- Sep 1, 2008
- Houston, Texas
- RAF 2000 w/Horizontal stabilizer, KB3 w/tall tail
- Total Flight Time
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.
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.