outrunner notes just not right flying a Pup like that, they do not even go that much faster but just want to climb like the homesick angel.  Also it is common for many early type aircraft to display some strange unwanted aerodynamic traits if flown to fast.

[Jon H]

I am a big fan of WWI models and the worst thing you can do is over power them. Flair pups with 70's or 80's are my personal nemesis. Use a 50! as JD8 points out running an engine much below half throttle is likely to lead to reliability issues. 

 

My little Nieuport 17 is 52 inches and powered by a saito 45 that is brutally overloaded with a 15x5 prop. I dont recall how heavy it is, but the plan says 6lbs and it is probably about that. Anyway flying it last night in flat calm conditions i needed barely any throttle at all even with my railway sleeper of a prop. it is whisper quiet and, its not thrashing its bits off.  In the future i have a similarly sized pup, triplane and se5a to build. They have already been assigned 40-50 4 strokes as they really do not need any more than that. 

 

The only time i use engines much bigger than needed are in 1/4 scale WWI fighters as you need a big prop on them so they dont look stupid. A 1/4 pup will fly fine on engines a small as a 90 but i always recommend (and will use myself) a 180 or 200 just so i can turn a huge great prop. My 1/4 stampe is the same way. 

 

Even so, it does require some discipline and skill from the pilot to not fly it like a pitts as well as the right choice of propeller. In the pup example with a 200 i would be looking at 21 or 22x6 to really load it down. 

 

If you plan to build a WWI fighter i recommend you pick exactly which one you want and go from there. Certainly any 50-60 ish inch model needs no more than a 50 4 stroke 

Edited April 15, 2022 by Jon - Laser Engines

[Doug L]

Great info, and plenty to think about. Thank you.

[PatMc]

Static thrust to weight ratio is meaningless except that the model will be able to hold a hover. Thrust will reduce with any horizontal movement as the model accelerates, at the same time drag will increase. When the drag & thrust are equal the model will continue at a steady speed. However the model may not have gained enough speed to achieve lift off.

For example this Futaba servo is rated at 4.3 Kg.cm torque (i.e. thrust) moving 60° per .16 sec (= 62.5 rpm). If it was modified to continually rotate a 1cm radius drum winding in a line winching a 4.3 Kg model the model would be travelling at 2 x Pi x 62.5 cm/min = 393cm/min = 0.24kph. 

In other words the servo can produce 4.3 Kg thrust but can only move the load at 0.24 kph. 

 

I only did the maths quickly on a calculator so can't guarantee 100% accuracy but near enough I think to illustrate my point.

Edited April 15, 2022 by PatMc

[Andy48]

On 15/04/2022 at 23:50, PatMc said:

Static thrust to weight ratio is meaningless except that the model will be able to hold a hover. Thrust will reduce with any horizontal movement as the model accelerates, at the same time drag will increase. When the drag & thrust are equal the model will continue at a steady speed. However the model may not have gained enough speed to achieve lift off.

For example this Futaba servo is rated at 4.3 Kg.cm torque (i.e. thrust) moving 60° per .16 sec (= 62.5 rpm). If it was modified to continually rotate a 1cm radius drum winding in a line winching a 4.3 Kg model the model would be travelling at 2 x Pi x 62.5 cm/min = 393cm/min = 0.24kph. 

In other words the servo can produce 4.3 Kg thrust but can only move the load at 0.24 kph. 

 

I only did the maths quickly on a calculator so can't guarantee 100% accuracy but near enough I think to illustrate my point.

I would disagree. Firstly you cannot compare a servo spec to the static thrust produced by a model aircraft. Torque is not the same as static thrust. Torque is a measure of the force that can cause an object to rotate about an axis. The conclusion you come to is also incorrect. It assumes a 1cm radius drum, a smaller radius will obviously produce a higher speed, or you could gear it up.

 

The static thrust of a model aircraft is the thrust of the combination of motor and propeller working together, and as we all know this can vary considerably depending on the characteristics of the motor and propeller individually.

Personally I have never found an example where one of my models  has not gained enough speed to take off if the static thrust is greater than the model weight. Most will take off on just half throttle, and never more than two thirds throttle. I suspect any model which requires a static thrust significantly higher than its weight would be very difficult to fly in any case and would have a very high wing loading. A RC model car will obviously fit this condition!

[Andy48]

On 15/04/2022 at 14:51, J D 8 said:

  Not having to fly at full throttle is fine most of the time, the problem comes with over powering an IC type like the pup and then having to fly at low power all the time. Engine cools and can start to miss and splutter and then be slow to respond to pushing the stick up or even cutting out.

 

The joys of flying electric. ?

[J D 8]

            The joys of electric flying,   Lets not start this eh.

[Frank Skilbeck]

On 15/04/2022 at 14:51, J D 8 said:

  Not having to fly at full throttle is fine most of the time, the problem comes with over powering an IC type like the pup and then having to fly at low power all the time. Engine cools and can start to miss and splutter and then be slow to respond to pushing the stick up or even cutting out.

 

 

I find if you put too big an IC engine in, then the residual thrust ay tickover can make landing seem to go forever, which if you are in a tight space is a pain.

[Graeme White]

7 hours ago, Frank Skilbeck said:

 

I find if you put too big an IC engine in, then the residual thrust ay tickover can make landing seem to go forever, which if you are in a tight space is a pain.

I have an irvine 53 in my travel air and it flys at idle. I have to cut the engine on final approach to get the thing to stop flying!

[Nigel R]

20 hours ago, J D 8 said:

            The joys of electric flying,   Lets not start this eh.

 

I think I see your problem, your balance plug is wired on backwards.

[Eric Robson]

Looks like you've nailed it!