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Thrust to weight ratios

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Old 06-11-2019, 07:35 AM
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john491
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Default Thrust to weight ratios

I am curious as to what everyone considers acceptable for thrust to weight ratio in their planes.
This is a bit vague I understand, and I am trying to keep this generic. We are talking about jets here, of every sort be they sport or scale, swept wing or straight. I try to keep in mind what I have available for power when I try to couple a plane and engine in my builds.
I have in the past tried for 1:1 as much as possible. It usually falls to about .8 : 1 from weight creeping up on me. Occasionally I end up as much as 1.5 : 1. I have found I have great difficulty getting something as low as .4 :1 in the air at all.
From what I seem to see here in the discussions the trend seems to be about 1.2 : 1, but that might be colored by the selection of threads I read.
My current dilemma is how to power a number of built airframes, and the fact that a number of them want the same turbine ( according to the standards I have found reasonable in the past), so now it is decision time for purchasing the next turbine.
I have a paved runway to fly from, with a grass strip next to it, though it is only 300 feet usable length. Most of my planes are from 13# to 21#.
What are people's thoughts on acceptable ratios.
John
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Old 06-11-2019, 08:29 AM
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gunradd
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it all depends on the plane. Some planes I want close to 1 to 1. On my scale planes for scale competition I have found I score better and fly them better if they are underpowered. Peter gold Smith is the master at flying underpowered scale jets and has won topgun several times by doing this.
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Old 06-11-2019, 11:23 AM
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john491
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Kris,

How much does your son's Cougar weigh? It would be a good example.
I saw where he went from a 45 Newton to a 60 Newton.
If his weighs the same as mine that's .588 thrust to weight gone to .762 thrust to weight.
You indicated it flew scale like with 45 Newtons but was more fun at 60, so unless scale flight is the objective sounds like maybe .75 might be a target minimum.
I was just wondering what others were finding to be minimum for fun, keeping in mind we can always really overpower the plane and then use that toggle on the left.
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Old 06-11-2019, 11:37 AM
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My DerJet Hunter is about 36lb empty/ 43lb fully fueled and is powered by a Merlin 140 at 31lb thrust.

T/W goes from about 0.7:1 at take off to just over 0.8:1 towards the end of the flight. It flies in a very scale-like manner, but you can definitely see an improvement in performance in the second half of the flight.

My Ultra Flash is also powered by a Merlin 140 and that is another beast altogether. About 1:1 at take-off and gaining performance all the time. That would be too much T/W for most scale models though for realistic flight performance, although it could be tamed by the restrained use of the throttle stick.

Paul
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Old 06-11-2019, 12:54 PM
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Originally Posted by john491 View Post
Kris,

How much does your son's Cougar weigh? It would be a good example.
I saw where he went from a 45 Newton to a 60 Newton.
If his weighs the same as mine that's .588 thrust to weight gone to .762 thrust to weight.
You indicated it flew scale like with 45 Newtons but was more fun at 60, so unless scale flight is the objective sounds like maybe .75 might be a target minimum.
I was just wondering what others were finding to be minimum for fun, keeping in mind we can always really overpower the plane and then use that toggle on the left.
John
Not sure what it weighed John but I think it was about 17lbs. It used lots of runway but after the first turn she got on step and flew nice but at a high throttle setting. The 60 makes it a whole different aircraft. He flies around at a much lower throttle setting and uses much less runway and the extra power if needed could get him out of trouble on a go around.
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Old 06-11-2019, 09:45 PM
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Well I completely see it different. To fly scale you need a tonne of extra power. We don’t have the momentum of the full scale, so we need power to drive the airframe through. If you fly with lower power you have to fly faster to have the energy needed to do vertical type manoeuvres. Throttle is everything and I do see people struggling to manage that, but done right the result is stunning. A lot of scale jets will be in the 450-500 knot cruise speed range, At 1/5-1/6th scale that is very slow, I agree we have to increase the speed a little for the correct look, but it’s still very slow compared to what many are used to. My DerJet Vampire is perfectly powered with 32lb thrust, most of the flight is 1/3 throttle stick position, but as soon as you pull the nose up power is slowly applied and the speed pretty constant. This model un managed is like a Super Bandit as it’s very slippery.

