View Full Version : Prop changes with temp.
11-27-2004, 11:04 AM
Kinda silly to make this post but if someone can give me a quick answer it would help settle my mind.
If you pitch your prob to give say 5100 (engine) rpm during the summer when the temp is in the mid-80s. Then when winter rolls around could the colder air (45-50F) reduce the max engine rpm to say 4900?
I say this is a silly question because I have changed 2 other variables that are almost certainly the cause in the drop in RPM. I have installed a new engine that has less then 10K miles (old one had better than 80K) and a new RAF airfilter which I think is more restrictive than my old one.
If one of you could verify that RPM would not drop significantly with such a small drop in temp then I will feel better as I tinker around with airfilters and timing to try and get my 200 RPM back.
11-27-2004, 11:53 AM
Tim: I can tell my prop rpm's go down with colder temps. There just are more molecules a movin along their way each bite.
This extra load on the prop is somewhat offset by a peppier engine as well..as it is sucking in more molecules as well. The extra load on the prop is greater though...by how much...I cant say.
11-27-2004, 05:01 PM
there is really not much of a need for a airfilter on a aircraft engine. Just a screen to keep Bugs and other trash out of the carb.
11-27-2004, 06:38 PM
Depends on your situation Ron,I do oil changes at 50 hours ,and if I don't clean the K&Ns at every 50,they won't be there cum the next 50 change.They'll be sucked into the carbs coz they clog up with grit every time I land.
Regarding the rpm changse with temp changes Tim,on average I'll gain upto 400 rpm from a cool morning[20c/70f] to a heated midday [45c/115f].
Thats the change you can get in the same day,and if you through humidity into the equasion as well...................... .
I reckon humidity plays a bigger part in air dencity than temp.
As an exsample,I just got back from next door,shift'n some cows for him and the air temp got upto 39c.When I was traveling home,cruise rpm was 4100.
When I took off to go over there this morning,it was bout 18c,and the same cruise speed needed 4400.
Same amount of fuel but less rpm at the hotter temp..........????
The cool STILL air at dawn and at 100' had X amount of humidity.
The hotter air had none.
11-27-2004, 06:41 PM
David, where and how you fly, Airfilters are a must!
11-28-2004, 06:01 AM
Tim, as the other guys have said. The cooler the air the more engine rpm. You are saying that you get less. That means there is some other reason or reasons.
11-28-2004, 10:16 AM
Hi Ron, The old engine guys have an old saying that "the boat that never comes to shore and the aircraft that never lands doesn't need an airfilter, all other engines should have a good filter." Thank you, Vance
11-28-2004, 12:15 PM
C'mon Ron, you know better than that, or you should. Filters, filter screens or filter foams are a must for air inducted carbureted engines. The air inducted should be as clean as possible.
I do agree 400 rpm changes in cruise are possible.
400 max rpm (full engine load at take off) loss due to temp change is nearly impossible.
This would mean around 180 rpm prop rpm difference.That is a lot of HP.
Give me the redrive ratio and the engine hp at the given rpm and I calculate within 10 percent margin the hp difference.
Also mention the temp difference.
11-29-2004, 10:27 PM
That would be very interesting JOS.
You'll need to be more specific with the variables tho,you need to allow for humidity %,barametric pressure differences as well as the temp.Thinner air means more prop rpm,but also less avalable power.And wot dose humidity do to engine efficiancy/power???
I haven't changed the prop pitch on the ferel for bout 4 years.............since me last bingle,and I now have a good idea of the rpm range from best to worst..............and it'd be more than 400 rpm difference between the two extreams.You'd also need to know where the power/toque peeks are in realtion to the rev range.[IOW,in the best,thickest air,it may not even be get'n to the peeks.]
Don't know the reduction ratio off the top of me head,wot ever the 80hp 912 cums with.
12-04-2004, 02:36 AM
I also agree with above said and thats why I use an In-Flight-Adjustable prop.
I have an electronic controller that pitches the prop according to a preset rpm in different settings, TO, Climb and cruise. This way I always have highst power available.
In almost each flight I turn into Manual mode, just to check if the automatic settings are optimal.
12-04-2004, 07:19 AM
That must be a sweet setup. I'll bet it helps with range at cruse too.
12-04-2004, 01:26 PM
The reason why I choose an IVO IFA prop was that I needed to get as low noise as possible because our regulations says it has to comply with a limit of 60 dba when cruising in an alt of only 500ft above the sensor, and that is very hard to obtain in a gyroplane. But with an IVO-IFA prop I can adjust the pitch so I can take off, climb and cruise, all with different settings. Setting for cruise was set to lowest noise level when doing the measuring thing but was not really suitable for economic cruise, but what the heck I got the approval. Donít ask what setting I use now ;)
I have always used this IVO-IFA so I canít say if I get better performance or not compared with other propellers, I can only say that I always have max engine rpm no matter what time of year or what weather conditions are, so in a way, yes I get better performance in TO and climb. With a 27ft McCutchen and high disk load (8,5kg/sqm) I get a poor 200ft/min or so climb but 80-120 mph cruise. 120 are a bit much in a semi-enclosed gyro so I keep it around 95 to 110 mph. Even at that speed it feels like the wind is trying to tear my clothes off. Quite exiting actually. I fly from grass and dirt strips so the only draw back I have with the prop is that it is sensitive to nicks, so I have to repair and balance it ones in a while.
I can recommend you to use an IFA prop. (if your regulations approve of it of cause)
12-04-2004, 05:55 PM
I think I have an answer to my drop in RPM. I will post in my 'engine out' thread under training as soon as I can type it all up. Thanks!
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