View Full Version : Derating an Engine
02-28-2005, 01:56 PM
OK gang, here's my delema...
I live and fly in Southern New Mexico. Key point being that most of my flying is above 5,000 ft MSL, and a lot of that with significant density altitude adjustment in the summer.
I'm currently putting together a Gyrobee, and at least initially want to keep it under Part-103. I have all the pieces accounted for except the engine and prop. The unanimous concensus of the pseudo-aeronautical engineers locally (most of us are engineers in other fields) is that I'll need at least 45-50 HP to work reliably.
Obviously a Rotax 503DC would do nicely... except for the weight. The new Hirth F23 might work, but it looks like a "boost" from 40 to 50 HP on the same basic engine. Seems intuitively pushing the envelope for reliability problems. Then there is the MZ202, which using the same basic engine but dropping to a single carb and lower RPMs becomes the MZ201 at 45HP.
By forgoing the full instrument pod for only a couple of engine gauges and a Hall wind indicator, the MZ202 would fit Part-103 with a couple of pounds to spare (actually 2.5, but I'll be conservative). I don't think I'll forseeably need 60HP, but by backing down the power curve I can get the 50HP I want at a lower-than-maximum RPM. Intuitively this would generate less wear on the engine and improve reliability.
Finally he gets to the point... What is the feasibility of using an adjustable stop at full throttle to limit the max RPM? If this is workable, I'll push-on through the fog. If not, is there another altenative?
02-28-2005, 02:25 PM
let me say this you will need all the HPyou can one day, but if you think this is wise just adjust the prop to load the motor at 5000 max. I see no good out of doing that but you can do it.
02-28-2005, 02:28 PM
Leave the throttle and rpms where they should be. To stay legal under 103, just slow down, ease the throttle back after your climb. On hot days you will need the extra hp to take off and climb. Better keep it.
02-28-2005, 03:29 PM
If you're looking to limit the speed of the aircraft to remain Part-103 compliant, then you can choose a prop large enough that will require a finer pitch to achieve your maximum desired engine rpm. This finer pitch will give you incredible thrust at lower airspeeds, while running out of thrust at some selected top airspeed where you simply won't be able to go any faster.
Here's an example:
My GyroBee has a Rotax 503 DCDI, a 2.58:1 B gearbox, and a 3-blade 60" ground-adjustable Ivoprop. In order to *barely* reach 6,400 rpm at full throttle and 50+ mph, the prop has to be set at a pitch, which (going from memory) is about 10 degrees measured at the tip. At lower airspeeds the maximum engine rpm is even lower (maybe 6200 rpm at 45 mph). Static thrust has been measured to be in excess of 325 pounds.
My Bee's all up flying weight is about 545 pounds. Maximum level flight speed at full power on a standard day near sea level is just barely 70mph. On hot summer days and 1000' MSL, top speed drops to about 65 mph.
However, in exchange for this limited top speed, I get very good climb performance at from 40 to 50mph. It's about 800 fpm on a standard day, and about 500 fpm on a typical summer day.
If I reduce the prop pitch to get the engine rpm up, the top speed suffers markedly. If I increase the prop pitch to get more top speed, climb performance suffers dramatically. So the prop pitch setting I currently use is a comfortable cross between good climb performance, reasonable top speed, and acceptable max engine rpm.
Now if I wanted greater top speed performance, a better prop to use might be one with 2 blades and a larger diameter.
Anyway, In my experience I don't see why you couldn't get a prop that's sized and pitched to keep you Part-103 legal on speed without sacrificing climb performance.
PRA Chapter 30
Western WA state
02-28-2005, 04:41 PM
Just don't tell anyone how fast it is, The FAA isn't using radar guns yet. :D
02-28-2005, 05:03 PM
Don't think he is worried about top speed. Sounds like he wants to know if his engine will last longer and be more reliable if he limits the RPM to a lower than max rpm. I don't think it will make much difference on a two stroke like the ones used on aircraft. They are already very derated.
02-28-2005, 08:04 PM
Seems like it was in the words of the Bard "Much Ado About Nothing". :o
At this point I'm looking at the PowerFin prop primarily for mass/weight.
It is ground adjustable, so I can tweek it more to my needs.
Back when I was in the CAP, the squadron had a 172 with a cruise prop.
At one of the annuals, the ol'man opted for a climb prop. Took us longer
to get across the state, but mountain searches were more comfortable.
Duh, I need climb, not speed in Part-103 anyway. :rolleyes: Or as we say around
here when the obvious is overlooked, "Snake bit again!".
02-28-2005, 08:13 PM
Oh yes, Ron,
That was what I was hoping would be a benificial "side effect" with the lower engine RPM. Sounds like it might only be a small or even insignificant one though.
02-28-2005, 08:25 PM
you don't want to run slow and load up a 2 stroke Rocky. They tend to run better at the higher RPMS. A four stroke aviation engine is another story.... Big cubic inches, low Rpms, long life
03-01-2005, 04:20 AM
Has Smoky Davis flown his gyro yet?
03-01-2005, 08:43 AM
Not a clue... or is that clueless? Nevermind...
It's been so long since I've been to the airport, I'm not sure I remember how to get there. :confused: Damn, I hate these rotating shifts. They get in the way of important stuff like living. Of course there is that little detail of "road work" on Picacho just to confuse things, too.
03-01-2005, 10:48 AM
The faa is testing a system that is going to be online very soon it is in the testing phases and is in the process of being implemented from south jersey to florida. It will id aircraft and give alt, heading, ground speed and a bunch of other big brother things. I saw it in action at the safety seminar Last wed.
John Hopkins developed the software and emry riddle developed the hardware. Big brother will soon be riding with everyone in one way or another.
you could take your mouse pointer, put it on a bogey and it would give you all the info.
Orwell is rolling in his grave.
Just don't tell anyone how fast it is, The FAA isn't using radar guns yet. :D
03-01-2005, 10:52 AM
03-01-2005, 12:03 PM
Rocky: simple answer from a desert rat next door (that also rides dirt bikes, flys r/c airplanes and other two-stroke fun); higher load, more heat, more wear, increased stress= greater chances of failure. Limit the speed by using the throttle. Optimize your prop for the power setting you'll use the most. All those four-letter-word pick-ups with horse trailers on the side of the road in the summer time should answer your question about engine loads!! :D
03-01-2005, 10:00 PM
Ron & Cody,
Thanks guys. This makes sense (scarry, huh). Sounds like I'm ready to
commit some coin to this part of the project now. Target of reasonable
RPMs with reduced loading is sounding beter all the time.
Jonathan & Scott,
"Can we say Fascist, boys and girls?"
03-02-2005, 08:17 AM
Dont mistake what I said as an agreement because I do not think you need anything more than is what in place now.. Tracking your flight from the moment you sw on the master to when you shut it off is an invasion of privacy in the most terrible way.
What the tsa let slip is that they are going to spend 28 million, 28,000,000 american dollars on the uat boxes and have EVERY PLANE THAT operates in the washington adiz have one installed. It will send number, position, all the data to a reciever which will log all the flights of that aircraft. I am not in favor of this. It will not stop another terror attack, even if you believe all the hogwash that is supposed to be the facts of that day and facts leading up to that day. I can only guess mandatory installation of such electronics will not be far behind. At the very least if you fly in sensitive areas.
A bomber could walk right into the capitol building and blow a lot of people up. It wont take an airline to cause damage to our already fragile "freedoms"
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