Thrust-To-TOW Ratio

[Vance]

When discussing the use of real-world thrust number actuals as a comparative index across the many types of gyrocopters one might consider, instead of just reported HP claims and ratings, some great info is contained in this article concerning power loss through shafts and transfer cases.

Drivetrain Power Loss - The 15% "Rule"- Modified Magazine

Understanding drivetrain power loss, or how the clutch, gearbox, diff, shafts, et al leach away horsepower and whether the “15% rule” is indeed universal

www.superstreetonline.com

It is easy to understand that a direct-coupled Lycoming will transfer significantly more power to the prop, and thus produce more thrust per rated engine HP, than, say, a Yamaha Apex with one of Teal Jenkins' three-gear tractor FW gearboxes mounted on it; or that a Mohawk Silent Drive Hy-Vo chain drive is far more efficient than any gearbox at all.

The many configurations of redrives/PSRUs used on aircraft powerplants of all types makes it impossible to nail down a single efficiency number that fits every case.

Therefore to be summarily dismissive of thrust testing as a suitable means of determining power delivery to the prop and predicting aircraft performance is foolish, if not arrogant.

As for me, what I have tested and found personally over many years of work is what I stated prior. Prop efficiency is a mathematical equation that does not lie. It is a straight-forward trig, physics and real-world measurable of either static thrust or true air speed, with the difference being in generating the propulsion of air from a standing-still, steady state vs forward aircraft motion creating an apparent movement of air through the prop before being propelled by it. In the latter case, prop efficiency is a real-world, hard facts number determined by comparing theoretical distance to be moved over a given time period according to prop pitch compared to an actual, tested, measured distance an aircraft moves over a given period of time, or AS, and taking into account airframe drag.

Static thrust tests are lots of fun because of the noise and drama involved.

They are also easy to perform with relatively inexpensive equipment.

In my opinion based on my experience; static thrust is not an accurate representation of horsepower output or particularly useful as a flight performance predictor.

I feel there are simply too many variables

Having watched many static thrust tests it appears to me that how the propeller is pitched may have a very large effect on static thrust measured.

The size of the propeller can also have a large effect on propeller efficiency.

In my opinion the reason people use a propeller speed reduction with all its attendant problems is to be able to use a larger propeller for more efficiency without having the tip speed become a problem.

In my opinion using static thrust combined with dry weight as a way to predict gyroplane performance is both foolish and arrogant.

The Predator regularly climbs out at a steady 1,100 feet per minute when flown solo with a light fuel load (1,100 pounds takeoff weight) and regularly climbs out at a steady 600 feet per minute with two up and full fuel (1,400 pounds takeoff weight.

The static thrust didn’t change and the dry weight didn’t change; only the takeoff weight changed.

Anyone who has flown a two place gyroplane solo and two up is familiar with how takeoff weight affects the performance of a gyroplane.

When I solo someone in a two place we put weight in the passenger seat so that the performance is closer to what the client is used to.

I read lots of big talk about potential performance of a Yamaha powered gyroplane. I have yet to see that promised performance realized.

 

[Martin W.]

So what does the average guy do as far as best prop pitch in flight ? Make small changes and test each time ? Or if it works OK just leave it as is ?
thanks

 

[Vance]

So what does the average guy do as far as best prop pitch in flight ? Make small changes and test each time ? Or if it works OK just leave it as is ?
thanks

Prop pitch depends on your mission, your aircraft and your engine.

In the Predator my propeller is pitched so I am not too close to red line (2,700 rpm) at 90kts.

I have a one of a kind fixed pitch Kato propeller and the first one he did for me give me great climb but I had to pull the power back at speed to keep from exceeding redline.

He made another one for me based on my input and I get a little less climb rate, more relaxed cruise, less nose and slightly less fuel consumption.

The top speed is about the same.

I move the throttle more in unstable air and this gives the aircraft a different feel.

With a ground adjustable propeller it is easy to try different settings and see how you like it.

Your criteria may be climb, speed, reliability, fuel consumption or noise.

Most propeller manufactures will have an idea of the appropriate pitch for an engine and airspeed.

Or find a similar gyroplane and pitch it to what works for him as a starting point.

That may be a good starting point and experiment from there.

The density altitude may make a difference on the pitch that works for you as will the takeoff weight of the gyroplane.

 

[WaspAir]

So what does the average guy do as far as best prop pitch in flight ? Make small changes and test each time ? Or if it works OK just leave it as is ?
thanks

Some of us have constant speed props, in which case you just set the control where you want it when climbing or cruising.

 

[Martin W.]

Some of us have constant speed props, in which case you just set the control where you want it when climbing or cruising.

Yes and thank you .... I am aware of what you fly ... (I have been lurking here for a while) ... . In your case the constant speed (pitch) is self governing right ? In other words the pilot cannot change the pitch in flight.

I recall a gyro with in-flight adjustable prop .... or maybe it was just one of those "new designs" that never came to fruition.