The type of turbine will also affect that, the higher efflux speed types don’t work so well in scale jets.
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Old 06-12-2019, 03:05 AM
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I have made several large jets with low thrust to weight ratios. The largest was the Arado 555 that had 6 of my own design engines of 3.2 Kg thrust. The plane weighed 35.5 Kg so had a thrust to weight ratio of 0.54:1 when all the engines were running at full power. On one flight was had 2 engines stop before takeoff and had to proceed regardless as we were almost at takeoff. When we landed we found only 3 were running that gave a takeoff thrust to weight of 0.36 :1 and in the air of 0.27:1. on my Boulton Paul P111 the thrust was 0.77:1 and that was fully aerobatic and nicely scale, mostly being flown on 1/2 throttle.
It is very difficult to give figures as what is OK with one plane will not be adequate with another. Full size aircraft fly with surprisingly low thrust to weight ratios. The BAE Hawk is in the order of 0.325:1 and the original Boulton Paul P111 was 0.5:1.

John

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Old 06-12-2019, 04:11 AM
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Originally Posted by Jgwright View Post
Full size aircraft fly with surprisingly low thrust to weight ratios. The BAE Hawk is in the order of 0.325:1

John
On average, transport and corporate jets are on average .33:1. Thatís Max Thrust to Max Gross Weight T.O. Most take offs are at closer ratios.

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Old 06-12-2019, 04:49 AM
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Originally Posted by Dave Wilshere View Post
Well I completely see it different. To fly scale you need a tonne of extra power. We donít have the momentum of the full scale, so we need power to drive the airframe through. If you fly with lower power you have to fly faster to have the energy needed to do vertical type manoeuvres. Throttle is everything and I do see people struggling to manage that, but done right the result is stunning. A lot of scale jets will be in the 450-500 knot cruise speed range, At 1/5-1/6th scale that is very slow, I agree we have to increase the speed a little for the correct look, but itís still very slow compared to what many are used to. My DerJet Vampire is perfectly powered with 32lb thrust, most of the flight is 1/3 throttle stick position, but as soon as you pull the nose up power is slowly applied and the speed pretty constant. This model un managed is like a Super Bandit as itís very slippery.

The type of turbine will also affect that, the higher efflux speed types donít work so well in scale jets.

Dave I see your point as well but the thing is with more power means more fuel and allot more weight. This means more speed and the scores suffer.

My my first year having a very under powered aircraft at top gun I won my class. Before with jets with good power best I could do is 12th.

My F15 weighed 75lbs full of fuel and had only 45lbs of power. It flew very scale like. The results speak for themselves. Also like I said Peter Goldsmith has been Mrtopgun several times and all with underpowered aircraft.


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Old 06-12-2019, 04:58 AM
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Most real jets of the -70 and -80 era had a thrust to weight ratio of around 0.6/1. Most model jets i see today are very overpowered in my opinion.

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Old 06-12-2019, 07:34 AM
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So my 1:4 scale panther jet itís about 65lbs and itís powered by the k-170 about 37 lbs off thrust the jet will perform all the scale maneuvers rolls and loops and fly just great, Iím planning to change the turbine to k-180 just because itís newer and kerosene start so there no need for bigger turbine,but in other hand I have twin power bvm rafale with 45 lbs weight and two 31 lbs turbines 62 lbs off thrust in 45 lbs jet this is just too much fun and crazy vertical .
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Old 06-12-2019, 07:52 AM
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john491
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So far it looks like .6 to 1 is about where the scale guys are scoring well, and it goes to about maybe .9 to 1.1 for really lively, exciting flying?

