caiorcu

These Chart (Thrust vs. 2 and 4 stroke engines), are from a search of
this website and are actual "fishing scale" measurements.

Jack Hyde

My Feedback: (1)

I is surprising how well the trust correlates with only displacement. It provides a quick way to estimate how large an engine is needed for a various planes.
Other rules of thumb:
10% nitro is equivalent to 10% more displacement (for engines that are nitro lovers)
YS engines act like they are 10 - 15% larger than they are

There are exceptions to all of these but they might get you into the right area when selecting an engine/plane combination.

BillS

Yesterday thrust was measured on a 50-glow engine at 6# 6oz and a 25cc gas engine at 7# 6oz. The small difference was very surprising. The gas engine is four times larger than the glow engine. There is more going on with thrust than I understand.

Bill

Jack Hyde

My Feedback: (1)

The thrust from your nitro engine is right on the graph prediction. Your gas engine is pretty feeble. Most fliers report 12 to 15 lbs thrust from 25cc gas engines. 25 cc is 1.6 cubic inches. The chart shows about 18lbs for a 1.6 ci 2 stroke on nitro. Apparently typical gas engines produce about 70 to 80% as much thrust as typical nitro engines of the same displacement. You reported less than 50% of what the graph shows.

BillS

Jack,

Yes the gas engine seems to be way down on thrust but I haven’t found enough other data to know what numbers are good or bad. Since 25cc engines are usually flown on airplanes up to 15# it would seem likely that the engine is not operating in the power band.

Some time ago in another thread you asked about thrust numbers. What engine sizes are of the most interest? If you are still interested I will spend a day at the field accumulating data.

Attached are pictures of my bench test contraption. It’s a little crude but seems to work.

Bill

Jack Hyde

My Feedback: (1)

I am interested in "little" stuff - props 12 to 16" , peak static thrust 6 to 16 lbs. I would like to see Zinger, APC, and MAS black at the same rpm. I fly slow 3d planes that weigh 4 to 6 lbs, use 4 to 6" pitch. I am also interested in 10% vs 30% or so nitro in the same engine. Most of the info on the forums is 90% emotional. Eg "my engine was a dog on 15% nitro but on 20% its a rocket".

I have also been looking at the smaller gas engines. I like the idea of gas but don't see hauling, storing or paying for a 25 lb plane in my future. Maybe a 10 lb.

BillS

Agreed and I’ll get some engineering style data, conservative and correct.

Bill

Jack Hyde

My Feedback: (1)

I will be watching for your results.

rmh

Looking at your pictures of gas engine on test stand -it is easy to see why your gas engine has low thrust.
The type exhaust system you are using cuts available power 30-50 % easily .
A good exhaust setup on a 1.6 gasser puts out the 18 lbs thrust you are talking about on glow setups you run.
Alky and nitro will always produce more power but always will use twice as much fuel
YS engines are all supercharged and typically have an increase of about 6 psi in the intake duct ( from crankcase to head)--over non supercharged fourstroke model engines -which supply fuel charge from the carb to the head - no double pumping as in the YS.
A10 lb plane with excellent performance is available using a piped gas engine -and also is VERY quiet My Funtana is 9.5 lbs ready to fly and has instant unlimited vertical capability from a dead stop -

laryboy

My Feedback: (2)

what is your setup that is very quite

BillS

Sounds like a backhand method of saying that tuned pipes produce more power.

Almost all modelers know a tuned pipe will enhance the power of a two-stroke engine. Most will choose a conventional muffler anyway. Incidentally the port timing is far and away too mild to pipe the engine.

I intend to run gasoline by choice.

Likely there is a very fundamental design problem with the ‘bench test contraption.’ Surprised the flaw was not noticed.

Bill

Dick,

I forgot to ask for your thrust, RPM and prop data.

rmh

18 lbs thrust prop Skorepa 18x6 pipe Skorepa OR MVVS/ Evolution 26 running about 8800 -in air close to 10.000 in level flight
the port timing is fine on any gasoline engine to see increase with a pipe - Most model engine pipe information -was related to speed setups and that information is about worthless for our use - 150-155 exhaust window timing is very acceptable . A proper pipe setup is very docile thru entire rpm band.
Pipes do increase power (they increase torque) and typically at a higher rpm
the in cowl devices reduce power - -
you can prove this by trying open stack tests . Some guys like the noise of in cowl devices . I prefer quiet setups.

BillS

Thanks for the thrust and prop and RPM information. From earlier comments one might guess you are running glow fuel with some nitro.

