Questions About Ballast

[kestrel]

Very good analysis and useful regardless of final choices. One point though...

Also notice that at very light conditions, the CG gets only about 2.5" from the main wheels (mains are at -1.5"). This makes the a/c likely to tip on its nose due to a small upward load on the tail.

This is only true when in a level flight attitude. Because the wheels are well below the height of the CG, in a three point attitude the mass is well behind the main wheels. I take advantage of this when trying to stop short. Once I have bled off some of my touchdown speed (it only takes a moment) I reduce my braking to lower the tail and can then use the brakes more aggressively without concern about nosing over. If the tail is kept up, you have to be more careful and limited on the brakes.

Bearhawk tails are generally difficult to lift by hand, but this movement of the CG can be easily seen on a J-3. It is reasonable, but still an effort to lift the tail off the ground. But, the higher you lift it, the lighter it gets.

[Mark Goldberg]

Bob has frowned upon adding ballast at the tail. He thinks it could make it more difficult to recover from a spin with the added weight in the tail. Mark

[Nev]

Nice work, and mirrors what others have experienced.

It's great having the utility of a very large CG envelope, but it does become somewhat self limiting.
If the empty CG is too far forward then the elevators tend to lose authority at low speed and forward CG.
If the empty CG is too far aft then the aft CG limit tends to be reached before MTOW is reached, and the tail wheel/ aft fuselage is subject to additional stress.
Moving the engine or prop further forward tends to decrease yaw stability due to the large cowl surface area forward of the CG.
Incidentally, I think having a light tail or a "tippy" aircraft is perhaps more a function of the main wheel position. Bearhawks generally tend to be heavier in the tail. It's a compromise either way, but probably the better of two evils. Damage at the aft end is usually less expensive to fix than damage at the forward end.

Speaking of my own aircraft (a 4-place), it handles very nicely when the CG is around mid-point (15-17"). However for backcountry flying I prefer to keep it fairly light, which generally dictates a more forward CG, and with that I need to keep power on at low approach speeds to maintain elevator authority.

[Mark Spickard]

Someone may find this interesting. In the following, the "Max Load" condition consists of 380 lbs in the front seats, 300 in the rear seats, 54 gallons of fuel, and baggage set to whatever is necessary to get to max gross weight of 3000 lbs. The "Min Load" condition consists of 150 lbs in the front seats, 0 in the rear seats, 5 gallons fuel and 0 lbs of baggage. CG limits are defined as 10.5 and 22.5". Moment arms are as follows:

Rfs =	9	Moment arm of front seat
Rrs =	46	Moment arm of rear seat
Rbag =	80	Moment arm of baggage
Rmf =	23.5	Moment arm of main fuel
Raf=	27	Moment arm of aux fuel
Rbal1 =	97	Moment arm of ballast position 1
Rbal2 =	141	Moment arm of ballast position 2
Rmains =	-1.5	Moment arm to main wheels
Rtail =	215.5	Moment arm to tail wheel

Assuming the aircraft is loaded to Max Load conditions, and the CG is at the the aft limit, this plot depicts where the empty CG needs to be as a function of empty weight. Notice that as the a/c gets lighter, the CG needs to move forward. Using a lighter prop or engine does the exact opposite. Also notice that at very light conditions, the CG gets only about 2.5" from the main wheels (mains are at -1.5"). This makes the a/c likely to tip on its nose due to a small upward load on the tail.

image.png

Assuming the CG is successfully placed as defined by the above plot, this following plot depicts how much ballast are needed so as to keep the CG at the forward limit when loaded to Min Load conditions. The blue line assumes the ballast are located against the aft wall of the baggage area. The red line assumes the ballast are located near the tail (141" radius arm). Putting them at the tail represents an increase in moment of inertia which will have an adverse effect on some flight characteristics. Moving ballasts from R=97 to R=141 has a 45% increase in the ballast inertia. Being that the ballasts are a relatively small percentage of the entire a/c inertia, this extra 45% may not be a deal breaker......maybe.

image.png

When at the Max Load condition and the CG at the max aft limit, the blue line depicts the allowed baggage weight…….assuming the ballast are not in the aircraft. It can be seen that as the aircraft weight changes by 200 lbs (1475 to 1675) the allowed baggage changes by 200 lbs. The red line depicts the allowed baggage assuming all required ballast are in the a/c and they are located at the aft wall of the baggage area. The green line depicts the allowed baggage assuming all required ballasts are in the a/c and they are located near the tail. Notice that for the two conditions where the ballast are in the a/c, as the empty weight decreases, the allowed baggage also decreases......counterintuitive, but reality. If you think you are going to need to fly the a/c anywhere near min load condition and thus decide that you need to bring your ballast with then reducing the empty weight of the a/c hurts load carrying capability.