John
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Old 06-12-2019, 09:22 AM
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Hmm, I am not sure I follow Daveís point. My 25kg (55lbs) 1:4 Mig-15, flies very well with a JetsMunt 140 (14kg/30lbs thrust) turbine. Scale like takeoffs with ęlongĽ takeoff run, and shallow climb. Once airspeed is up and running, it is all about managing mass and throttle. Yes, speed will drop in vertical maneuvers, but so will the full size as well. The full size did not have power in reserve to maintain airspeed during verticals. I tend to plan ahead more with half power to weight ratio, which also contributes to a more scale fashioned flying style.
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Old 06-12-2019, 10:44 AM
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We would have to quantify airframe type, drag-big inlets or small since this is a big factor on drag. Big wing area so it flies on the wing, or big lift area like the F15, but big drag, so power is helpful.
Bigger turbines use less fuel...you will hardly use full power and if you do it will be for a very short time. Smaller turbines need full power longer, so less fuel can be carried with more power...Xcalibur with a P-60 around 7 mins, Xcalibur with a P-100 8 mins. Joker with a P-60 around 6.5 mins, Joker with a P-100 7.5 mins.
I fly my P-120SX powered CARF Hawk 12 mins with a std tank, because I fly it scale, it just about never had full power, especially take off.
Mig 15...low drag airframe, my P-100 Global Jetclub Mig 15 flew for 10 mins, because full power was not needed.
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Old 06-12-2019, 10:52 AM
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listen to the audio, at no point in this flight was I at full power, I roll on so slowly I never get past 118,000 even on the longest up lines, this is not the best flight as the first part is way faster than I’d normally fly-I hadn’t flown it for 7 months, so forgot how little power it needed. But also look at the video time and slow passes mid way through.
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Old 06-12-2019, 12:52 PM
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Originally Posted by gunradd View Post



Dave I see your point as well but the thing is with more power means more fuel and allot more weight. This means more speed and the scores suffer.

My my first year having a very under powered aircraft at top gun I won my class. Before with jets with good power best I could do is 12th.

My F15 weighed 75lbs full of fuel and had only 45lbs of power. It flew very scale like. The results speak for themselves. Also like I said Peter Goldsmith has been Mrtopgun several times and all with underpowered aircraft.


Nice!!!!

Is there a vid of this beast flying your routine?

i do agree on the overpowered, other than more vertical with scale Jets what is the gain?

Top speed is no where near proportional to thrust even with a sport jet.

I used to do USAC competitions and there was a point of diminished returns, same here ... Fluid Dynamics no doubt rhymes if not repeats.

66:1 is minimum

Last edited by erh7771; 06-12-2019 at 10:20 PM.
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Old 06-13-2019, 04:16 AM
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[QUOTE)

i do agree on the overpowered, other than more vertical with scale Jets what is the gain?

Top speed is no where near proportional to thrust even with a sport jet.

[/QUOTE]

Its not proportional on any jet !
.
So just for fun during coffee time :

Drag,and therefore required thrust, varies as the SQUARE of the speed ( IAS). D = cd x 1/2 rho V 2)

So, doubling thrust gives a 1.4 increase in speed . ( 1.4 x 1.4 = 2)

To double speed needs a 4 x thrust increase. ( 2 squared = 4 ) .

Exhaust velocity can have a significant, beneficial, effect on any jet where high speed is sought, as the higher the exhaust velocity the slower the thrust decay with speed. That is before ram rise intake recovery begins which can further improve matters !

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Old 06-13-2019, 04:35 AM
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Originally Posted by erh7771 View Post
Nice!!!!

Is there a vid of this beast flying your routine?

i do agree on the overpowered, other than more vertical with scale Jets what is the gain?

Top speed is no where near proportional to thrust even with a sport jet.

I used to do USAC competitions and there was a point of diminished returns, same here ... Fluid Dynamics no doubt rhymes if not repeats.

66:1 is minimum
Here is some video on one of my first flights on it. I was messing around not flying my scale routine though. Someone had video at Topgun of it but I cant find it. You will see the bomb drop on the first pass. My topgun scale routine I flew it slower and had the gyro turned up allot higher. This video was done in allot of wind also.

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Old 06-13-2019, 06:27 AM
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I just finished a scale jet weighing in at 33lbs dry. She flys great on a P120sx.
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Old 06-14-2019, 01:50 PM
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Originally Posted by gunradd View Post
Here is some video on one of my first flights on it. I was messing around not flying my scale routine though. Someone had video at Topgun of it but I cant find it. You will see the bomb drop on the first pass. My topgun scale routine I flew it slower and had the gyro turned up allot higher. This video was done in allot of wind also.

https://youtu.be/1zhl4c0rCv0
nice routine

​​​​​​No lack of power there
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