Increasing Angle of Incidence

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

keith103

Well-Known Member
Joined
Nov 12, 2013
Messages
127
Location
Aurora, Denver,CO
This is more of a hypothetical question, but I am interested to learn.

The positive angle of incidence on the MiniMax airplane's wings is made possible by rigging the bottom the rear spar 1 inch lower than the bottom of the front spar. Obviously we need to keep the fuselage horizontal before measuring this differential and only then attach the wings. I think this translates to an angle of incidence of about 3 degrees measured across the chord on the flat bottom of the wing.

I was just trying to imagine what would be the result if we were to lower the rear spar by 1.5 inches instead of the standard 1 inch, to provide possibly another 1 to 1.5 degrees of incidence making a total of 4.5 degrees instead of 3 degrees.

I am certain total drag will increase. Will the stalling speed reduce or increase ? How about landing speed ?

Will the plane require a more powerful engine to overcome the increased drag? Will there be other changes in how the plane handles ?

This is a basic aerodynamic question, but I referred the MiniMax only because I wanted to try it on a MiniMax.

Why would I do this ? I surely would not do it if I were to fly from point A to Point B, because the increased drag would reduce performance and range. But if I wished to loiter in the local flying area while flying at 50 feet, and gaze down to admire the farmland or the river, then it may be more fun flying slower.

Thanks
 

jedi

Well-Known Member
Joined
Aug 8, 2009
Messages
2,805
Location
Sahuarita Arizona, Renton Washington, USA
You are correct in thinking it will benefit the slow flight at the expense of the "high speed" flight but I do not think that is the primary change to be considered.

The IMHO largest positive change is the improved over the nose visibility in slow flight.

Other factors to consider is the change in takeoff and landing characteristics caused by the additional 3 degrees AOA when in the 3 point attitude. This may allow a shorter takeoff run if the rotation angle is limiting the lift off speed. It will also make the full stall landing more critical towards reaching the stall speed prior to touch down to avoid the typical J-3 bounce. The ground handling effects can be simulated or negated by changing main wheel diameter or tail wheel modification.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
8,489
Location
Saline Michigan
Rules of thumb:
  • The wing flies while the fuselage goes along for the ride
  • The tail works relative to the wing
  • The engine and prop should pretty much be aligned with the relative wind at the airspeed of most importance to you for prop power
So, what can setting the wing at any incidence mean?

The airplane flies at 1g in wings level climbs, level flight, and descents at an AOA based upon airspeed and weight. Less airspeed and/or more weight takes more AOA. More airspeed and/or lower weight and less AOA. Stick the fuselage under the wing with the wing at higher incidence, and it flies more nose down over the whole range of flight conditions. So, I suggest a change in your frame-of-reference: You are shifting the fuselage under the wing.

You can see more over the nose, but maybe the fuselage is draggier.

What else changes? Downwash off the wing and general flow of air from wing over the tail is about the same too, regardless of fine adjustments of fuselage to wing angles. If you are going to put the wing on the fuselage at higher incidence, you should probably make a similar change to the horizontal stabilizer incidence so that they work together as planned. So now your view from the frame-of-reference is to hold the wing and tail angles and rotate the fuselage under the wing and tail...

Engine angle to the relative wind should be about directly into the wind. On tractor airplanes, zero yaw and zero pitch to the relative wind in cruise has been used but most of us like our airplanes better with a tiny bit nose down and nose left (conventional prop rotation) - there is another thread active right now on this. So now we have the wing and tail and prop incidence angles being held while we rotate the fuselage underneath the wing and tail and prop angles.

Now for ground interactions. If the airplane sits on all three wheels of a conventional (traildragger gear) at a full stall landing, and you reduce the deck angle 1.5 degrees, you will now do a full stall landing on mains only. If instead, the tail hits first on a full stall landing, you may convert it closer to a three point landing. If your engine CG is in the same position relative to the fusealge, the nose down fuselage will have the prop tips closer to ground in all maneuvers. I do not know how much prop clearance you had in the plans, but it will be less with the deck rotated nose down. Maybe on wheelies and on firm touchdowns with gear legs deflecting, you will be picking up more debris than before, or maybe even have prop strikes.

See all these tails to changing something on the airplane. You get to investigate all of this and see what it does for you or does not do for you on your airplane. Welcome to airplane design. Are other people doing this? How have their airplanes fared? Good? Bad? Prop damage?

