Prop control in a C208

Ask questions about flying techniques here. Real or Simulated - the principles are the same!

Prop control in a C208

Postby tjander » Sat Sep 21, 2013 8:11 pm

OK I have read a few topics on here about prop pitch and how it controls the RPM not the pitch, now with that said how is feathering a prop not changing the angel of the blades. It is my understanding that feathering a prop reduces the amount of air flow over the blade allowing it to pass through the air with little friction. So if I am wrong, please correct me. However, I have noticed in flying the C 208B at 5000 agl that I gain my most airspeed from a RPM setting at about 16oo RPM's and the MP set at near the red line. What I don't understand is how can a propeller turn slower and produce a larger air speed. Seems inverse to logic or am I completely off base here...
tjander
Ground hog
Ground hog
 
Posts: 2
Joined: Sat Sep 21, 2013 7:50 pm

Re: Prop control in a C208

Postby Moach » Sat Sep 21, 2013 9:39 pm

User avatar
Moach
Captain
Captain
 
Posts: 994
Joined: Thu Nov 28, 2002 3:22 pm
Location: Vancouver, BC

Re: Prop control in a C208

Postby Brett_Henderson » Sun Sep 22, 2013 7:29 am

tjander wrote:OK I have read a few topics on here about prop pitch and how it controls the RPM not the pitch, now with that said how is feathering a prop not changing the angel of the blades. It is my understanding that feathering a prop reduces the amount of air flow over the blade allowing it to pass through the air with little friction. So if I am wrong, please correct me. However, I have noticed in flying the C 208B at 5000 agl that I gain my most airspeed from a RPM setting at about 16oo RPM's and the MP set at near the red line. What I don't understand is how can a propeller turn slower and produce a larger air speed. Seems inverse to logic or am I completely off base here...


The FSX turbine modeling is a mess.. so it's not worth getting into.. and even if it were accurate; turbine engines have almost 3-dimensional power curves.. biased by not only RPM, but altitude, too. To a point, they get "better" with altitude, where a piston has a linear fall-off with altitude. So maybe, at a relatively low altitude (5000msl), a C208 gets peak IAS pushing the power turbine, while limiting the prop RPM (they aren't mechanically linked in this turbine, unlike a piston engine, where prop RPM and engine RPM are the same)

In general (especially piston engines), you are correct in assuming that higher thrust at lower RPM is counter-intuitive. Light GA piston engines will always generate more HP, at higher RPM, and the constant-speed-prop "governs" RPM much like a pilot "governs" airspeed while climbing.. ala constanty adjusting wing AoA to maintain a specific airspeed (prop AoA to maintain a selected RPM). Alll these prop-blade pitch changes happen, without the pilot touching the prop-control.



Now... Feathering differs per the aircraft.. and it's well outside of normal operations.

For most turbine engines, it's a setting independent of what you'd use in flight.. as is a genuine, "Beta".

For most light GA piston aircraft, "feathered" is simply the theoretical, minimum RPM... normally used to minimize drag from a "dead" engine.. and must be set while there are still enough RPM (wind-milling), for the oil pressure needed.

Also, if it's not an engine failure, but a prop-mechanism failure, there are spring-loaded settings that will set the prop blades to either full-coarse, or full-fine. Full-coarse for most twins, for an effective feathering (minimize engine-out yaw).. full-fine for most singles, so that you can still fly (albeit slowly, with the engine screaming).
Brett_Henderson
Major
Major
 
Posts: 3606
Joined: Fri Oct 08, 2004 7:09 am

Re: Prop control in a C208

Postby tjander » Sun Sep 22, 2013 3:31 pm

Thanks for the info... I do find controlling altitude easier with prop control as it is more responsive then MP. Basicly, I set the MP at redline and use prop control to make adjustments to altitude.
tjander
Ground hog
Ground hog
 
Posts: 2
Joined: Sat Sep 21, 2013 7:50 pm

Re: Prop control in a C208

Postby hyaxasa » Mon Nov 09, 2015 10:34 pm

Seems inverse to logic or am I completely off base here...
สล็อต
hyaxasa
Ground hog
Ground hog
 
Posts: 14
Joined: Thu Oct 29, 2015 5:57 am

Re: Prop control in a C208

Postby Flacke » Tue Nov 10, 2015 2:11 am

tjander wrote:Thanks for the info... I do find controlling altitude easier with prop control as it is more responsive then MP. Basicly, I set the MP at redline and use prop control to make adjustments to altitude.


Thats not how real aircraft are operated. You normally only use max. MP for climbing at a rapid rate. You set Manifold Pressure for takeoff, climb , and cruise settings.
The propeller is set at high RPM for high power settings such as climb and then both MP and Prop RPM are reduced to a cruise setting or a cruise/climb setting. This is a basic explanation.
You should continue to research Prop and MP management until you have a really good understanding of how to use both. You will enjoy flying your flight-sim airplanes more when you set power properly for each phase of flight. You are on the right track because you took the time to post your questions in here. Keep learning, and soon you will be a power setting ace.
User avatar
Flacke
2nd Lieutenant
2nd Lieutenant
 
Posts: 266
Joined: Sun Apr 21, 2013 8:25 pm

Re: Prop control in a C208

Postby Shane Stachwick » Thu Dec 03, 2015 3:33 pm

tjander wrote:OK I have read a few topics on here about prop pitch and how it controls the RPM not the pitch, now with that said how is feathering a prop not changing the angel of the blades. It is my understanding that feathering a prop reduces the amount of air flow over the blade allowing it to pass through the air with little friction. So if I am wrong, please correct me. However, I have noticed in flying the C 208B at 5000 agl that I gain my most airspeed from a RPM setting at about 16oo RPM's and the MP set at near the red line. What I don't understand is how can a propeller turn slower and produce a larger air speed. Seems inverse to logic or am I completely off base here...



Basically, you're setting your prop blades at a steeper angle so they can grab a bigger bite of air. A steeper blade angle (and thus a lower RPM) reduces engine stress and improves fuel efficiency at the expense of a bit of airspeed and throttle response.

Feathering the blades turns the edge of the blade towards the oncoming air (perpendicular to its normal orientation), allowing your dead prop to cut through the air with the least resistance possible. This isn't something you would typically do unless you have to fly (or glide :pray: <<t ) with one engine out.
Shane Stachwick
2nd Lieutenant
2nd Lieutenant
 
Posts: 108
Joined: Thu Mar 06, 2014 10:59 pm

Re: Prop control in a C208

Postby logjam » Thu Dec 03, 2015 7:14 pm

Feathering a prop reduces or eliminates drag so that in case of engine failure, maximum effect can be gained from the good engines with minimum opposite force or drag. Propeller angle of attack only works between a tiny envelope which has been determined by intensive testing to achieve the best results. That's one of the reasons for having test pilots. Results have then been obtained under RL conditions. <<u <<s
User avatar
logjam
Major
Major
 
Posts: 1293
Joined: Tue Mar 26, 2002 3:10 am
Location: Lillooet, BC Canada

Re: Prop control in a C208

Postby tomcruzz » Tue Apr 18, 2017 3:28 am

while limiting the prop RPM (they aren't mechanically linked in this turbine, unlike a piston engine, where prop RPM and engine RPM are the same)
สล็อต
บาคาร่าออนไลน์
tomcruzz
Ground hog
Ground hog
 
Posts: 8
Joined: Tue Apr 18, 2017 3:24 am


Return to Flight School

Who is online

Users browsing this forum: No registered users and 224 guests