Making your Bearhawk too light - the best way to ruin a good airplane!

[Battson]

In your recent reworking of your plane, did you add profiling to the horizontal stabilizer, making it more efficient? Any other aerodynamic or weight possibilities adding into this mystery?

No other changes apart from those listed.

The main point of this thread is to alert builders to the consequences of focusing only on weight, without thinking hard enough about the balance. This could have been easily addressed during my rebuild, had I realised the consequences at the time.

I honestly believe you want the CG right at the forward limit, or slightly beyond, with the plane empty. That has been my experience.

[Battson]

Battson, can you elaborate on this for the benefit of those of us who haven't decided on a prop yet? I'm sure you are seeing lots of differences in performance with the aft CG at light weight, electronic ignition, wing tips, etc., but how much of that do you attribute to the prop and why?

The main benefits of the prop are from the blade design and low rotational inertia (ironically). There are a few main points:
- The speed with which it spools up and the shape combine to provide much better thrust for takeoff.
- Climb performance is also improved but hard to say if that's the wingtips or prop.
- It also works as a kind of speed brake, because of the large blades and ability to spin up quickly at the end of a descent.
- The vibration in the airframe is much reduced too, again a benefit of the lower weight.
- Lastly, ramp appeal is improved.

The main downsides are:
- the impact on CG, which can be addressed easily during the build
- the vulnerability to stone chips when taxiing / taking off, off airport

[marcusofcotton]

To the people using fixed pitch props..... sell them immediately! Quite apart from the performance sacrifices, you might as well have a two place aircraft.... Seriously. The empty CG needs to be forward of the limit to get any real utility from the aircraft once you start loading it up.

Heath's Bearhawk (Calendar pics: Jan '16 on wheels @ 1366#, Jul '17 on floats @ 1636#) has a Catto three blade that he's been real happy with. I'm thinking he probably loads it up pretty well at times with family trips. No idea on his cg location.

[nichzimmerman]

The new prop performs a lot better, definitely worth keeping.

Battson, can you elaborate on this for the benefit of those of us who haven't decided on a prop yet? I'm sure you are seeing lots of differences in performance with the aft CG at light weight, electronic ignition, wing tips, etc., but how much of that do you attribute to the prop and why?

[marcusofcotton]

In your recent reworking of your plane, did you add profiling to the horizontal stabilizer, making it more efficient? Any other aerodynamic or weight possibilities adding into this mystery?

[Battson]

It sounds like the easiest fix is to put the old prop back on. It performed great and it makes more sense to me to have the extra weight in the prop than ballast in the nose bowl. Of course I'm sure you just paid a fortune for the new prop so that might not be something you want to consider just yet.

The new prop performs a lot better, definitely worth keeping. I am going to try and move the weight around and get some CG back (or CG forward, should I say) that way. I can extend the engine mount if I have to, big job but possible.

[Battson]

The LE of the wing.

[Battson]

Battson,
If your math is correct, an o-360 Bearhawk would be unflyable. I don't have the measurements, but 10 kg out of the prop does not equal 2 90 kg people in the rear seats from a cg perspective. The Surfly ignition is only slightly lighter than mags. I think I read 5 or 6 lbs each.

Just a swag: the prop looks about double the distance from he cg than the rear seats. 10kg out of the prop would mean 20kg less weight available to sit in the rear seats.

It doesn't need to EQUAL the 90kg person, it just tips you beyond the balance point whereby a full grown human male is too heavy and overbalances the plane! I don't know a way to chop 20 to 30 kg off a person and take them flying....

Test flying has confirmed the CG moved aft considerably. I did find a small mistake in the maths, but it only accounted for a few centimetres.

[JimParker256]

Where is the datum for the 4-place?

[Bdflies]

I'm with you! Haven't done the hard numbers, but at first blush - something doesn't pass the litmus test of "what am I missing here?"

Bill

[Rollie]

It sounds like the easiest fix is to put the old prop back on. It performed great and it makes more sense to me to have the extra weight in the prop than ballast in the nose bowl. Of course I'm sure you just paid a fortune for the new prop so that might not be something you want to consider just yet.

[svyolo]

Battson,
If your math is correct, an o-360 Bearhawk would be unflyable. I don't have the measurements, but 10 kg out of the prop does not equal 2 90 kg people in the rear seats from a cg perspective. The Surfly ignition is only slightly lighter than mags. I think I read 5 or 6 lbs each.

Just a swag: the prop looks about double the distance from he cg than the rear seats. 10kg out of the prop would mean 20kg less weight available to sit in the rear seats.

[Archer39J]

Yeah we're still talking about a 540 here right?

[jaredyates]

If you'd like for us to check your math, post your weight at each wheel at the first weighing, and at the recent one.

This is just arithmetic, not like the black magic of oil temps and cooling drag. You've still got way more weight forward of the datum than any of we 360 operators, signs are definitely pointing to a measurement or calculation discrepancy. We should be able to account for the weight changes in calculations to verify the measured results.

[jaredyates]

I'm thinking the same thing!