Billski
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
8,041
Location
Rocky Mountains
The positive angle of incidence on the MiniMax airplane's wings is made possible by rigging the bottom the rear spar 1 inch lower than the bottom of the front spar.
This only adjusts the AOA relative to the fuselage, not the air you fly thru. To get a more complete picture of how this changes the performance of the plane you also need to consider what it does to the decalage - the angle between the wing and the horizontal stabilizer.

It sounds like a little more study of aerodynamic basics would help? I know it is a book for model airplanes, but it does a good job of presenting the fundamentals in an easy to understand way.

Model Airplane Aerodynamics

Edit:
If you get serious about this area of study you will also need to study some related engineering concepts. This is my recommendation for the first step in that direction:

Jeff Hanson on YouTube
 
Last edited:

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,380
Changing the incidence will not lower the stall speed. It adds drag that might even raise the stall. Stall speed depends on weight and how much lift the wing can generate at high AoA, not on incidence.
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,914
Location
USA.
When Piper designed the PA-12 they changed the incidence of the wings and stab so the fuselage did not fly with the tail up as high as the J-3. Fuselage flew less nose down with less drag. The PA-12 also has a different lower drag airfoil for the wing struts, streamline covering over the LG bungees, and wing fuel tanks. Change in fuel tanks so it can be soloed from the front seat. All of the drag reduction resulted in 17 mph higher cruise than the J-3.
 

challenger_II

Well-Known Member
Joined
Jul 15, 2009
Messages
610
Location
Fisher County, Tx. USA
Two items to consider:

The angle of incidence is not measured from the bottom of the wing. It is measured from the trailing edge through the center of the radius of the airfoil.

If One were to increase the angle of incidence at the tips, this will increase the probability of the tips stalling before the inboard portion of the wing, i.e: tip stall.
 

keith103

Well-Known Member
Joined
Nov 12, 2013
Messages
127
Location
Aurora, Denver,CO
Thanks all, for sharing your thoughts. A consideration is that since the thrust line is not being changed, the fuselage should continue to travel the same path as before. In other words, if the fuselage is allowed to re-position so that the wings get the same old angle of incidence, then effectively we are not changing the angle of incidence. Instead, just the fuselage is pointing more nose down.

I guess the horizontal stabilizer and elevator trim may need adjustments to maintain same attitude of the fuselage as before, so as to make the new incidence effective.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
15,865
Location
Memphis, TN
The plane will want to balloon with speed increases. You will make more lift per speed because of the angle, but it will take a lot of trim to stay at one speed. Speed up and trim will go out the window. Slow down will reverse it. Incidence is set as what the plane needs to lift its weight. If it was double the weight, you could increase the incidence to counteract the weight, you don’t have the excess lift of the half weight one so lift has to come from somewhere if you can’t go faster. Add unnecessary incidence and the airframe has to dissipate the excess lift with trim. At an extreme you could use it like you only wanted to fly at 30 mph. You can tailor it for one speed. A good average speed range needs the least amount of incidence you can get away with.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,380
Thanks all, for sharing your thoughts. A consideration is that since the thrust line is not being changed, the fuselage should continue to travel the same path as before. In other words, if the fuselage is allowed to re-position so that the wings get the same old angle of incidence, then effectively we are not changing the angle of incidence. Instead, just the fuselage is pointing more nose down.

I guess the horizontal stabilizer and elevator trim may need adjustments to maintain same attitude of the fuselage as before, so as to make the new incidence effective.
I think you need to review the difference between angle of incidence and angle of attack. Two very different things. As I said before, you won't get more lift at lower speed by increasing the angle of incidence. That wing will generate the same amount of lift at any given airspeed and angle of attack. There is no free lunch. All you will achieve is a nose-down fuselage position that will likely increase drag and slow the airplane, and slowing the airplane reduces lift.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,380
Two items to consider:

The angle of incidence is not measured from the bottom of the wing. It is measured from the trailing edge through the center of the radius of the airfoil.

If One were to increase the angle of incidence at the tips, this will increase the probability of the tips stalling before the inboard portion of the wing, i.e: tip stall.
That's a washout change. Incidence is measured at the wing root, and any washout is specified from that point.

