ST G90. static thrust?
 

hello all
all you G90 owners help me out with some statistics on the amount of thrust that your engine is giving out?
thanks
regards
James

RE: ST G90. static thrust?
 

James,


This is one of the easiest things to measure.

Just use a spring-draw scale, like the type used to weigh laundry, hook it securely to the tail of your plane and hook the other end securely to a fence-post, or to another stationary upright.

Get the help of someone to read the scale, with the engine running full-bore. It may fluctuate, so the average is the right number.


It will completely convince you that for this, all thrust calculators are worthless (but not for prop-size-RPM-HP calculations. For this some of them are great).

RE: ST G90. static thrust?
 

The guys in the Tach Readings forum on RCU report peak rpms of 10,500 to 12,000 for their G90's for 13x6 props. This gives static thrust 8 to 9 lbs for most props.
I used the the reported rpms and a thrust vs rpm map I have developed for a variety of props. I plotted thrust vs rpm using software " Prop Select " and plotted as much measured data as I could find on the map to verify or correct the curves. I have found Prop Select predicts thrust well enough for me. I agree thrust is easy to measure. Much of the data on my maps was measured by me using a fish scale that I calibrated with known dead weights. Most people don't need to know thrust to a very high degree of accuracy as they use it to predict how an engine/prop/plane combination is going to perform which can only be done approximately.

RE: ST G90. static thrust?
 

I have some data that may help:

APC 14x6 @ 10300 rpm = 9.5 lb.
APC 15x4W @ 10000 rpm = 10 lb.

I think the G-90 can do slightly better than these 2 figures.

RE: ST G90. static thrust?
 

This engine responds good to a tuned pipe. My ST90 on a tuned pipe turns a APC 15x6@10000, APC 15x4W@12000 and APC 13x8@12000rpm. I am currently running a Master Airscrew 16x7 to keep the rpm & noise down;)

RE: ST G90. static thrust?
 

The 15x6 and 13x8 should be much closer in RPM -- the load in not that far off.

RE: ST G90. static thrust?
 

My digital 20# fish scale tells all. When comparing prop changes.

I can do verticals without ever leaving the ground, with it.

A tachometer and a fish scale. Simple.

RE: ST G90. static thrust?
 

My digital 20# fish scale tells all. When comparing prop changes.

I can do verticals without ever leaving the ground, with it.

A tachometer and a fish scale. Simple.

DO NOT tie the scale to any of the tail surfaces!! Use the wing in a harness so it does not break loose on the biggies. Run the harness lines over the tails to prevent it from lifting up and breaking a prop. Use a steel stake with a ringed eye.

RE: ST G90. static thrust?
 

STG,


This is not accurate.

Within the prop sizes most of us use, the load (and hence, RPM) is kept about the same, when 1" in diameter is equivalent to 2" in pitch.

This means a 13x8 prop would be spun by an engine at about the same RPM, as a 14x6 prop of the same design.

APC props I have tested exhibited a difference no larger than 100 RPM (to the 13x8's advantage), on an MVVS .91.


This is also true of 12x4, 11x6 and 10x8 APC props (on an MVVS .49).

Differences may change a bit more, since high pitch props tend to cool the engine better than low pitch props that have the same load.


And as to thrust; even though the HP output is the same, at identical RPM levels, the static thrust is greater with large diameter, low pitch props, than with small diameter, high pitch props.


This makes one wonder why some thrust calculators use HP x 6, to calculate static thrust...

RE: ST G90. static thrust?
 

STG,
As Dar has described it the the APC 13x8 is more equal to an APC 14x5 or 14x6.