Propeller efficiency at Takeoff

[flomo]

Hi everybody!

can anybody tell how to estimate the propeller efficiency at take off?
Are there any reference data available?

I am doing an conceptual design of a low speed aircraft and want to use the slipstream velocity as a kind of "extra high lift" device, but there for a need the prop efficiency to do get a equivalent ideal efficency with finally will give me the slipstream velocity.

any hints how to estimate prop efficiency for take off?

tnx for any advice!

greets flomo

 

[rb1957]

apologies for the dumb-a$$ answer, but ...
it depends on the propeller (particularly the propeller airfoil section).

btw, i think it is common practice to account for the increased dynamic pressure in the slipstream.

 

[flomo]

no woes for the answer and i am sorry if my question is a bit naive or so.

i know that efficiency depends on the specific propeller used (advanced ratio, power coefficient..). my problem is that i dont know yet which kind of propeller will be used (and so i dont know the propeller characteristics). I coundlnt find a commercially available propeller which fits to the engine selected, and it is beyond my abilities to design a propeller from scratch.

therefore i was wondering if there is a kind of benchmark for takeoff I could work with. like: propeller efficiency at cruise is about 0.8 for most aircrafts (its still not the truth and depends on the specific propeller engine combination but will give a good enough estimate for the first design)
so do does any know if such a benchmark exists and what value can be expected? i dont know enough about propellers to know if it is even possible to break it dont to such a benchmark.

its intersting to hear that the slipstream will be acountted. do you know where i could find guidelines how to do that. i already did a literature research but could find anything. so i kind of invented my own concept, but i dont know if is correct.

tnx in advanced for every answer. this design i am working on is for my university project, and it slowly drives my crazy

greets flo

 

[Manticore]

It depends more on the diameter of the prop than anything else which is why helicopters have huge rotors and jets are useless for STOL. Check out Chris Heintz Design College at

http://www.zenithair.com/kit-data/ht.html

for some useful articles on low speed aircraft.

 

[rb1957]

torenbeek "synthesis of subsonic airplane design" has a section on propeller selection.

as a guide to prop diameter ...
Pto/D^2 = sqrt(Pto/Vcr)/4
Pto in hp
D in m
Vcr in kph
(sorry about the units !)
so that D^2 = sqrt(Pto*Vcr)*4
D = 2*(Pto*Vcr)^(1/4)

but i think you need to find some texts on propeller design, or else use a real world propeller.

good luck

 

[flomo]

tnx for your replies so far!

I looked at the website of zenithair but havent found something helpful yet.

and i already selected a prop diameter (D_prop=1.22m) based on the maximal possible tip velocity, prop rotation , power_required for other flight phases, an estimate for prop efficiency and available shaft power.

so maybe i should further clearify what i want to do:

what i basically need is the power that will be transmitted to the fluid (air) during take off.

the only way i know to calculat it is the following:

take off means full throttle, so i check which is the max. shaft power available based on my selected engine, a calculated P_available with:

P_available= prop_efficiency * Pshaft

with P_available I should be able to calculated the velocity of the propellerstream... and so on
BUT! I dont know the propeller efficiency, and i even dont have a rough estimate for it (equals=0.1? or 0.2? or 0.01?!)

or maybe anybody has an idea how to calculate P_available during take off without prop efficiency?

greets flo

ps i sorry for the amount of spelling and grammar errors!

 

[rb1957]

propeller efficient is a function of J, the general shape is something like parabolic.

a fixed pitch prop has a narrow band of peak efficiency, and a variable pitch has a much wider band.

i'd think that even a poorly designed prop (ie one off its design point) would manage 75% efficiency. you might want to assume 75% at take-off and 80% at cruise, then you'd have two points to fit a curve through.

i take it you're using a multi-engine design.

 

[Manticore]

If you're using a fixed pitch prop suitable for cruise most of the blade will be stalled at zero speed. Efficiency increases as you accelerate but I doubt if you would even be getting 50% at take off (my CH701 could be pulled off the ground at about 25Kt but you had to accelerate in ground effect for a few seconds before it would climb - I was using a 56" prop on a Jabiru 2200 (direct drive, 3300 rpm max) and later found I could have improved take off performance considerably using a 72" prop with a geared engine of the same horsepower).