image.png

The following plot is the same as the above except the ballasts are located in the baggage area at R=80. Notice that as the a/c empty weight is decreased by 200 lbs, the additional allowed baggage is only increased about 40 and 94 lbs for the red and green lines respectively. Again, if you think you are going to need to fly the a/c anywhere near the min load conditions and thus decide that you need to bring your ballast with then reducing the empty weight of the aircraft doesn't get you much.


image.png

This plot depicts the lbs/seat capability at the Max Load case. This is simply the sum of all weight in the seats and the baggage area all divided by 4. The blue line is with no ballast in the a/c. The red line is for an aircraft that uses aft baggage ballast, the ballast are in the a/c but they have been moved forward to the baggage area (R=80) . The green line is for an aircraft that uses tail ballasts, the ballast are in the a/c but the ballast have been moved forward to the baggage area.

image.png


From the above plots we see that as the aircraft gets lighter, if you want to use the weight savings to add more fuel and cargo, the empty CG must go forward. This is because as the aircraft gets lighter, we can add more weight to the tanks, rear seats and baggage area.....all of which are behind the CG, all of which move the CG rearward. A point is hit where the CG goes out of range. The only way to fix this is to remove fuel or cargo (and then not getting the max usefulness from your a/c), or to modify the a/c so that the empty CG is more forward. We typically get just the opposite.....we reduce aircraft weight via a lighter engine and lighter prop, both of which move the empty CG backward. How do we get the aircraft lighter for more cargo weight, and at the same time get the CG more forward? Keep the tail light, Use a lighter prop but put in a prop extension. Use a lighter engine but move it forward. Use a light battery but move it to the firewall. Using a CS prop is bad in that its is heavy but very good in that it is way out at the nose. The handiest variable is the battery. Can use a heavy or light battery, and can put it on the firewall or way aft. In general, just keep the a/c as light as possible, then move the battery and any other components to get an unloaded CG that is sufficiently forward that the a/c can be loaded to max weight and still stay within CG limits. We also see from the above, as the aircraft gets lighter, and we push the empty CG more forward, when loaded light the loaded CG gets too far forward and rear ballasts are necessary. In order to keep required ballasts to a minimum, don't put the empty CG any further forward than absolutely necessary for the max load case. Also, consider putting the ballast near the tail. When near the tail, less weight provides the same change in CG. Also notice that as the a/c weight is increased 200 lbs (1475 to 1675), the allowed baggage actually goes up if the ballast are brought along and located in their normal positions. If the ballasts are moved forward into the baggage area (R=80) then the amount of effective baggage only changes about 40 lbs for aft baggage ballasts, or about 93 lbs for tail ballasts.. Also, a heavy 1675 a/c doesn't need any ballast......big engine and heavy but the effective cargo is only 40 lbs less and don't need to carry any ballast ever. Also, the heavy case has a much more rearward empty CG, which makes it less likely to tip on its nose.

Summary: If you build real light and use a parallel valve engine you may be able to get empty weight down to around 1475. In order to get to the Max Load condition and stay within CG limits, the empty CG would need to move to about 0.8" It would be difficult to get the CG this far forward when putting a light engine up front. It would also lead to a tippy a/c. The end result is that you probably would not be able to reach the defined Max Load condition. If you did somehow get the CG this far forward, the Min Load condition would require about 170 lbs of ballast at the aft baggage wall in order to stay within CG limits. If you go on a trip and put sufficient load in the back that you don't need the ballasts, you may want to bring them along anyway just in case a need arises to fly near Min Load conditions. If you do bring your ballast with, and you locate them in their normal positions, you can actually carry more baggage in a heavy aircraft than you can a light one.

If you build real heavy and use an angle valve engine you may have an empty weight as high as 1675. In order to get to the max load condition and stay within CG limits, the empty CG would need to move back to about 10". This may be difficult even if a lead-acid battery is moved way back in the tail. If you did get the CG this far rearward, the min load condition would not require any ballast.......max load would put the CG at the aft CG limit, min load would put it at the forward limit.

Now consider something more in the middle......maybe 1575 lbs. This could be a light airframe with an angle valve engine, C/S prop, lead-acid, etc. This would require a reasonable empty CG of about 5.7" (I believe the prototype 5 has an empty CG a bit below 4"......and if the mentioned design changes bring the engine back a couple inches then 5.7 seems easily doable). It won't be tippy. At Min Load conditions it would require about 80 lbs of ballast at the aft baggage wall which could be provided via 2x5 gallon collapsible containers filled with water.....which weigh about nothing when empty. Assuming you can always find water, your ballast are always at your destination waiting for you. Allowed baggage at the max load condition is a respectable 425 lbs. With 54 gallons of fuel, you have about 275 lbs/seat of useful load.

[Redneckmech]

Thanks for sharing!

[DavidGA2]

https://www.kitplanes.com/wind-tunnel-2/

Try this link for the article.