Anyway , here is my point and actual question regarding all the debate over static thrust pitch vs best in flight pitch ..... it would be great if a gyro with flight adjustable pitch would set it for best static thrust .... then fly and report flight performance at that setting .... then in flight reset pitch to ultimate and once on the ground do another static measurement and compare to the first.

I for one would be interested in the results. I know very little about prop pitch .... I have only trained on helicopters

(My guess) a fine pitch at high RPM would give best static thrust .... and courser pitch for speed ... and somewhere in between is the sweet spot for fixed pitch gyros .

martin

 

[Vance]

Yes and thank you .... I am aware of what you fly ... (I have been lurking here for a while) ... . In your case the constant speed (pitch) is self governing right ? In other words the pilot cannot change the pitch in flight.

I recall a gyro with in-flight adjustable prop .... or maybe it was just one of those "new designs" that never came to fruition.

Anyway , here is my point and actual question regarding all the debate over static thrust pitch vs best in flight pitch ..... it would be great if a gyro with flight adjustable pitch would set it for best static thrust .... then fly and report flight performance at that setting .... then in flight reset pitch to ultimate and once on the ground do another static measurement and compare to the first.

I for one would be interested in the results. I know very little about prop pitch .... I have only trained on helicopters

(My guess) a fine pitch at high RPM would give best static thrust .... and courser pitch for speed ... and somewhere in between is the sweet spot for fixed pitch gyros .

martin

Some inflight adjustable props allow you to simply adjust the pitch; some automatically adjust the pitch for a specific pilot selected rpm.

Because a Sport Pilot is not allowed to fly a gyroplane with an inflight adjustable prop they are not popular in the USA.

The Sport Copter II I flew had one that would hold a pilot specified rpm.

It was set for 2,700 rpm and that made the exhaust quite loud and the throttle very responsive.

Your guess is accurate Martin; less pitch for static thrust and more pitch for speed.

Adjusting a propeller is fairly easy so experimenting is easy.

Horsepower is torque times rpm divided by 5252.

If you prop is pitched so it won’t achieve RPM for peak power it won’t do well in a static thrust test.

 

[WaspAir]

Yes and thank you .... I am aware of what you fly ... (I have been lurking here for a while) ... . In your case the constant speed (pitch) is self governing right ? In other words the pilot cannot change the pitch in flight.

It's not like a governed helicopter always at 100%. There's a broad range of rpm from which to choose.
As with a high performance airplane, you pull the prop control until you have the rpm you want, and the prop governor will vary the pitch slightly to maintain that rpm through changes in load (within practical limits). For takeoff, shove it full forward for fine pitch / high rpm to get 2700. To cruise, pull it back to say, 2450 for less noise and vibration at coarser pitch in "high gear". Your power setting depends on the manifold pressure you command with the throttle. The pilot sets the average pitch as desired in flight and the governor continuously fine-tunes it to hold rpm where you put it, subject to changing your mind through the phases of flight.

 

[NoWingsAttached]

A prop efficiency of 85-95% is not realistic. It's usually closer to 60%. Please see this paper: https://engineering.purdue.edu/~andrisan/Courses/AAE490A_S2010/Buffer/AIAA-46372-872.pdf

My friend, I believe either myself or you may have misinterpreted the paper. I did not labor through the entire article, but scanned it for salient information. Perhaps someone else who has read it thouroghly will have something else to say, but from what I gather the given 62% propulsive efficiency number arrived at in the excellent paper is what was measured with a particular tractor Luscombe and which includes total airframe drag, etc.

You can install the same engine and propeller on a pusher aircraft and get a totally different number. Does that mean the prop itself is any more, or less efficient on either application? The prop certainly hasn't changed, so why should its efficiency, as a stand-alone quantifiable force, be said to have changed? It should not. The prop efficiency remains unchanged, regardless of application. It is the aircraft that changes the dynamic with its inherent drag.

Prop efficiency taken alone is what I was referring to. In a tractor test like the one in the paper, the airframe interferes with propulsion in a much more and very different way than on a pusher gyroplane which has literally nothing other than the tail empennage in the prop wash.

Prop efficiency in our world is rather simple and straight-forward: theoretical distance a propeller at a given pitch will screw through the air vs the actual distance traveled, which we can measure with indicated air speed. Take the angle of the prop blade, figure out how far it would travel froward through the air per rotation with no slip at a given rotational speed, and compare that to how fast it actually moves you through the air at a known propeller RPM. That is the efficiency, of course.

In a static thrust test, airframe drag is not part of the test and only the prop efficiency and HP-to-the-prop are factors in the resultant. However, drag as a factor in flight testing is somewhat reduced in the analysis due to the acceleration of the air particles from standstill in a static thrust test compared to apparent speed of air particles through the prop while in flight. (Which also explains the ability of the engine to turn 500 Engine RPM, or ERPM, faster at maximum (WOT) in flight relative to static testing conditions - why we pitch a prop to turn no more than 500 ERPM below red line WOT while tuning on the ground).

When someone claims that static thrust testing has no relevance to actual in-flight performance, they are likely referring to the effects of airframe drag and the fact that one type of rotor may provide better lift performance over another. These factors are difficult to pin down, and not nearly so easily quantifiable as the two basics dealt with in this thread: Dry weight + 225 lbs = nominal TOW, taken together with reproducable. measured static thrust.

