New Bearhawk owner with some burning questions

[Baloo]

Hi Everyone!

This is my first post here as I am a new Bearhawk owner. I recently purchased a beautifully built 4 place from a local builder who did a fantastic job building a beautiful airplane. I am currently in the process of getting some tailwheel training as my previous 4000 hrs have all been in nosedraggers ranging from C172s & Twin Otters to Learjets.

Before I get to my questions here are a few tidbits about the aircraft to better help get answers
- 4 Place Bearhawk
- Lycoming IO-360 200hp Engine
- Dual MGL Odyssey Gen 2 Efis
- Empty weight 1560lbs
- Goodyear 26" Tundra Tires and an Alaska tailwheel.
- Most of the flying so far has been done close to sea level 3000ft and below.

Questions:
1. The airspeed indication on the MGL efis units seems to be a bit off. I have run them through calibration with a Pitot Static test kit from the local AME and they check out ok on the ground with a few calibration tweaks but in the air they seem to have quite an error. In cruise I am struggling to get anything more than 90kts indicated. It seems to be consistently under reading in the "high speed" level flight regime and slightly over reading in the "slow flight" regime. The pitot and static ports are located on a long pitot tube on the left wing. Any ideas?

Is 90kts reasonable for this aircraft with the big tires? From experience it feels like I am moving faster. Ie. On downwind with 24" and 2400rpm I am indicating 75ish kts but experience and the seat of my pants tells me I am going much faster. The approach speed with flaps 25 seems to be around 55kts is this correct?

2. Tire pressure: What is the optimum tire pressure for these wheels? For the time being I am operating mostly on pavement until I get comfortable with the aircraft.

Thanks in advance for your answers it looks like you have a great community here.

Baloo

[N942VT]

You should see more than 90 kts. My set up is an O360 constant speed, just put the 26" back on for the winter and cruise is right at 110 kts, stall at 36 kts. See if you can confirm with GPS speeds. You can run those tires as low as 15-18 psi but keep an eye on the valve stem to watch for the tire slipping. Congrats on the new plane, that one always looked like a good one.

[Baloo]

Thanks 2VT! I have been looking at the gps as well and unless I have a tailwind in every direction I am flying it it would seem that I am getting closer to 100-108 kts at a modest power setting down low. Any idea why even with the pitot static calibrated on the ground with the testing equipment it is still miss reading in the air?

What kind of altitudes are you seeing 115kts with your setup? I was stalling clean around 45kts and flaps 25 at 42kts. So the slow speed end of things seems to be indicating correctly but anything over 60kts seems to be a bit out of wack.

[Battson]

Welcome to the group, Baloo,

As you know, the pitot system works on pressure difference. With the right test gear you can easily test this on the ground, and simulate any altitude or airspeed. You are looking for pressure leak-downs, and correct differential.

What you're unable to simulate with test gear is the airflow conditions around the static ports.... we have seen bad airspeed readings on Bearhawks before, due to poor choice of static port placement. Dual static ports on either side at an aft fuselage station are known to work. Ports like a Cessna on the boot cowl are known to give "mixed" results, some are fine, others aren't.

Two Victor Tango is right, you will see no problems with 15psi in those tires as they are, and that is plenty low enough for most operations. I stick around 15 most times. For VERY rough or soft surfaces you can go as low as 8psi safely, but you would want to install bead-lockers (see instructions here) if you plan to use full brake pressure regularly at really low tire pressures.

[N942VT]

110 kts is at 23 squared at 3000 ft or less. Your GPS sounds more accurate if your seeing 108 kts. Is your Alaska Tailwheel the baby bushwheel, those are more draggy than you think. I switch back and forth from winter to summer with 31" Bushwheels and baby Bushwheel to 26"Goodyears and standard tailwheel and the difference approaches 20 kts.
Good point on the static ports, I have one on each side just behind the baggage compartment station and that seems to work well.

[Battson]

My static ports are in the same place.

[Bcone1381]

"Usually placing a dam/riser in front of the port will lower the pressure across the port (think flute). This RAISES indicated speed."

Here is link to a discussion on the Vans Airforce Website that might provide a fix with out moving your static port.

http://www.vansairforce.com/community/showthread.php?t=78489

[Baloo]

Thanks for some great feedback!

My current setup is a pitot/static tube. I was thinking that it might be an incorrect mounting angle. Let's say it was mounted at too low of an angle relative to the chord of the wing. In slow flight it would be receiving more ram air than one might expect and in cruise the low mounting angle might prevent it from taking in all the ram air.

Maybe this is far fetched but it is the only scenario that explains fast indication in the slow regime and slow in the fast regime, unless I'm missing something.

Re: Tires, I had them up to 17psi and felt like tigger on a pogo stick when landling I've since reduced the mains to 10.5psi and the baby bush wheel on the tail to 8.5.

I'd love to get more feedback!
B

[Battson]

Where are your static ports? On the pitot tube itself? Do you have a pair, or just one port?

You have already pressure tested both the pitot and static systems?

If so, I would retro fit a pair just aft of the rear bulkhead, and see if that helps.
It would be a VERY easy modification, as far as mods go.
Run the static line under the floor cable tied to the structural tubing. Use Aircraft Spruce rivet-on static ports, put them over the fabric on the outside, rivet them onto a simple 0.025 (ish) Aluminium back plate on the inside of the fabric, which is in turn riveted onto the stringer. Put one port on each side in the same place, to cancel out any yaw effects or horizontal movement of air due to turbulence. A very, very easy mod. Probably 2-3hrs work at most.

[whee]

Congrats Baloo! I get totally lost when you guys start talking in kts. If Google figured it right the BH I have time in was a 105kt airplane on small tires but it was missing some fairings. I think Battson has a great suggestion on moving the static ports to the aft fuselage.

I'm curious what brought the weight to 1560lbs. Not that it is overly heavy, I'm just curious.

[Baloo]

Thanks for the awesome feedback.

I have ordered a pair of static ports : https://www.aircraftspruce.ca/catalo...aticports6.php

In the meantime I have disconnected the static line from the pitot static tube and have left it loose in the wing cavity (unless you all think this is a terrible idea) I figure that I will fly it once taking static air from inside the wing and see if that changes my readings for the better.

In terms of static port placement is there any advantage to putting them on the empennage instead of just forward of the main cabin doors aft of the firewall (this is where they were located on my Cessna) it is also very close to the instrument panel and is aluminum which will make for very easy installation.

In terms of weight, it is a bit heavier because the builder put aluminum to the rear sill of the doors to avoid fabric damage while climbing in and out also the interior is completely finished in carpet, floors walls and headliner. It's a bit heavy but looks like a million bucks.

[Battson]

As I mentioned before - placing the ports in the usual Cessna location has mixed results on the Bearhawk. Sometimes it gives a similar effect to what you're seeing now, according to the old Yahoo Bearhawk group members. I would stick with the proven location of behind the aft baggage bulkhead, to be safe. You don't want to put holes in the wrong place!

I don't see any issue with doing one test flight with the static line drawing from inside the wing, provided it can't disappear into the wing.

[Baloo]

Ok thank you! I will report on how it goes in the wing and new location aft of rear baggage bulkhead.

[Battson]

Just to confirm - you already pressure tested the existing set-up? (being careful not to damage the transducer, must disconnect and plug)