Static thrust

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BrianW

Active Member
Joined
Jul 2, 2018
Messages
44
Location
Altus SW Oklahoma
If you want to know engine power, a stationary dynamometer will give you power versus RPM. If you want to know thrust in climb and cruise, you need to fly accurate timed climbs no wind, and timed cruise distance. The RPM you make in these two phases of flight tells you a lot about the prop you chose....
 

fly2kads

Well-Known Member
HBA Supporter
Joined
Jan 2, 2010
Messages
1,627
Location
Justin, TX
I think what the problem here is, is that no one wants to invest (spelled waste) the time and money building several propellers for their airplane in order to tune performance.
I don't know that would call that a waste (I think of it as an R&D expense), but I think your point is essentially correct. The math and procedures for design and analysis of propellers is pretty well defined. One principal challenge is that it is fairly complex, and many people find it intimidating. Another challenge is that it is very much a "garbage in, garbage out" process, in that the quality of results is dependent on the quality of your input data. Further complicating that challenge is that some of your most important input data are often unknowns: how much power does your specific powerplant installation actually produce; how much drag does your airframe really have, etc.? To get the best results, you need to either obtain better data, be prepared to iterate on a solution, or some combination of both.

There is a strong temptation to look for an "easy button" to simplify this process. I understand this desire. Static thrust is relatively easy to understand and measure, so people may find it tempting to look for a way to use it, either directly, or as a proxy of some sort. Unfortunately, as we have discussed here, that's not particularly valuable in most normal cases. Another "easy" solution is a ground adjustable propeller. Start with something that gets you in the ballpark, and tweak the blade angle to make it better. Will it provide an optimum solution? Most likely not, for the exact same reasons that constant speed props aren't universally applicable, as discussed above.

Each builder needs to decide how much they want to optimize their setup, and how much resources (time and money) they want to put into obtaining it. If you want a cookie-cutter solution, the most popular airframe/engine combinations have been worked out pretty well. If you are building some unique combination, an airplane built in limited numbers, or something altogether new, you have some decisions and work ahead of you.
 

Dana

Super Moderator
Staff member
Joined
Apr 3, 2007
Messages
9,257
Location
CT, USA
Another challenge is that it is very much a "garbage in, garbage out" process, in that the quality of results is dependent on the quality of your input data. Further complicating that challenge is that some of your most important input data are often unknowns: how much power does your specific powerplant installation actually produce; how much drag does your airframe really have, etc.? To get the best results, you need to either obtain better data, be prepared to iterate on a solution, or some combination of both.
Thatone

When putting a new prop on my Hatz, even with performance data for the existing prop it was a crapshoot, the new propeller was reworked three times before I was (mostly) satisfied.
 

Jsample40

Member
Joined
Jan 18, 2020
Messages
8
Location
Western North Carolina
An easy and relative inexpensive solution is to acquire an IvoProp ground adjustable propellor. I have a 54" two blade unit and have recently replaced a 25hp KFM 107ER twin cylinder light sport aircraft engine with the next size up. The KFM Maxi is a 30 hp engine which maxes out at approx 6500 rpm.
The Ivo prop allows me to "try / test" various prop pitch settings and see the actual result by either using a large pull scale, or flight testing (including differences in take off, cruise speed, and top speeds). The pitch can be easily and quickly adjusted with a wrench and measuring tape (to check for the central neutral pitch position.
An additional benefit of the IvoProp is if one of the blades is damaged, it can be replaced relatively cheaply, as compared to having to buy an entire new propellor. For those folks attempting to "tune & tweak" for the optimum prop pitch with various engines, this may be helpful. And lastly.. if the prop is determined to be too long, it can simply be cut off to appropriate lengths with a typical hand or power saw.

Disclaimer: I own an Ivoprop for my Ridge Runner 1, but am not compensated by the manufacturer for my recommendations.
 
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