Quantcast

Propeller position research?

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Pilot-34

Well-Known Member
Joined
Apr 7, 2020
Messages
583
Could anyone point me at some research a layman can understand in regards to the optimal propeller position in regards to the wing ?
For instance would a 60 inch propeller be best centered. 30 inches above and 24 inches forward of the leading edge of a wing ?
The same measurements below?
Or is inline better ?
Do flaps or leading edge slots make a difference ?

I suspect it’s a complicated matter but is it explored anywhere on laymen’s terms ?
 

cblink.007

Well-Known Member
Joined
Jul 7, 2014
Messages
506
Location
Texas, USA
Prop position is driven by configuration, power, performance, stability, control, etc, etc, etc. Very broad question with no one solution.

Same with wing considerations!
 

fly2kads

Well-Known Member
HBA Supporter
Joined
Jan 2, 2010
Messages
1,697
Location
Justin, TX
Propeller location is primarily defined by overall aircraft configuration. An optimum position would be at the end of a magic shaft that provides no impediment to either the inflow or outflow from the propeller. Since props are driven by engines of some flavor, and carry bulky things like pilots aloft, we don't have that option. Therefore, put it where it makes sense, and do your best with it. All of the factors Mr. 007 mentioned apply. Picture a whole tube of air in front of and behind the prop, slightly bigger in front and smaller to the rear, and you'll get a picture of the volume space acted on by the prop. Realize that, theoretically speaking, half of the velocity change by the prop happens in the inflow to the prop, and the other half as it flows aft. Work to keep that slightly tapered cylinder flowing smoothly, and you'll be okay.

In this picture of a C-130 taking off on a humid day, you can see the four propeller stream tubes thanks to condensation forming in the vortices at the prop tips.
C-130J_vortices.jpg
 

Pilot-34

Well-Known Member
Joined
Apr 7, 2020
Messages
583
I appreciate the advice about configuration and such but that’s not really the question I want to explore. There are so many interactions there that I’m not sure there’s a configuration that is optimal. I’m sure if it was we would only have one kind of airplane.
In fact Im concerned that simply the placement of the propeller relative to the wing might approach infinite configurations and considerations

But for now let’s assume that we have a self supporting self driving propeller that we can place in any location relative to the wing.
Would there be an optimum location for it ?
Would that location change if our primary focus was a short takeoff? Would it change if our primary focus was speed ?
 

Swampyankee

Well-Known Member
Joined
Dec 25, 2015
Messages
1,431
Location
Earth USA East Coast
The NACA did a series of experiments to find the best vertical location of propellers in relation to the wing; I believe they found the best was to have them below the wing by some amount.

Here are some articles to check out:

I suspect that horizontal location is far more important for noise than for drag. I also would be fairly certain that issues like torsional loads on the wing, which would worsen as engines are mounted farther from the wing's torsional axis, length of landing gear, Vmc, and engine packaging are more important than the few drag counts from being at a less than optimal vertical or horizontal location.
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
11,729
Location
97FL, Florida, USA
Would there be an optimum location for it ?
That question still is too general for a single answer. What does “optimized” mean?

For one sub-set of aircraft, aerobatic, the thrust line and the wing chord line both should pass through the aircraft’s CG. Other considerations, such as visibility, ease of entrance and egress, drive compromises.


BJC
 
Last edited:

Mavigogun

Well-Known Member
Joined
May 29, 2016
Messages
80
Location
Progressive Texas
That question still is too general for a single answer. What does “optimized” mean?
One design might seek to maximize thrust, another more concerned with forestalling flow separation on the wing at high angles of attack, or washing the empennage, or easing take off rotation, as examples. What is the prop tasked with?

The question seems akin to asking “what is the optimal number of legs for an animal?” The answer depends on whether we are concern with cheetahs or dung beetles or monkies.
 
Last edited:

Pilot-34

Well-Known Member
Joined
Apr 7, 2020
Messages
583
That question still is too general for a single answer. What does “optimized” mean?

For one sub-set of aircraft, aerobatic, the thrust line and the wing chord line both should pass through the aircraft’s CG. Other considerations, such as visibility, ease of entrance and egress, drive compromizes.


BJC
Check for maneuverability thrust line in line with CG
 

Pilot-34

Well-Known Member
Joined
Apr 7, 2020
Messages
583
One design might seek to maximize thrust, another more concerned with forestalling flow separation on the wing at high angles of attack, or washing the empennage, or easing take off rotation, as examples. What is the prop tasked with?

The question seems akin to asking “what is the optimal number of legs for an animal?” The answer depends on whether we are concern with cheetahs or dung beetles or monkies.
I kinda thought Focusing on short takeoff and speed where pretty narrow.
Lift and thrust.
 

cblink.007

Well-Known Member
Joined
Jul 7, 2014
Messages
506
Location
Texas, USA
I kinda thought Focusing on short takeoff and speed where pretty narrow.
Lift and thrust.
Not quite that easy. Not by a longshot. But we all desperately wish it was!

