Thrust Testing / Prop Pitch

[Mike484]

There's a very interesting discussion going on about thrust testing and prop pitch on the First Titanium thread. Since I think we need to respect that thread and not hijack it, I have started this thread so it can be discussed.

I see a lot of people here give a lot of credence to static thrust testing for possible representation of prop performance. It actually means very little in reality. No one flies statically with no airspeed. As soon as you pass 20 mph, that number is meaningless actually. A proper propeller designed with its chord and twist optimized for a certain speed range will not perform well at all at static condition. Just FYI. I used to do these tests all the time with great care, till finally I did some digging around to understand what is really happening. After that I throw these results right in the garbage can where they really belong.

Very true Abid, not to mention, as the prop travels faster thru the air in flight... it makes less and less thrust until the prop is hitting the airflow with no angle of attack and the machine will not go any faster. To me, the static thrust is only a good indicator of whether the machine has hardly any thrust, a fair amount, quit a bit or a hell of a lot.... the best indication would be a race to an altitude, such as 5000' with the same weight in each machine.... but then, the drag of each machine is different too....:yo:

Is there an on-ground test that you feel would offer a better comparison and allow people to get a side by side evaluation of two props?

When trying to find out the best propeller for a particular aircraft I feel rate of climb and fuel consumption have more value than a static thrust test.

I have never been that excited about how fast I am able to accelerate for a stop.

I have also found that some propellers work over a wider speed range and I feel there is value in that.

Yes. Do the same test on the same aircraft in very similar weather conditions at the same weight. Do all the tests using a MAP sensor and OAT sensor to calculate power percentage, instead of using RPM. Do rate of climb, speed range for straight and level and takeoff ground roll tests.

Look at
http://www.stolspeed.com/id/62

for instance.
I have tested many props on Searey carefully including Duc (French) prop models (2 different ones), Sensenich, Sterna, of course Warp drive, etc. That Kool prop listed in above report is actually Lugo prop from Ukraine and is extremely similar or the same as Aero prop (also from Ukraine and from the same city) that I have used for thousands of hours. They are good but not as good as Sterna and GA prop I am using right now. Also, Ukraine is having issues in that area and supply is becoming problematic unfortunately.

I think the static thrust test became popular for gyros in the era of the single-place stick machines, when thrust at low speed was a much bigger priority than thrust as 90 knots.

All the new tandems are aimed at the cross-country crowd, and they all take some runway to get off the ground, but cruise efficiently.

It is my observation that best rate of climb for most gyroplanes is between 55 and 65 miles per hour Paul.

A static thrust test doesn’t tell much about how the propeller will work at best rate of climb.

A static thrust test doesn’t tell much about how the propeller will perform at cruise of even 70 miles per hour.

In my opinion the thrust test became popular because it was easy and a way to compare gyroplanes that were otherwise not comparable.

Testing an aircraft takes time and patience and is what is supposed to go on in the phase one testing of an experimental.

I spend very little time accelerating a gyroplane from zero air speed.

Vance, many small gyros powered by Rotax two-strokes, especially ultralights, are so draggy they can only dream of such speeds. The last single-place I flew weighed 340 pounds, was powered by a Rotax 503, had no windscreen or pod, and the minimum power required speed, Vy and best glide were all 37 MPH. On a machine like that at elevations like ours, the terms "cruise" and "static" begin to converge!

That's not what Fara is saying. The thrust produced at 0 airspeed at full throttle ( while tethered to the ground) is not comparable to the thrust with airspeed. An engine and prop combination can be optimised for a static trust test but not create as much thrust as a combination optimised for a certain cruise speed which may not register as well on a static thrust test.

The pitch of the blades, the aerodynamics of the aircraft, pusher or tractor and the way the engine delivers he power are all variables that can be changed to get the maximum for what you are trying to achieve.

I am not sure how you should determine how efficient ( iow thrust) a engine and prop combination is because it should be measured at a set airspeed (not static) and therefore a better measure would be to determine it mathematically from empirical flight testing

Yes they are a measure of which prop produces more static thrust for sure. What I am saying is that that measure is a bit useless and unfair if you are looking for a good prop for say your MTO. Even though gyroplanes are slow they are not flying at 30 to 40 mph

I find the best thing a static thrust test is good for, is to see that you have the prop pitched correctly, so the engine doesn't over rev, or struggle to reach the revs you want. However, if you want a motor, say a 582 to max out at 6800 rpm, you would have to set the prop while the machine is tied or on the trailer, stationary, to about 6400 rpm.... if you set it for 6800 on the trailer, it will over rev in flight at wot.

The thing is that people can cheat a static thrust (pull measurement) test by setting the pitch for best static thrust.... but that wouldn't be best in flight...then re set the pitch of the prop again before flying.:yo:

But even using static test to set the prop pitch doesnt work that well either as it doesnt account for the forward movement through the air and the subsequent change of AoA of the prop blade so therefore pitching your prop based on a static stationary test is only optimising you thrust for being stationary.

To pitch your prop and/or optimise the thrust (at given airspeed) require empirical flight testing so that the optimal pitch can be calculated.

Static tests are not necessarily the best way to compare fixed pitch props because the prop pitched for maximum static thrust will have a course pitch compared to the one optimised at 60 knots airspeed.