My earlier ‘bench test contraption’ presented a deceiving but large effective frontal area to the engine and the test results were distorted. After attending to the frontal area the numbers are up 50% (9 to 11 #) with no other changes. It is cold today and the test rig felt dangerous so only a few minutes of rough testing was done.

If you have a cylinder off anytime soon a pencil trace of the exhaust port would be very interesting. What is the exhaust duration on the engine you are using?

How are you measuring in air RPM?

Bill

Retested one prop after securing everything and the thrust numbers are:
Old test - 7500 RPM MA classic – 7# 6oz thrust
Today’s test - 7400 RPM MA classic – 10# 8oz to 11# thrust

rmh

Most ignition systems we use, have an internal memory which records highest rpm (part of the Evolution26 ignition)
I make certain I am flying level - then let engine unload for a few seconds then land idle down and kill engin - then read the LED with a tach .
as for timing - I am not certain what this particular model has - really it is not the critical factor in using tuned systems . it may be 160 - never checked it .
My ZDZ's have a similar setup but is a direct readout thru a plug in tach

BillS

Jack,

I am finding that thrust measurement is sensitive to the method used. The scale should be tied to something stationary and not held. Frontal area of the airplane makes a difference and can sometimes be significant. Within any given run a 5% plus/minus deviation might be seen on the scale although an RPM deviation is not observed. By the time I’m finished with the little gas engine data collection might be more accurate.

Nevertheless a digital fish scale is/was a valuable measuring instrument. I also use an inductive tack on gas engines.

You are very correct about the vast amount of emotional data.

Bill

Jack Hyde

My Feedback: (1)

The prop wash blowing on the large wall of wood behind the motor mount produces a force that works opposite to engine thrust and gives lower force at the scale than if the wood were not there.
I have measured thrust by just tying the tail of a plane to a stake with a fish scale between the plane and the stake. The wheels offer a little resistance that is a small source of error.

BillS

Intellectually I knew the mount would make a difference but never would have guessed that it would make a 50% difference. Learned something important.

Based on my observation it would not be surprising to see a 10% thrust difference when moving an engine from one airplane to another.
On the airframe is a good method when the engine is on the airplane. The engine is for a 35-pound twin and I prefer to work out issues before mounting in the airplane. If engine issues are unsolvable different design decisions will be made.

Bill

SNIGGEMANN

My Feedback: (10)

I also had the same experience with test stand design. I have a converted 25cc Homelite that was only pulling about 7 lbs on my stand that had considerable frontal area. When I re-built the stand to have a very small frontal area the thrust went up to 11 lbs with a 16X8 APC.

BillS

Thanks for the verification. By the way I got a few more RPM with a ‘K’ series prop than the APC prop.

These were older test numbers and now the ‘K’ series is up to 8000
7300 with 16x8 MA old style prop
7800 MA series K 16x8
7700/7800 APC competition prop 16x8


Tomorrow I am going to test a 72 glow on a 4* with wing off and wing on. I am betting there will be a significant difference in thrust.

Bill

BillS

Jack,

Some conservative test data:
Irvine 72
Looks like it has a stock muffler with exhaust extension.
Prop: 13 x 8 MA classic
RPM: 9200/9300 RPM
Thrust: 8# 9oz
Fuel: 20% nitro

The engine is on a 4* 60, which does not have a cowl. No attempt was made to retune for maximum RPM since the airframe drag is big and airspeed changes little from half to full throttle. The thrust varied between 8# 6oz to 8# 13oz and settled in at 8# 9oz.

Thrust number correlates with your chart.

Bill

Jack Hyde

My Feedback: (1)

I would think a 13x8 is way too much load for a .72 . The port timing on a typical 2 stroke that size gives best power at higher RPMs , like 12000 to 13000. OS 61's turn a 12x6 or 13x4 about 12k for about 8 lbs thrust. Your results look good , but I would use a smaller load on that engine. If you like an 8 pich use a 12x8, rev higher , go faster. I am a put aroung 3d flier and would try around 13x4 , 14x4 13x6. I like what I hear about Irvines, just can't buy them at the usual sources. I am tempted to go to Just Engines for one. Do you like your .72?

BillS

The engine is extremely well made and handles very easily. I like the engine.

The engine came on an airplane with a 12 x 7 prop, which appeared to turn too many RPM. I don’t remember what it turned but it seemed like too much.

Prop recommendations were found somewhere for the engine and they seemed bizarre. 11x 10, 11 x12, 12 x 9, 12 x11, 13 x 9, 14 x 8, 15 x 6
I tried a 14 x 8 MA and got 8100/8200 but thought the engine was too heavily loaded.

The airplane is overpowered and the speed is drag limited so I didn’t experiment much.

Bill