A matter of thrust
 

Some of us use a “fish scale” attached to the rear of the plane to calculate the thrust of our planes while accelerating the engine.

How accurate can this method could be?

All planes have some weight and maybe we should add some friction to the final equation.

Besides the correct equation I want to know if the fish scale method is some kind of trusty.

If so, how do we consider a good power-to-weight match? (1 to 1?)

The wing loading affects the flight of our models too, should we have to consider it?

I have finished a dynaflite SE5a kit which is around 12 pounds the engine is a Magnum 4 stroker 1.20 size using a top flite 15 x 6 prop. (Omega fuel 15%)
This engine is tacking 9150 rpm and the fish scale reads 7 pounds.

Any ideas?

Thanks

Alfred.

RE: A matter of thrust
 

The fish scale only tells you how hard your engine/prop combo is pulling in a static condition. The conditions under which our planes fly are dynamic, so that fish scale stuff don't mean much unless you fly 3D. Most of us just make a educated guess and take a handful of props and experiment to find what works best for our combination of plane/engine.

If you want to get scientific, some are using telemitry to get real time readings on engine rpm, aircraft speed, etc.

RE: A matter of thrust
 

I have never been happy with the new style Top Flight props. If you want a wood prop get a Master or a Zinger; I would go with the Master Airscrew for the 4 stroke. I would think though it would have the power to fly fine unless you want to hover. I fly a Top Flight SE5a with an open rocker OS .60 and I bet I dont have as much power to weight as you.

RE: A matter of thrust
 

I had handy that prop. but I looked all over the place and found out that I am out of props. for this engine and my Saitos 100 (which are now resting in my shop) that use the same size (15 x 6 and 14 x 8), I will have to go to the LHS on weekend to buy a wooden master airscrew.

Thanks.

RE: A matter of thrust
 

I have spent toooo much time with mechanical tools till I have got to the Pe Reives
calculator - that is the solution and works Very Well even if you DO NOT have purchased
the engine nor the propeller not the Airplane yet ... - as a good illustration what the
Physics + Mathematics could do for us ...
And if you get some different practical results then it is better re-measure because
that calculator is too good anyway. Download it from
http://www.mvvs.nl/
and on the top you will see 'Propellers' - click there

And then compare with what you measured and let see what do you think about
the comfort too.

Cheers,
Nick;)

RE: A matter of thrust
 

Static thrust is just that - thrust at static condition which is really only relevant when hovering or accelerating from a standstill. As soon as airspeed is involved, the thrust changes. The prop that that gives the highest static thrust will pull you off the starting line fastest and do the best at hovering. If that's important then you're all set. Otherwise fly them and decide empirically which works best, as that is more realistic. With your method you will probably find the prop that will pull you out of low speed antics the best. If that is important then this is good.

MJD

RE: A matter of thrust
 

Thanks Nick.

Fish scale measurements are quite OK to define static thrust, which can then be used to calculate the engine horsepower. Due to engine vibrations, it is NOT needed to calculate friction (same like tapping a barometer to alter reading), but it is needed to calculate plane drag in the prop slipstream. (My calculator calculates HP and uses this to calculate static thrust and thrust when fitted in a plane. Calculations have been verified to be accurate within 5% (low)using wind tunnel tests. This 5% low means non-optimistic power and thrustresults, whichare more than adequate for our use.
Static thrust is a special case in prop calculations, where efficiency per definition is ZERO, and all engine power is converted into a donut shaped vortex flow.
From this one can guestimate (sort of controlled calculate) the performance of the plane at speed.
When I used my calculater to guestimate the prototype Spitfire, my calculated topspeed was off only 4 mph (calculated speed too low)from exact measured plane speed.

PS.
When I calculated the topspeed of the sundowner class, I got 98 mph, which was spot-on with radar gun measurements of the same plane that I used to calculate.

RE: A matter of thrust
 

- completely depends on the desired flight regime of the model. 1:1 is excessive power unless aggressive aerobatics are part of the game plan.

- Well, yeah - bigtime. The wing loading, airfoil, and weight define major characteristics of the aircraft.

- Lift counteracts weight. Lift comes from the wings, and the production of lift results in induced drag. In addition to that you have all the other drag of the airframe. If it flew on engine thrust alone or was required to have some level of vertical performance the 7 pound to 12 pound ratio would matter. The issue is - does the engine deliver sufficient thrust to pull the airplane faster than stall speed in level flight by a comfortable margin. If so then it will fly fine. Chances are you are okay.