In the aircraft design process, the very first thing you need to do is write down some questions, such as:

1. How many people do I was the aircraft to carry?
2. How fast/far do I want to fly on a fuel load?
3. Do I need short / unimproved field capability?
4. What kind of powerplant do I wish to use?
5. Metal, wood or composite?
6. Does this thing need IFR capability?
7. Do I have a size restriction?
8. How much can I allow myself to compromise?
9. How big is my budget?
Etc, etc, etc.

It does not appear that you have any background in air vehicle design....and that is perfectly okay! I was there too, many years ago!!

So, a good book to help get you started is "Simplified Aircraft Design for Homebuilders" by Dan Raymer. Of most senior engineers I have had the priviledge of receiving mentorship from, Dan had a very uncanny way of explaining things in laymans terms, which helped me greatly during my formative years. In this paperback book, you follow him on a journey as he literally designs a small twin. The book is pure gold, and even though I now hold a graduate degree in aeronautical engineering with 20+ years of aviation experience, I still reference this book on many occasions!!

Aircraft design, development, construction and testing is a process of continual give and take, and I can tell you that it is the most difficult, frustrating, tirade-inducing, satisfying, enrichening and rewarding experiences you will ever enjoy!!
 

Pilot-34

Well-Known Member
Joined
Apr 7, 2020
Messages
583
Ok Guys I got my degree in truck driving no engineering experience etc.

So sometimes I need to examine things in the smallest possible division.
Since I’m obviously not experienced in everything.

The Custer channelwing.
Makes me suspect that the more accelerated airflow I can get moving over the upper half of a wing the more lift I can generate at the slowest speeds.
but I suspect that the channelwing suffers from a couple things.
The biggest thing of course is the entire upper half of the propeller thrust area contributes no lift.
Second thing is the middle third is using a lot of thrust to generate A lift vector that is not predominately vertical.

It seems like the Antonov 181 uses the upper portion of the lower half thrust to generate lift by being forced against a flap.

So how do I build a wing with the advantages of the Custer channel wing without all the crazy structure?

Just putting the propeller in line with the Center line of the wing would seem to have the downside of forcing a lot of air against the bottom of the way where the Custer seems to emphasize thrusting all the air over the top of the wing.
Plus for a flying boat the higher above the water the better.
 

Mavigogun

Well-Known Member
Joined
May 29, 2016
Messages
80
Location
Progressive Texas
“Makes me suspect that the more accelerated airflow I can get moving over the upper half of a wing the more lift I can generate at the slowest speeds.“

Sounds counter intuitive, at first, typically associating airflow over the wing with airspeed. What might a wing-spanning array of small fans deliver? I recall such a strategy is presently being pursued for electric VTOL craft.
 
Last edited:

fly2kads

Well-Known Member
HBA Supporter
Joined
Jan 2, 2010
Messages
1,697
Location
Justin, TX
What might a wing-spanning array of small fans deliver? I recall such a strategy is presently being pursued for electric VTOL craft.
NASA is also testing the concept with its X-57 test aircraft. They are going through a multi-phased program to modify a Tecnam P2006T with a high aspect ratio wing and a distributed electric propulsion system along the whole span.
NASA Armstrong Fact Sheet: NASA X-57 Maxwell
 

Pilot-34

Well-Known Member
Joined
Apr 7, 2020
Messages
583
I wonder how high above the wing accelerated airflow affects lift ?
I suspect a propeller centered a few inches less than half a blade above the centerline of the wing will achieve much the same as the Custer channelwing .
Where in the slipstream would make the most effective flap ?
 

cblink.007

Well-Known Member
Joined
Jul 7, 2014
Messages
506
Location
Texas, USA
I wonder how high above the wing accelerated airflow affects lift ?
I suspect a propeller centered a few inches less than half a blade above the centerline of the wing will achieve much the same as the Custer channelwing .
Where in the slipstream would make the most effective flap ?
Again, brother, there are different props for different applications, different wing airfoils, etc, etc., so your question is too general to get an answer.

Just like any good math problem, we need constants in order to properly solve for the variables...

So you are interested in a seaplane, so...

1. How many people on board?
2. How far/slow/fast do you want to go?
3. Single or multi-engine?
4. Any particular engine you wish to use?
5. What kind of takeoff/landing distance?
6. Will this aircraft be used in a high DA environment?
7. Is there a particular configuration you would ideally want to use (ie conventional, unconventional, etc)?
8. Et cetera...

The physics brought forth by your requirements will drive the solution.

We are only trying to help & educate...
 
Last edited:

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,601
Location
Memphis, TN
The question to ask too is how accurate is the Custer data? Cream tends to rise. I suspect whatever aero advantage it has, 5% more horsepower on a simpler airframe will always win out except in a bet With the right limits.
 

cblink.007

Well-Known Member
Joined
Jul 7, 2014
Messages
506
Location
Texas, USA
The question to ask too is how accurate is the Custer data? Cream tends to rise. I suspect whatever aero advantage it has, 5% more horsepower on a simpler airframe will always win out except in a bet With the right limits.
Look and you shall receive....


Big download...20+MB...

I downloaded it, but havent had a chance to dig into it. But a first glance indicates that the chosen airfoil on the test aircraft, under power, generated a massive nose-down pitching moment, adversely affecting low speed pitch stability.

I'll read it tomorrow while sitting on standby at DCA tomorrow!
 
Top