"Tip stall" is not a common term here. It's common in RC circles, and it carries the misconception that the wing isn't stalled until the tips stall. If that was true, all we would need is the tips, not the whole wing. Most wings are designed to start stalling at the roots, at the trailing edge, and work forward and outward as the AoA increases. A rectangular wing with no washout will stall that way with no washout, but many such wings will have some washout to tame the stall further. The typical spam can will almost never stall the tips; the wing's area largely stalls and drops the nose, leaving the tips, and ailerons, still flying. It's a safety feature that costs some performance.
 

challenger_II

Well-Known Member
Joined
Jul 15, 2009
Messages
610
Location
Fisher County, Tx. USA
You are correct... to a point. What the OP is suggesting is to adjust the aft strut to increase the angle of the outboard section of the wing, in essence, adding wash-in. this will cause the outer sections of the wing to stall before the inboard section.

"Tip Stall" isn't a common term, here, because designers, and manufacturers, have been designing in wash-out to aircraft since the First World War, to tame down the stall characteristics of aircraft. We, as a group, have become accustomed to it, and we do not discuss it, as we take it for granted. As for your comment about "the wing isn't stalled until the tip stalls" that is a fallacy that no one I have ever dealt with has proposed.

Model airplane enthusiasts discuss it, because many of them are on the learning curve of what makes an airplane fly right. At the end of the day, my 26" Aeronca Tandem requires much the same aerodynamic massaging as the full-size TA-65 in the hangar next to me to fly right.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
8,489
Location
Saline Michigan
A consideration is that since the thrust line is not being changed, the fuselage should continue to travel the same path as before. In other words, if the fuselage is allowed to re-position so that the wings get the same old angle of incidence, then effectively we are not changing the angle of incidence. Instead, just the fuselage is pointing more nose down.
That is not the way it works... The fuselage pretty much goes where ever while the wings fly in the air and need a set AOA for any given airplane weight and airspeed. To fly in steady unaccelerated flight, the wing must make lift equal to the sum of airplane weight, tail download, and any other vertical forces present. To make the same lift from the same wing flying at the same airspeed takes the same AOA for the wing. The fuselage basically does not lift, nor do the aerodynamics pay much attention to the direction of the fuselage. But the the wing does. You fly the wings, and the fuselage goes along for the ride, not the other way around.

Go fly an airplane, hold altitude, fly it fast and note the deck angle by referencing the horizon to the windshield frame. Pull the power back, hold altitude, the airplane will now fly level with the deck angle increased to raise the AOA enough for the wing to make the same lift with less airspeed.

We are not making this up, and you can look up the topic.

I guess the horizontal stabilizer and elevator trim may need adjustments to maintain same attitude of the fuselage as before, so as to make the new incidence effective.
You can raise the fuselage angle relative to the wing, and leave the horizontal tail fixed on the fuselage, and it can still be flown, but now instead of having the elevator streamline at something close to min drag behind the stabilizer, the stab will now be more nose down (relative to the wing), and trying to make more downforce than before. Since we balance the airplane with elevator, we would end up running the elevator more trailing edge down than before and usually that makes for more drag than with the elevator trailing the stabilizer. Better scheme if you want to change incidence of the wing is to maintain the angle between wing and stabilizer while you adjust fuselage angle. Same deal occurs with the powerplant.

Billski
 
Last edited:

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,380
As for your comment about "the wing isn't stalled until the tip stalls" that is a fallacy that no one I have ever dealt with has proposed.
I ran into it among way too many RCers. They kept talking as if the wing wouldn't drop until the tip stalled, which is patently not true. All it takes is enough wing area stalled that it can no longer support the weight on it, and it will fall. And one side or the other may fall first depending on coordination, propeller blast at the wing roots, propeller torque reaction, and so on.

Here are common wing stall progression patterns on wings with no washout or airfoil modifications:

1637376686576.png

You can see that the rectangular wing, besides being easier to build, has a natural stall pattern. Everything else needs various washout or spanwise airfoil changes to tame the stall. Sometimes stall strips need to be installed on the inboard leading edges to start the stall there before it starts farther out. The Bonanza is a famous example of that.
 
Last edited:

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
15,865
Location
Memphis, TN
RC planes won’t stall if they can fly on that tip lift. It’s not different, it’s degree of stall progression. It’s application not different physics. Because no one else is riding to feel a stall, RC planes are more stable despite what they seem. They are also way easier to iterate to get right. I have plenty with no washout on double tapered wings. And I have some rectangular with washout. Of course you get the aerobatic guys who put the stall strips at the tips for better snaps. On a different forum, someone was told by Delmar Benjamin that the R2 would tip stall if he moved the ailerons below 120 mph. 20 above stall. Good times.
 

