Source: 2023Q3 Beartracks, Russ Erb
Starting in the mid-1990s, the Bearhawk was first introduced to the homebuilt scene when Bob Barrows, the designer, would famously park the prototype at airshows such as AirVenture and Sun ‘n Fun with the flaps full down. It was an impressive sight with those long span flaps deflected down 50 degrees. “Everyone knows” that flaps allow an airplane to land slower and thus shorter. If you are trying to sell a design as a STOL airplane, showing a lot of flap is certainly one way to do it. (Personally, I built the airplane because it was a high wing four place, not because of STOL back country capabilities. The wheel pants give away that this airplane mostly lands on pavement.)

The reality is that the flaps don’t actually deploy that much while in flight. While the actual deployment is sufficient, as we will discuss later, it just doesn’t look like it does on the ground. Anyone who has flown a Bearhawk with the flaps deployed and bothered to turn around and look at them while airborne will notice the difference. It also turns out to be a good thing that the flaps don’t deploy any more than they do.
A while back, Jared Yates was talking with Bob Barrows about how much the flaps actually do deflect while in flight. They both knew it was less than the flaps deflect on the ground, but no one had ever actually measured the deflection. Seeking to find out, Jared contacted his favorite Bearhawk flight tester to investigate this issue and find out what we could learn about it.
I had always assumed the difference was because of “cable stretch” but hadn’t bothered to actually investigate this hypothesis. Now that someone else was asking the question, it was time to design a flight test program.
The Difference Is Visible
Since you may not have your own Bearhawk to go fly and see for yourself, let’s take a look at some pictures taken during our flight test. Each of these pictures is taken the with the camera in roughly the same position, so you can compare the pictures directly.
In Figure 2, the flaps are three notches down on final approach at 65 KCAS, which is supposedly 40 degrees of deflection. Note the size of the gap between the trailing edge of the flap and the trailing edge of the wing root fairing and compare it to Figure 3.
In Figure 3, the flaps are three notches down, but with the air loads removed. Notice how much larger the gap is between the trailing edge of the flap and the trailing edge of the root fairing compared to Figure 2. This very noticeable difference in deflection in the air and on the ground for the same flap lever position is what led to this investigation.


Flight Test
The test item for this test was Bearhawk #164 “Three Sigma”, which is a four place Bearhawk of the original design (Model A). The flaps of the Model A stopped at a wing root fairing about 12 inches from the fuselage. Later Bearhawk designs have flaps that extend right up to the fuselage.
The flaps were mechanized in a Piper style design. A flap lever pulled a cable, which pulled an arm on a torque tube at the wing root, and another arm on the same torque tube pushed a pushrod that deflected the flap. Flaps were held up/pushed up by air loads (lift) in flight, and were held up by springs while on the ground. The flap cable ran from the flap lever by the pilot seat, underneath the floor, and up behind the rear cabin bulkhead. Through a fitting and three turnbuckles, the flap cable formed a “Y” with the upper two branches running to the top of the fuselage and forward to each wing root, where they attached to an arm on each torque tube. The total length of cable from the flap handle to the torque tube arm, minus fittings and turnbuckles, was approximately 172 inches.
Results
The measured flap deflections are shown in Table 1 and in Figure 4. (Flap deflections measured at flap limit airspeeds)

Notches	Nominal Deflection Degrees	Ground Degrees	Flight Degrees
0	0	4.3	-1
1	15	14.6	2.2
2	25	29.1	6.4
3	40	42.8	17
4	50	55.1	N/A