If you seek 1:1 thrust:weight on the ground as a rule, you'll be overpowering models like the SE-5 big time.

MJD

RE: A matter of thrust
 

Hi!
First of all you are using too small props for your 1.20 four stroke!
15x8 or better 16x6, is what you should use if you fly at sea-level!
5% nitro is just perfect for all engines except Y&S! 15% nitro is a waste of money!

Then....forget static thrust! It means nothing! It's what happen in the air that counts!!!!
So you try different props until you find one that works good... or ask someone who knows!;)

RE: A matter of thrust
 

Jan, I know for the type of flying you do static thrust means nothing but in the world of 3D it means a lot. (I do many types of flying including pylon racing and yes static thrust pretty much means nothing there) Many of the maneuvers in the 3D world are virtually stopped and instant acceleration is important. Remember we're talking about hovering and harriering etc.

RE: A matter of thrust
 

Nick thanks for your advise, I already downloaded the chart, which I consider a great job of investigation and, Pe Reivers, thanks for your words I feel a lot of respect for your enthusiasm and efforts to help others in this hobby.
I also thank to other posters that are trying to help in this very confusing term of thrust of our model engines.
At these days I have been busy trying to transform my engine test stand into a test stand capable of measuring static thrust (redundant after seeing the Pe Reivers charts.
In this particular case I feel that I am into the comfort zone because of the wing loading, so the model does not need a big power plant and been a world war one replica it is obvious that I am far from practicing 3D type of flights.
What I am really interested to know is that this thrust device constructed with a fisher scale is really worth the time to build it.
In example, I have two Saitos 1.00 size that I have rebuilt and I want to check how they perform and if there is some difference on them, also want to test different props. and check the thrust on them. Finally, I have a Saito 91 that is kind of tired, I want to measure the actual numbers, change the ring and bearings and see the difference.
Of course I can use Pe Reivers charts but messing with those engines is kind of fun too.

Pe Reivers I could not place the prop constant, it was late yesterday and could not stay a lot of time doing the task, I understand that I only have to change the numbers placed in the green cells, is that OK?

I appreciate the help of all of you and expect more info regarding this complicated issue.

Best regards

Alfred.

RE: A matter of thrust
 

Hi Alfred,

To be very honest myself I never really got the confort of exactly(!) repeated performance
of even the same engine due to weather for example etc. It seems that after all that many
of the RCU fellows make the final decision based on sentimental preferences too...;)
In this sence I found also comfort in the Pe's Calculator, it is not sentimental and it is
always giving me the same result, meaning, a very good initial solution which proves
very frequently to be achieveable in practice. Also I find pleasure experiementing with
engines and thus we should maybe make clear difference between the more-pilots
and the more-constructors members of RCU, for example based on the amount of
fuel burned resp. in the Air or on the Ground. You might find some more answers
how to measure in the books of
http://www3.towerhobbies.com/cgi-bin...?&I=LXL802&P=0
and
http://www3.towerhobbies.com/cgi-bin...?&I=LXLGR8&P=0
but reading them as well many pulbications by Cl.Lee in Magazines
might "ground" you for quite a while ... the engines are so interesting
thing and surely many of us like experimenting... Last summer I had
about 4:1 fuel amounts burned on ground:air ratio ... even beside the
heavy use of the Pe's calculator. Both, Calculate and Measure seems
to be nicely complimentary while trying to learn more about my engines.

Cheers,
Nick

RE: A matter of thrust
 

Alfred,
The green cells are for input. The other cells cannot be selected.
You can change altitude and/or air density, even to the extend that the air is as heavy as water for ship's propellers.
The calculations are based on the propeller as airbrake. There may be slight differences in propellers in their drag/lift ratio, but these are quite small and do not justify the use of more complex calculations that would require more complex inputs as well.

A thrust measurement would add to the data of a good rpm meter, but like many will tell you, static thrust is not very important, and will suffer when using high-pitched props (P/D >1), because the prop will stall when operated static. Yet it will function very well as air brake.

For engine work, a good rpm meter and a thrusted (pun intended)set of test propellers will tell you all you need to know about engine performance, and power/rpm curve generation. Thrust is a variable that deminishes when the plane speeds up, until thrust equals plane resistance attop speed in calm air and horizontal flight. Plane drag is again depending on airfoil, plane drag, and wing load.
An aeronautical engineer can tell you all about that. Yet, rule of thumb calculations which cram all variables into a single constant go a long way, and are good enough for 98% of our model flight applications. After all, we do not want to calculate fuel consumption across the atlantic to the last ounce.