Vic Bottomly

Member
HBA Supporter
Joined
Nov 24, 2020
Messages
19
Location
Clarkston WA
There are guys who modify super cubs to increase angle of incidence to shorten take off and landing rolls. The idea behind it has a couple things in support:

1. In regular configuration, a 3-point landing is still at a higher speed than level stall speed. In other words, a normal 3-point does not approach the stall angle of attack because the tail wheel hits first. That is supported by noting that you can take off in a 3-point attitude, so obviously the wing is not stalled at that angle.

2. Increasing AOI also allows one to lift the tail to see the stumps you want to avoid while still having a climb angle of attack on take off, or in flare on landing.

Everyone agrees, it will slow cruise speed down. Everyone who does it also changes the stabilizer angles to match the increased AOI.

The same basic result can be had by increasing tire size of the mains. Of course, you do that and land in rough places, you end up wanting a bigger tailwheel, too. And, cruise speed goes down.

'Round and around it goes.... (leaving off discussion of slats and modified flaps).
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
8,489
Location
Saline Michigan
I ran into it among way too many RCers. They kept talking as if the wing wouldn't drop until the tip stalled, which is patently not true. All it takes is enough wing area stalled that it can no longer support the weight on it, and it will fall. And one side or the other may fall first depending on coordination, propeller blast at the wing roots, propeller torque reaction, and so on.

Here are common wing stall progression patterns on wings with no washout or airfoil modifications:

View attachment 118139

You can see that the rectangular wing, besides being easier to build, has a natural stall pattern. Everything else needs various washout or spanwise airfoil changes to tame the stall. Sometimes stall strips need to be installed on the inboard leading edges to start the stall there before it starts farther out. The Bonanza is a famous example of that.
The infamous NACA study that produced the above illustration was a classic case of correlated variables producing misleading results:

Rectangular wing used same chord and same % thickness at root and tip with good progression;
Tapered wings used same camber curve but thicker foil (by %) at root than at tips with poor progression;
Tapered swept wings also used same camber curve but thicker foil (by %) at root than at tips with even poorer progression;
Elliptical also used same chord and % thick throughout with a decent progression (the tiny Re of the tiny tips is known to bias stall outward);
Missing was any attempt to build with tips at same and greater % thickness at tips.

The result is called "correlated variables" and easily results in drawing a conclusion about one variable when a different variable could easily account for the behaviors observed. This is classic poor experimental design and is railed against in training folks on the topic. Generations of airplane designers have "learned" this erroneous conclusion.

Starting with the PA24 Comanche, we have had strong evidence that a tapered wing using the same % thickness throughout can have excellent stall behavior without fixes and washout. Since then, a number of airplane designs have been flown with tapered wings and good behavior. Our own Bill Husa (Orion here on HBA.com, now passed away) has advocated both same foil and % thickness from root to tip and built airplanes this way with excellent behavior.

Let's try to quit spreading the false conclusions coming from a flawed study.

Billski
 

rv7charlie

Well-Known Member
Lifetime Supporter
Joined
Nov 17, 2014
Messages
2,167
Location
Pocahontas MS
It's been a while, but IIRC, the Questair Venture designer (who also designed the Piper Malibu) exploited the inverse of the flaw in that study. I believe that he actually used a thicker % airfoil in front of the ailerons than the % thickness of the inboard section of the wing. Also incorporated a rather ingenious variation on the wing slot....
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
8,489
Location
Saline Michigan
According to what I have handy, the Questair Venture has the 23017 root and 23010 tip, meaning 17% thick root tapering to 10% thick tip, so it would appear to have continued with design feature used in the NACA study.

The technique of using a foil at the tip with a higher AOA for stall is sometimes called aerodynamic twist or aerodynamic washout. This is as opposed to physically using lower incidence at the tip than at the root. Both tend to drive stall behavior that initiates at the root and progresses to the tip.
 

rv7charlie

Well-Known Member
Lifetime Supporter
Joined
Nov 17, 2014
Messages
2,167
Location
Pocahontas MS
Apparently either my memory is flaky on that subject (certainly is in other areas), or the magazine article where I read it back in the early 1990s was in error. The current iteration of the company is local to me, so I'll try to stop in & look at an example the next time I fly over there.
 
Top