Earthstar Forums

If I buy an HKS 700e for a JT2, do I want or need a 3.47 to 1 ratio? Or a 2.58? What are the pros and cons?

Noise is lower with the higher ratio (slower prop tip speeds) . Speed is
higher with the lower ratio (Higher prop rpms). Take off distance is
shorter with the higher ratio. Propellers for each ratio should be
different,.Choose a higher diameter for the slower turning prop. Testing
with different props is the only true way to find out how much you gain
either way


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Thanks Jaime. I pretty much understand the math. I am curious about the experience. I have built a list of the seven or eight JT2 owners with HKS engines, but have very little contact info for them. I'll keep working on it. Thanks again.

I do not have a JT-2
ButI have had several ULs and have flown dozens of ULs and have
experienced the 4 different gearbox ratios in Rotax engines and many, many
different propellers. I have tried, for example, an (the same) Arplast with
2, 3 and 4 blades and IVo props with 2, 3 and double triple (6) blades


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Jaime, how was the 6 blade? Was it quieter? I like quiet and am willing to put up with a slight performance decrease to get it.

Glad to hear that !
Me too!

The 2 quietest propellers I have tried are the 4 bladed Arplast and the 2
sets of triple IVO props

No big difference in performance or quietness, but I have a very slight
preference for the Arplast
It may been the aesthetics

With the IVOs, what you do is to put together 2 triple bladed propellers,
but you must use 4 to 1 ratios for something like 66-68" diameter inch
props with a Rotax 582

If you use 3.5 to 1 ratio gearboxes, you would have to use smaller diameter
props


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Hi
3.47 to 1 is preferred
Mark

Sent from my iPhone

With all respect, Mark.
Preferences are about choices
It all depends upon the emphasis some put to one, or other, quality or
quantity
For a fast aircraft such as the Odyssey, perhaps the 4:1 ratio is not the
most recommended choice, as you will "eat up" the actual angle of attack of
the prop pretty fast with a fixed prop and not be able to cruise as fast,
if you want a fast ground acceleration, but then, you might be comfortable
with a very poor acceleration during take off (very high angle of attack of
your blades. or even stalled angle of attack at the beginning of the roll)
in order to achieve a high cruise speed and still have a quiet prop.
The higher the ratio, the harder it is to have both, good performance at
takeoff and at the same time, a fast cruise speed
The lower the ratio, the nosier it becomes or, if you shorten the blade
diameter, the less efficient the prop becomes
It all comes to choices and there in no way to decide reasonably unless you
try them all and adjust the facts to your airstrip and personal tolerance
to noise





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Hi Jaime
I totally agree with you, what I meant was that despising between the 2.58 and the 3.47 is a mater of preference.
I like less noise. And good performance so I like 3.47.
Happy Flying
Mark

Sent from my iPhone

I can't help but believe that Marks many decades of trial and error with his own design prompted his advised selection.
In my experience which includes the experimental flight test division at Lockheed Palmdale, and that of my long deceased father in law who was a chief acceptance engineer at Muroc;[pre-dated Edwards]wind tunnels,theories and drafting boards take a back seat to flight test experience.
Neither Mr.Douglas,or Mr.Northrop [and many others]were graduated engineers.
I found this to also be the case when racing sophisticated formula cars,flat track motorcycles,midgets,and even FVee.
While not entirely applicable in this scenario, i can't help recalling MR.Packards response to a reporter when asked about his car"ASK THE MAN WHO OWNS ONE"