Have fun.

RE: A matter of thrust
 

The OP was talking about an SE5, so I don't think hovering and harriering have anything to do with his question. For 3D flying the static thrust measurement has significance. The optimum props for that of flying generally work really well at flying around slowly too, just like a car drives around great all day in first or second gear so long as you aren't interested in cruise or higher airspeeds. Chances are a prop that works well at that is going to also be a decent choice for a low wing loading draggy bipe as well. So static thrust isn't the worst quick-look data for that or similar iarcraft, but is not of any universal value. Imagine the fits pattern and pylon guys would have in the air if they based prop choices on static thrust. Even medium to higher wing loading scale or sport aircraft will noticably suffer compared to choices made empiracally and/or with the normal airspeeds in mind. I'm not trashing the usefulness of static thrust data in the right application, I am just trying to stress that a lot of people give too much importance to the issue when valuable thought energy might be of more use elsewhere.

MJD

RE: A matter of thrust
 

Pe Reivers, then I am using the chart in the right way, thanks.

I do not want to be redundant but just want to make sure that I am in the right path.

I have many other engines, generally I play with them in a test stand and try to get the best by changing props. measuring rpm and checking the heat.

I have a couple of Saito 1.00 which are using the same sizes of props. and identical as those that my Magnum 1.2 uses and practically with the same rpm numbers, then I shall assume that the thrust on those saitos and magnum is basically the same?

Thanks.

Alfred.

RE: A matter of thrust
 

You can safely assume that the engines will perform the same way.

Having said that, you should never extend assumptions beyond the window of observation. (rule # 1 in statistics)</p>

RE: A matter of thrust
 

Pe Reivers thanks for your help and congratulations for that great work in your chart, so easy to use and it is perceptible that a lot of work has been invested on it.
Nick, thanks for your suggestions, as you can see I have a lot of fun working with my engines and I like to read a lot about them. I also wrote some letters to Clarence Lee some years ago and surprisingly one of my questions was published at the Airmodel News magazine, I am not exactly an engine expert but having fun with them and understanding its behavior is enough for me.

Blessings

Alfred.

RE: A matter of thrust
 

Here’s another wrench in the works…..
Some “square” props (diameter & pitch numbers same) or over square props (pitch number greater than diameter) tend to partially stall or are in turbulence when ran statically. This means the prop is slipping or not realy “hooking up” until the plane is in flight. This can lead to false or misleadingly high static horse power numbers from high tach readings. For example an APC 10X10 is very prone to this and has gotten me on a few engines.

This can be recognized when you fly a plane at speed and you actually hear the engine slow down compared to the static run. If the plane is actually clean enough (low aerodynamic drag) and there is enough power available it may continue to unload and you will hear the pitch rise again after that un-stalling or “hooking up” sag point.`

RE: A matter of thrust
 

All things being equal, the props aren't really stalling. They have run out of the air they were designed for. Give the prop disc more airflow, and they become more efficient.

RE: A matter of thrust
 

Same thing different way of saying it. The phenomenon can be visualized, for anyone that needs an analogy otherwise just ignore me, by imagining taking off from a slippery intersection.. your tires slip, the engine revs up with nly a partial load, the vehicle accelerates poorly due to lack of traction, then as the vehicle gains speed the tires regain bite - the engine now loads up back to the rpm where you'd expect it to be. And as FF says, you may then later hear the engine get to the original point or beyond if the power is available to actually pull that speed in the airframe.

MJD

RE: A matter of thrust
 

Perhaps, and I think the Pe Reives calculator takes this into account for most props. My point was simply; don't celibate high numbers with the calculator with certain props.

All things considered, the Pe Reives calculator is one of the best tools I've used for this hobby.

Whatever you say just don't call it cavitating, that'll really stir the pot. :D

RE: A matter of thrust
 

A good analogy.

RE: A matter of thrust
 

It is good analogy but not quite if a resonance pipe is used, then the things get "seduced"...;)

RE: A matter of thrust
 

MJD,

I would agree that the analogy of the tires running out of grip would work. However, the engines react differently when getting enough grip as opposed to getting enough air. Model engines speed up as they unload. How much do they speed up? There is no hard and fast rule. Most people I've heard from give a headroom of about 1000 RPM.

RE: A matter of thrust
 

BLW,

I just question here the role of the tuned pipe for "draging" the engine toward the "optimum" RPM
determined by the pipe's parameters. Is this what you mean or ?

Nick