Inarguably, these two basic quantities are the easiest to determine and the most reliable in every case regardless of gyrocopter make and manufacture.

As mentioned, however, airframe drag (including the rotor disk) is a quantity which can be measured, and these days now for free, with smart phone G-force apps.

Everyone I know recognizes the general, stand-alone, prop efficiency number to be 90-95% for most propeller designs under the best circumstances - until a certain prop tip speed is reached, whereupon the laminar flow over the blade starts to separate and the system loses efficiency. 80% efficiency is considered to be the lowest acceptable number.

For a prop diameter of 68" I see excellent efficiency up to 2850 - 2950 PRPM, and somewhere around 3000 PRPM efficiency starts falling off measurably. Anything over 3100 is marginal and at 3200 PRPM it all becomes wasted fuel and engine wear. At a little over 3500 PRPM we reach tip mach speed shock wave construction and the whole thing goes to hell in a handbasket: complete loss of laminar flow and what is akin to cavitation. Bad juju. I know of some gyro pilots who have experienced this in flight. They reported a sphincter adjustment was required upon landing.

 

[NoWingsAttached]

I don't entirely follow the significance of expressing this ratio as an angle. An L/D ratio can be converted to an equivalent glide angle, and that gives some additional insight into how it might fly (e.g., a 19:1 L/D is about a 3 degree approach angle), but the angle computed here doesn't appear to me to carry a comparable meaning, adding anything over and above the information of the bare ratio itself.
For an F-15 fighter with 58,320 lb thrust and empty weight of 37,500 lb, the arctangent of that 1.555:1 ratio is 57.3 degrees. Does that tell us something that 1.555:1 does not, or provide further insight about its flight? Maybe I missed something.

I was attempting to arrive at something we could imagine in a relative way as a climb angle.

A ZeCA of 45°, ratio of thrust to weight is 1:1, means that you can hang on the prop alone, without any lift from the rotor. Anything over 45° ZeCA should be able to overcome gravity and climb on the prop thrust alone and go vertical.

The jet with a 57° ZeCA can and does climb vertically, very easily, and so fits the ZeCA model. What is its best climb angle? Might be what ZeCA predicts. Maybe I should rename the model BCAP, for Best Predicted Climb Angle.

Perhaps I could also add a column for simple Thrust: TOW ratio.

 

[WaspAir]

An aircraft capable of a sustained vertical climb (such as the F-15, my Bell 47, or a hot air balloon) has a best climb angle of ninety degrees and its best predicted climb angle should also be ninety.

 

[NoWingsAttached]

This is absolutely hilarious - it reminds of a few years ago when Vance posted on RF a picture of himself flying at some little local airshow and telling everyone here he was photographed doing a 90° banking turn.

I pointed out that the RW beneath him in the photo was at a 20° angle, so he was actually doing a 70° banking turn, not a 90.

Who knows what angle the gyro is actually climbing out in this one? There's no reference in the frame. Guess he learned his lesson?

Vance loves it when I point out his lies and foibles.

Like that other time when I pointed out that he was not a CFI, but that Autogyro was advertising on their website that he was. They stripped him of his company gyrocopter and his affiliation with them for a year or two until after he actually did get his CFI license - after flunking the test on the first attempt. (This is the reason Vance trolls me and attacked me so viciously when "the Ghoul" falsely posted that I crash landed on a crowded beach while showing off, endagering hundreds of bikini-clad tourists, and Vance quickly piled on and relentlessly attacked in that scandal.)

Wonder what his score was when he finally did pass his CFI? Funny, he's never told us what it was...

Ah, well. Back to work, actually engineering, inventing and fabricating aircraft parts, powerplants, and airframe stuff from scratch. Gee, wish I could be an armchair warrior like Vance. SO knowledgeable. SO entertaining. SO honest. At least I still have my entire brain, intact, inside my skull. Duh.



Dear Vance: How many gyrocopters have you built from scratch? From plans? From a kit? Torn down to the last nut and bolt and rebuilt? How many have you altered the head placement? Rotor thrust line? Prop thrust line? Changed engine types? Changed PSRU types?

How many PSRUs have you designed and fabricated from scratch successfully?

What have you ever done that was innovative in the least for a gyrocopter?

Inquiring minds wish to know, since you present yourself as an authority on so many levels and on so many subjects in your 16,000-odd posts here.

 

[StanFoster]

Greg....Vance has way more impressed me with his "lesser" brain...What a cheap shot of you using that against Vance for him losing 30 percent of his brain....In my simple mind, your ranting post shows so much cross firing in your own synapses, I would seriously want that checked out.

Vance to me is a true legend, and my life has been greatly enhanced by letting my inferior self be rewarded with so much of his time shared with me.

I have never seen such a tenacious personality as Vance, and I can only imagine what he would be able to do if he were given back a full brain like you and I take for granted. But I assure you this, Vance has done FAR more than you and I combined, and with only 70 percent of his brain.