[DavidGA2]

Thanks Bill...that is very helpful. Even though I have Bearhawk 4-place plans, I am finishing a plans built that was enlarged in every dimension including being stretched 27". It is similar to the 5-place and is built for a M14-P engine. I realize this will make me a test pilot since my fuselage is very different from the plans. It is helpful to see how the new 5-place tested. I have attached an interesting article that discusses the effect of extending the tail on allowable CG. Thanks again!

[Redneckmech]

Has the Balance been released for the Model 5? I have plans for a 4-place and I know the CG range is 10.5 to 22.5" from the leading edge for it. However, the 5-place is 24" longer and the extra leverage normally results in a wider CG range. This would be very desirable as the longer moment arm for the rear seats and baggage compartment will move the CG further back with load than a 4-place.

Per my BH5 plans the CG range is 9.0"-21" PXL_20211123_140727013.jpg

Bill

[DavidGA2]

Has the Balance been released for the Model 5? I have plans for a 4-place and I know the CG range is 10.5 to 22.5" from the leading edge for it. However, the 5-place is 24" longer and the extra leverage normally results in a wider CG range. This would be very desirable as the longer moment arm for the rear seats and baggage compartment will move the CG further back with load than a 4-place.

[way_up_north]

I purchased Model B wing planes today with full intention of scratch building BH5 on 3k anphib floats. Knowing nothing about the BH WB. 1st reaction is add a double wall rear fuel tank along with a high volume transfer pump. However I am talking out of turn because I know nothing of the problem as this moment.

Welcome to the club...lots of great guys here to help each other out...and whats great is the factory is always there to sell you parts to speed things along. I`m sure as we move along with this build many interesting ideas will come up to deal with the ballast...

[way_up_north]

I was reading online about ballast and aircraft... it looks like the Bearhawk 5 has joined the ranks of million dollar aircraft....like the Cessna Citation private jet

"One 180-pound pilot flying solo needs 140 pounds of ballast in the nose baggage compartment to just barely pull the CG inside the aft limit. Two 180-pounders still need about 20 pounds in the nose. Fortunately, Cessna delivered the aircraft with eight 20-pound ballast bags of sand, so getting the CG inside the envelope didn’t require scrounging up ballast from the FBO."

source
M2entoring

New owners shake hands with a new Citation

[svyolo]

I am just waiting for a starter solenoid to wire up my power supply stuff FWF. I got my ETX-900 a few days ago. The starter cable, alternator cable, and ground straps weigh almost as much as the battery and my power wires are about as short as humanly possible. Knowing what the starter cables weigh, and knowing if I put a battery in the tail I would need to go up 1 or 2 sizes on the wire if I mounted it in the tail, I think just bolting lead in the tail would be more efficient , lighter, and cheaper than running huge cables back to the tail.

15 or 20 feet of 1-0 or 0-0 battery cable to the tail would be a lot. Depending on the airframe, you might need to run another of the same back to the engine block to ground it. My 2-0 wires are about .35 lbs per foot.

[Collin Campbell]

Thanks for the kind words. Guess I feel kind of over rated though. Anyone that has been to Oshkosh probably knows what I mean...you think you have built a real nice plane until you look around at all the outstanding workmanship on display there. Good idea about the video manual though...someone should definetly do it. No doubt I will continue to build...right now I am in the process of rebuilding Bob's LSA...after that who knows? A Companion? We'll see..

[way_up_north]

Another thing I might add is that in refining the plans, Bob tells me he is going to tilt the firewall back (2 1/2") the same as the 4-place. (The prototype firewall is vertical) So this change will allow moving the engine back a bit...at least 1 1/2" or maybe 2". As it is now the engine is as close to the firewall as we could get it and still get the mags out. I am thinking most people will use the IO-540, which as Rollie mentioned is a bit lighter than the IO-580, so the need for any ballast will be lessened if not required at all. Having a tool box just aft of the baggage area for essentials would make a difference too. Speaking of the battery...we used the Earth X ETX 900, weighs only 5 lbs and that thing cranks the engine like you would not believe! 900 cold cranking amps...think it would crank my diesel tractor!

I was reading the old Beartracks and saw pictures of your work on other planes....please consider making videos if you build another Bearhawk....all you need is a tripod to hold the Iphone or camera..or even just hold the camera and film...and just talk about what you are doing and why...a video diary....Jared could make up a DVD package to sell or download...

$100 per section wing, fuse, finishing, engine...etc...it would sell for years and its an income stream you can leave for the grand kids....etc..

we really need a video build manual as supplement to the manuals

Youre a master fabricator and craftsman...you came from the RV world ..the RV-6 was not an easy build either...


to other fabricators on here reading this...please consider it also



Thank you for your efforts in this project...the plane looks amazing

[auburntsts]

Agree 100%. I replaced my Odyssey PC925 battery at 24 lbs with an EarthX ETX1200 at 7 lbs last year. I'm sold on their LiFePO4 technology. My battery is in the tail and I've had zero issues with it cranking my IO-540.