Nerfing the Tornado

[Ganbatte]

So... We all know the tornado with its signature 140 cm 3-blade props delivers way more power than most people need most of the time, that's a given. And it's also lighter than the lightest moster build.

But looking at the engine files page (I don't know if this data comes from AC or not), moster is listed at 75kg static thrust with a regular 125 cm 2 blade, and tornado is listed at 80 kg static thrust when a 125 cm 2 blade is mounted. Not much more; only 5 kg difference. Reference: https://www.custom-air.co.uk/Custom_Air_Paramotors/paramotor_engine_files.html

So let's consider the feasibility of nerfing a tornado down to (or maybe even slightly below) moster thrust levels while keeping it easily returned to high thrust configuration if desired. Swapping prop is *way* simpler than altering the reduction ratio.

e-props site, with tornado entered in the search box, lists a 120 cm 2-blade offering, but no performance data. Another option is to ask a wooden prop shop to run off something slightly custom.

So the question: Has anyone any actual experience, or professional opinion, how suitable that 120 might be for this project, or likewise the wooden option? Any actual, hard knowledge why this might be a bad idea?

Thanks in advance!

 

[AndyB]

I have a 130 cm on my Tornado and it still knocks me over if I put the power on too quickly.

[Ganbatte]

Sure, that's even more prop than the 125 cited on the motors page.

[Blackburn Mark]

11 hours ago, Ganbatte said:

nerfing a tornado down

I'm not understanding the logic for doing so unless you are strapped for space on a trike or microlight

[Ganbatte]

Indeed, the nature of the beast ( forums read by humans) is if you ask a "how" question, inevitably someone will ask "why would you want to do that anyway?". But it is a "how" question.

Super short answer: easily reconfigurable versatility.

.

[Blackburn Mark]

17 minutes ago, Ganbatte said:

if you ask a "how" question, inevitably someone will ask "why

Ah, my apologies... I shall endeavor to be not the least bit interested  

[Ganbatte]

Not gettin' on your case, and accusing you only of being a human, just observing an inevitability. But I am hoping someone knows something germane. Sorry if I sounded snippy.

[Blackburn Mark]

I grasp the issue my good man and its not one I am unsympathetic with under the "usual" circumstances. 
If however, I am going to attempt a collaboration on a conundrum that is very easily resolved with a lighter touch on the throttle, I would prefer to know its for a good cause.

Take a look at the EMG 6 fitted with the Polini 250... Due to space restrictions, they had a low aspect ratio high pitch propeller made that could make use of the power... not sure if it was made by Culver props (USA)... cant remember, I would have to have another look.

I am currently attempting to avoid the EMG 6 propeller diameter restrictions in one of my designs so am obviously biased towards the larger diameter props

Edited September 4, 2020 by Blackburn Mark
Spelling

[Blackburn Mark]

https://electricmotorglider.com/2015/05/15/propeller-design-for-the-polini-250/

Apologies, the prop was made in-house (Rainbow Aviation)... its a 122cm propeller mounted on a Polini Thor 250 (lot of power)
The small radius can be attained but at the expense of efficiency

Edited September 4, 2020 by Blackburn Mark

[Ganbatte]

In this case, I'm attracted by the low total package weight of the tornado, but under the wrong circumstances (foot launch, minimal gear, low pilot hours) the excess power isn't a benefit but a hazard. Under other circumstance (higher cumulative pilot hours, laden trike), it's a benefit.

If 2 strokes had a linear power curve like 4 cycles, a simple throttle stop would do the business. With more sophisticated (read: electronic) engine controls, a power profile could just be dialed in. The traditional approach to covering a wide range of use cases is to just buy multiple motors.  I'd rather not buy multiple motors. If buying a few props would do the job, that's far preferable. The published 125cm/80kg data point suggests this should be feasible. And I'm not concerned with fuel efficiency, so that can be traded off for the desired versatility.

Make more sense now?

[Blackburn Mark]

8 minutes ago, Ganbatte said:

Make more sense now?

If I understand you correctly, I would have thought the easiest/cheapest way to "nerf" maximum thrust (without using throttle or tuning) would be to "over-run" the re-drive with an oversized pinion pulley (assuming there is the space for one)... The propeller-drag-curve would meet the motor-power-curve below the motors max output of 7500rpm
Overdo it and you might have CHT problems.

[Ganbatte]

That's altering the reduction ratio as mentioned in the original post. It might be cheaper, wouldn't be easier than swapping props, as propswap is done in the field routinely. Having 2 or 3 props from which I can choose on a given day seems to me an ideal solution, *if* I can determine appropriate design parameters.

[Blackburn Mark]

1 minute ago, Ganbatte said:

swapping props

The problem there would be, it would output the same thrust minus any inefficiencies of a smaller higher pitched propeller.
OR
You motor would over-rev the mismatched prop with very little back-pressure (possible piston slap...?)

[Ganbatte]

Yes, might have to tinker with the pitch. Which is why it's not a total no-brainer.

[Ganbatte]

If the motors had speed governors, this would all be moot. But we're dealing with primitive, simple engine designs, despite what their cost might suggest.

[Blackburn Mark]

I think you misunderstand...
A matching "pitch" prop would transfer the motors maximum power into thrust no matter what the diameter... or it would allow the motor to over-rev.

If you could afford an adjustable pitch propeller, you could "over-pitch" it which would be very similar to over-running the re-drive with the same CHT concerns.

[Ganbatte]

Oh, I understand. Recall I'm willing to trade off efficiency if necessary; I don't mind "wasting" some of the engine's power into turbulence instead of thrust if that's what it takes to achieve lower thrust without burning up the engine.

[Blackburn Mark]

Then an adjustable pitch prop "might" cover all your bases.... It seems a bit brutal but it would be a whole lot better than knocking a piston skirt off with piston slap

[Blackburn Mark]

PS: An "over-pitched" prop usually gives better cruse efficiency at the expense of static thrust.
Saying "over-pitched" might be a bit misleading.... We ought to be saying "higher-pitched".... I was using the term in reference to paramotors and standard fixed pitch for a given model.

Bush pilots often attempt to compromise between static thrust (short takeoff runs) and cruise efficiency (flight range)