Welcome Guest. Sign in or Signup

3 Answers

RPM and power

Asked by: 13650 views Aircraft Systems

hello everyone

i am a student pilot. i fly a single prop piston engine aircraft. anyone please clear following doubts in my minds....

  in constant speed propeller aircraft if power is increased i mean boost is increased how the speed of the aircraft or its lift is increased, despite the RPM of the engine is same? suppose i was flying at power 20 inch with 2500 RPM at speed 90 knots. i increased the boost from 20 inch to 25 inches and rpm is same(2500) i get speed almost 105. RPM is still the same. it means prop is rotating at the same speed. how do i get more speed or lift?

  i will really appreciate your kind guidance.

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

3 Answers



  1. Nathan Parker on May 17, 2012

    Please note that increasing power doesn’t mean you get “more lift”.  Some people use that terminology to mean “climbing”, but a climbing airplane doesn’t have any more lift than an airplane in level flight.  (In fact, it has less.)
     
    But, to answer your question, when you increase the throttle in an airplane with a constant speed propeller, the pitch of the propeller blades increases so that the RPM stays the same.  The propeller blade rotates at the same speed, but its AoA is larger so it produces more thrust.
     
     

    +3 Votes Thumb up 3 Votes Thumb down 0 Votes



  2. Lucas on May 18, 2012

     
    Also think of a car: in first gear at 2000 RPM you might be going at 15 MPH, but if you switch to second gear at 2000 RPM you would be going at 30 MPH. Now it doesn’t really work that way, but the concept is similar. Pretend that the propeller lever is your cars accelerator and your throttle the gear shift and there you go: increase the gear and the airplane flies faster without affecting RPM.
    So like Nathan said increasing the throttle increases the angle of the blade of the propeller (putting it, if you want, into a higher gear), increasing thrust and therefore speed.
    Lucas
    http://www.Pilottrainingsolutions.com
     

    +3 Votes Thumb up 3 Votes Thumb down 0 Votes



  3. Wes Beard on May 18, 2012

    Khoda,
    The way I think about it is like this.  If the propeller were to go through a solid gel like object.  The path it would make would be different depending on the blade angle.  If the blade angle is fairly flat compared to the gel object, it is going to take a lot of rotations for the propeller to work it’s way through the gel.  As the blade angle is increased, the number of revolutions required decreases but the amount of energy required to twist the blade will increase.
    It is the exact same thing,  when the propeller moves through the air, the energy required to rotate it through the air is directly related to the blade angle.  This explains why a propeller will remain at a constant RPM even though power is increased in the engine.  The extra energy supplied is consumed by the extra rotational force needed (due to a higher blade angle) to keep the propeller at that RPM.  Since, the propeller takes fewer rotations (due to a higher blade angle) to go a given distance, the airplane will have an increase in speed as well
     
    Obviously, there are speeder springs and flyweights that are designed to move the blade angle to keep the RPM constant.  These components are the supporting cast for the end product of a constant RPM and not the direct reason why the RPM stays constant.  The amount of energy supplied to the propeller plus the propeller blade angle will equal the rotational rate.

    +3 Votes Thumb up 3 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.