In Figure 4, the black dots show the nominal flap deflections, i.e. the values shown on the plans.
In-flight data at four notches of flaps were not collected, because at an airspeed of 55 KCAS I was not able to pull the flap handle more than about halfway from the third notch to the fourth notch because of high aerodynamic forces. Tests later with a force gauge estimated that the required force to pull on the last notch of flaps was in excess of 75 pounds, which was more than my (apparently wimpy) arm could pull. I tried multiple times with no success. I do not routinely use the fourth notch of flaps, but I have been able to deploy it in flight on a previous occasion. I don’t know what was different this time. Bob Barrows told me he has to slow to almost stall speed to get the fourth notch out. I’m not sure how smart that is close to the ground. He also said the new arrangement with the flap lever raised from the floor makes this somewhat easier by changing the muscle groups required.
The most significant thing seen in Figure 4 is that the in-flight deflections are around half of what was expected based on the ground results. This is consistent with what we saw earlier in Figures 2 and 3.
On the ground with no air loads, my flaps have a resting deflection of 4.3 degrees while the flap lever is in the zeroth notch. I made this adjustment years ago when I realized that with the flaps adjusted to have the trailing edge line up with the wing root fairing on the ground, airborne the flaps did not move at all when pulling to the first flap notch. Thus, I shortened the flap cable slightly so that the flaps would actually move slightly at the first notch. In flight, the air loads push the flaps up to the up stop, stretching the cable as required.
Bob Barrows told me that when designing the Bearhawk, he expected that the actual in-flight flap deflection of the flaps would be less than the deflection seen on the ground. He also expected that the flaps would deflect more as the airplane slowed down because of lessened dynamic pressure, causing the flap drag coefficient to increase. The flight test we did was only done at the flap limit airspeeds so as to capture the minimum deflections.
Analysis
My first suspicion was that the flap cable was stretching under load, resulting in reduced flap deflection. There are two ways stranded cables stretch when under load: elastic stretch and structural stretch. Elastic stretch refers to stretching caused by elongation of the wires in the wire rope. That is, the metal strands act like a really stiff spring. Conversely, structural stretch is caused by the adjustment of wires and strands, lengthening of rope lay, and compression of the core. Structural stretch is the larger of the two by roughly a factor of 10. Even so, the structural stretch should be less than 1 percent of the total cable length.
Can 1 percent stretch or less explain the difference seen in flap deflection? Yes it can. Because of mechanical advantage, the cable doesn’t move much when the flaps are deflected. The cable is about 172 inches (over 14 feet) long, so 1 percent is over an inch of stretch. A more detailed analysis predicted a total cable strain of just under 1 percent, as expected. (Bob notes that twisting of the Flap
Torque Tube also accounts for some of the system flex).
This article is a summary of the results. For the full story on how this test was done and the full analysis, go to http://erbman.org/trailingedge/2308%20in%20flight%20flap%20deflections.pdf.
Can We Do Better?
If by “better” you mean get more in-flight deflection, then probably not with the current design. But I contend that there is little to be gained with more deflection.
The aileron control cables and elevator control cables are each a complete loop, which we pre-tension with turnbuckles. This absorbs the structural stretch in these cables so that we can then make precise control inputs. With no other guidance to go by, I adjusted my cables such that when the control surface hit the stop, I could not push the control stick enough (with a reasonable force) to cause slack in the return cable. Knowing what I have learned in this investigation, I feel confident saying that I put enough tension into the cable to use up all of the structural stretch, leaving only the elastic stretch.
There is no pre-stretch in the rudder cables other than what you put there with your feet because the cables are not a complete loop. This is acceptable because, unlike the elevator or ailerons, there is no significant aerodynamic lift on the rudder in its neutral position, and the required precision of control is less.
The cable that we use for the flaps is not pre-stretched, which is apparent because you can grab the strands and easily untwist them. The Bearhawk flap cables don’t see enough tension for the structural stretch to take a set, which allows the cables to remain flexible, which is important when pulleys are involved.
You could always shorten the flap cable to get more in-flight flap deflection, but this will cause two problems. First, depending on how much ground deflection you add, you may add enough that the cable won’t stretch enough for the flap drive to hit its hard retracted stop, meaning that all of the flight loads on the flap will be carried by the flap handle mechanism. Second, doing this preloads the flap system, so it will take even more force on the flap handle to move the flaps into the desired deflection.
My operational experience would point to the conclusion that the flaps are fine as they are. My investigation into the best configuration for an instrument approach showed that two notches of flaps was the minimum deflection needed to control airspeed for the final approach and landing. For 13 years I have generally used three notches of flaps for final approach, which allows for a slower approach speed and produces sufficient drag to maintain an approach angle steeper than the 3° ILS/PAPI/VASI glideslope if desired. Yes, the flaps don’t deflect as much in-flight as the plans claim they do, but the actual deflection is sufficient for the mission as designed. Thus, no changes are recommended.
Besides, full flaps still look cool at fly-ins.