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

[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.

[JimParker256]

Where is the datum for the 4-place?

[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.

[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.

[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?

[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]

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.

[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

[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.

[Mark Goldberg]

I have been following this thread but reluctant to comment as my CG calculations etc are very lazy and lacking. I did have one idea that I ran by Bob before posting this.

Of course Bob didn't think too much of my idea so I will not even mention it. But Bob, being the design engineer that he is - came up with some very interesting ideas/thoughts.

As far as lengthening the nose - two inches would fix the CG issue but Bob thought it a poor idea because yaw stability would for sure worsen. You would have your old CG range but at the cost of a plane that wallows in the sky much more. It has been tried by others and it was not good.

Bob recommended that you look at lightening the tail of the plane. The tail is so far back that for the 20 lb less prop - you could reduce the weight in the tail by 6 lbs and it would be the same as before the prop change. How could you do that? Bob said the tail is 3X as far back from the datum as the prop is forward.
1) go with the round tail spring. Simon has one and the adapter to use it. But be sure to contact me about this if you decide to go this route.
2) Remove the heavy fabric on the tail surfaces and replace with Oratex. There would be a 2-3 lb minimum reduction in weight on the tail from just this. The elevator especially since using the lighter fabric would allow you to remove lead counterweight that you currently have to balance the heavier fabric.
3) Floorboards in the baggage area - replace .032 that came with your kit with .020 kevlar
4) What do you have for the back bulkhead of the baggage compartment? If it is aluminum, then you could replace with canvas as a lot of guys have done.
5) I know you have electric trim which you are probably very happy with - especially if it been well behaved for you. But the servo in the tail does add weight.

That is all the specifics of what Bob said. But his emphasis was reducing weight in the tail. Mark

[Battson]

As far as lengthening the nose - two inches would fix the CG issue but Bob thought it a poor idea because yaw stability would for sure worsen. You would have your old CG range but at the cost of a plane that wallows in the sky much more. It has been tried by others and it was not good.

bergy FYI - you might be interested.

[Battson]

Bob recommended that you look at lightening the tail of the plane. The tail is so far back that for the 20 lb less prop - you could reduce the weight in the tail by 6 lbs and it would be the same as before the prop change. How could you do that? Bob said the tail is 3X as far back from the datum as the prop is forward.
1) go with the round tail spring. Simon has one and the adapter to use it. But be sure to contact me about this if you decide to go this route.
2) Remove the heavy fabric on the tail surfaces and replace with Oratex. There would be a 2-3 lb minimum reduction in weight on the tail from just this. The elevator especially since using the lighter fabric would allow you to remove lead counterweight that you currently have to balance the heavier fabric.
3) Floorboards in the baggage area - replace .032 that came with your kit with .020 kevlar
4) What do you have for the back bulkhead of the baggage compartment? If it is aluminum, then you could replace with canvas as a lot of guys have done.
5) I know you have electric trim which you are probably very happy with - especially if it been well behaved for you. But the servo in the tail does add weight.

That is all the specifics of what Bob said. But his emphasis was reducing weight in the tail. Mark

Thanks for sharing Bob's thoughts Mark. He is always full of good ideas!

I am tempted by the tail spring change.... however on days like yesterday when we load our plane to the maximum and take-off from a very rough airstrip, I think about all the punishment which the tail spring tolerates. Having a heavy-duty spring is my main concern.

[Mark Goldberg]

The four place tail spring is 6150 steel hardened to around 60 Rc. It is made from 1 inch bar stock which tapers down to 3/4" where it fits into the tailwheel. You can run the numbers. It is my understanding from Bob that going this route does not lessen the toughness of the tailsprings. MG

[Battson]

The four place tail spring is 6150 steel hardened to around 60 Rc. It is made from 1 inch bar stock which tapers down to 3/4" where it fits into the tailwheel. You can run the numbers. It is my understanding from Bob that going this route does not lessen the toughness of the tailsprings. MG

What is the weight saving with the round spring?

[Mark Goldberg]

I do not know for sure Jonathan. But Simon has both and he could probably help by weighing them. In your case you would need to include the adapter with the weight of the round spring to know the weight savings. I had a set of the flat springs but just took them to my ranch where a customer is supposed to pick them up later this week. So I do not have a set to weigh. If for some reason Simon can not help - let me know. Mark