A static test will say the former has more thrust than the latter but it will be reversed at 60 knots, it just depends where you want the engine/prop combination to be the most efficient.

 

[Mike484]

Whenever we are testing a new built or modified (engine or prop change), we always do a static thrust test to set the pitch for the max RPM and thrust. This is before the first flight.

Sounds like we are doing this wrong. What is the proper way to set your pitch?

 

[JAL]

Whenever we are testing a new built or modified (engine or prop change), we always do a static thrust test to set the pitch for the max RPM and thrust. This is before the first flight.

Sounds like we are doing this wrong. What is the proper way to set your pitch?

I dont think thats wrong at all, it is the easiest way to pitch your prop. Setting pitch for max revs at static gives you the best acceleration from a standing start but once you start moving through the air the prop starts to lose efficiency faster you go.

It was just a comment that the static tests are not necessary a good way of comparing thrust as it depends on the what purpose the prop is pitched, either for take-off, cruise or going flat out (or for the static test itself).

Its just what you have to accept with fixed pitch prop, at a given pitch and rpm the prop will only produce maximum thrust at one airspeed, either side of this airspeed the prop loses efficiency.

 

[Vance]

A matter of personal taste.

A matter of personal taste.

Wrong seems too strong a term to me.

For me one of the goals is to not over rev at maximum speed.

If I have a properly pitched prop it will just reach maximum rpm in a slight descent.

The engine will also get into the power band on climb out when I have a propeller pitched to my taste.

I would recommend trying different pitch setting and document how it affects the performance.

Things like time to climb, engine rpm at VNE and a pleasant cruise rpm.

I find the settings I like the best and try to understand the compromises.

It is not necessarily about what appear to be the best performance numbers

The Lycoming IO-320 on The Predator has a nice wide power band so being a little over pitched doesn’t degrade climb performance too much and makes for a more relaxed cruise and lower fuel consumption.

 

[Joe Pires]

From what I read, most of us will not be able to do a meaningful comparison of different props. I for one am unlikely to purchase multiple props in order to do flight testing. Which means we are back to anecdotal information or hoping someone else happens to have a very similar machine and very comparable all up numbers and chooses to do the tests.

 

[Doug Riley]

Static testing isn't too bad a way to compare props' relative climb performance (analogous to low, stump-pulling gear in a car). Gyros climb best at rather slow airspeeds, so the test at zero airspeed isn't that far off. The RPM should not reach redline static, for the reason that Vance mentioned.

For a "yank and bank" gyro, you want a prop that's optimized for climb (that is, one that redlines the engine at, or just above, best rate-of-climb airspeed). Ditto for a low-powered gyro such a a Gyrobee: adequate climb rate becomes a matter of safety; a fast, low-RPM cruise is a lower priority.

The higher your cruise speed and the more you wish to optimize for cruise, though, the less useful a static test is. A cruise prop for a relatively fast aircraft is apt to have so much pitch that it's partly stalled static. In that case, it'll give wimpy thrust numbers in a static test. You can feel it un-stall and "come alive" as you pick up airspeed in actual flight.

With a cruise prop, the goal is to avoid SO much pitch that climb really suffers, and also to avoid "lugging" the engine at either climb or cruise. The formal way to test for "lugging" is to rig a manifold-pressure guage.

War story: my old Air Command 447 (bought used) came with an aftermarket Catto prop that had, apparently, way more pitch than the factory prop. Max RPM at a 45 mph climb was about 6200, and the climb rate was lousy in summer weather. Over 70, though, the thing just took off. Part 103, not.

Unfortunately, the HTL, no-HS configuration made the aircraft so nose-heavy and unstable at 70-80 that it was unpleasant, and possibly dangerous, to fly at those speeds in any turbulence.

 

[Steve_UK]

""""We can now report that the in-flight pitch adjustable IVOPROP propeller has been fitted to 5 Calidus and owner’s feedback has been extremely positive, giving even better take-off performance than expected. So much so, that all new Calidus on order are being delivered with them fitted. The fine pitch adjustment gives superbly short take off run, whilst coarsening it in flight gives a lower engine rpm, faster cruise, and more comfort."""


but only if you're allowed to have one.

 

[Georgi]

Just wonder ( from your many experiences ) about the best prop (size)/pitch combination for the single sit gyro with Rotax 503 and total flight weight around 500 lb. And ,probably, for take offs + climes ,because I can cruise ( legally ) 63mph. And it is not comfortable for me to fly faster in an open gyro.
I have Rotax 503 DISingleCarb , two blades 60"/30 wooden Culver prop, 23'7"
SportRotor.
I noticed many singles are using three blades props. Why not two blades?
Thank you. Georgi.

 

[C. Beaty]

I noticed many singles are using three blades props. Why not two blades?
Thank you. Georgi.

It all depends upon how much power you have, Georgi.

Some airplanes with turbine engines have props with 10 blades.

Two blades are more than enough to absorb the power of a 503.

But if you want to fly sideways, 3-blade props will shake less.

 

[Georgi]

Aha,Chuck.
So , these guys can really dance around.

 

[C. Beaty]

Georgi, if you installed a teeter hinge on your 2-blade prop, you could fly sideways without rattling your eyeballs.