Thanks: 
Likes: Yes, because the aft CG reduces the down force requirement on the tail. This in turn reduces the wing loading on the wing which will reduce the stall speed.Originally Posted by CharlieN
Yes, because the aft CG reduces the down force requirement on the tail. This in turn reduces the wing loading on the wing which will reduce the stall speed.
N1PA
I took his statement to be about cruise speed, which is faster with aft CG. Yes stall speed can be slower as well.
So did I! Confusing statement.
"Put out my hand and touched the face of God!"
Aft CG should be qualified. Slightly aft of neutral? I'll agree. Maybe even aft to the limit of a type certificated plane, but that limit is conservative, and in my perception performance at different CGs varies with different weights. Load it aft to the limit and to gross and see how slow it flies. Just don't do it too close to the ground. When heavy and aft loaded speed is your friend. Conversely when light and CG is on the forward limit speed is your enemy.
Colorguns liked this post
Just to give folks an idea of how I handled the bolt in weight for CG adjustment. I'm not saying anyone should do this, or do it this way, I'm just throwing it out as an idea.
These are the lead plates. They are a trapezoid shape, about 5" x 9". Thickness determines the weight. I have 5 pounders and 10 pounders.
When Javron built my kit, I had him weld in a couple of extra cross members to bolt the lead to
Top side view
Hope this helps
Bill
Very Blessed.
My 2+2 was almost at the front of the CG. After reading Bill's post I strapped a 5lb piece of steel to the T3. I could tell it made a difference. I welded up a form, found some old lead pipe and cast a bunch of 2lb blocks . Under the horizontal stabilizer I have an inspection cover. I made up a little 1/8 plywood platform that I glued on the longerons. I put the 2lb blocks in a shaving kit bag. I started with 10lbs and went up from there. I ended up with 22 lbs back there. The plane flies better than it ever has, I can drag the tail wheel before the mains, couldn't do that before the weight back there. The blocks weight 2lb, not 2.8
Bill, this looks like the most ideal way to make adjustments. Unfortunately I don't have those extra bars in place to attach to. I really lie the idea too of removable panels back in the area. Thanks for the response.
Bill Rusk thanked for this post
Skywagon and ChalieN are both correct. An aft CG permits a slower stall speed and a faster cruise speed.
The horizontal creates a downward force to balance the nose heavy airplane. The tail is in effect flying upside down creating a downward force. If say for example a 100 pound downward force is needed to balance the system, the tail must generate that force to keep the plane level. The wing then is required to carry that additional load of 100 pounds for the plane to fly. We worry about ounces when we build. We also know that lift generates drag hence less downward force (downward lift) by the tail creates drag requiring more power. Hence an aft CG requires less downward force by the tail and less drag, therefore more top end speed.
We concentrate on making these things fly slower, sorry I didn’t specify that I was talking about the stall speed Not clear on my part.
Keep in mind that as the CG is moved further aft the whole system becomes more unstable and too far aft can create an unrecoverable spin. A lesser issue is that It can even make the plane more unstable on landing.
Just like Bill Rusk did he has shelves welded in the tail to bolt weight in solid. That is the key. SOLID if it moves it could flex the tubes and crack brake them. Happened to me and on rebuild added the shelves.
After the time I went over in some heavy snow and my CG was forward, so on rebuild I found to get the plane to fly with little trim as I moved slower and added flaps, to make the plane “correct” I needed to add about 8# in the tail. Now many are screaming don’t add weight! Best thing I did. It now doesn’t need a lot of trim from take off 1 notch to 70 them off to cruise back to landing. 75 1 click. 60 2 click 50 3 click and not trimming needed to land.
Balanced aircraft, real joy to fly, safer on landing and ground, so a proper balanced plane are fun to fly even if heavy EWCG with 0-360 and all the mods of 1320# CG is 13.54 add the normal crop we carry, tools, ropes, etc and the CG is getting close to 14.2 or 79.8” depending on what number you use. This is where it likes to fly, then just add people, gear, etc. hard to get to true rear CG. Fun to fly!!!
It is in the 337 that I added weight in tail at rebuild, written by my IA-DAR in 2015
I guess I have 2 questions. Why not just trim as needed instead of adding weight to the tail? Downforce from the stabilizer will do the same thing as lead in the tail. Second question is has anyone really done stalls before and after adding weight to the tail and was there a noticeable difference in stall speed? (3 or more MPH) The reason I ask the second question is most every cub pilot I know would be sticking 20 lbs in the tail if it gave them a noticeable stall speed improvement I just don't see it happening.
DENNY
Denny, Trimming will relieve the stick pressure but not reduce the down force applied by the tail assembly. This in turn equates to an equivalent amount of additional weight which the wing must support. That extra lift requirement raises the stall speed by an appropriate amount.
If the CG is moved aft by the addition of a small amount of ballast at the tail, the stabilizer will be trimmed more in line with the wing and elevator in cruise by raising the leading edge, thus reducing drag and increasing cruise speed.
My Cub did not have enough up elevator travel to ensure a complete stall until I added lead to the tail. Then I was able to get a clean break. It would not spin without the ballast as it was elevator limited.
This scenario is not as effective in airplanes with a fixed stabilizer and trim tabs as the stabilizer and elevators are only streamlined at one CG and speed condition..
So did the plane fly slower or was it just a mushing stall vs clean break? I understand it does not reduce downforce (unless you trim nose down), that is why you would trim nose up to add downforce and drag. Say you need 20 lbs of downforce, you can get that with the lead or the stabilizer, the downside of the stabilizer is it adding drag, bad for speed. However, adding drag helps with landing. What I am getting at is until you have run out of nose up trim you should not need to add weight. Some planes may need this but if the OP has paperwork issues with adding lead to the tail, why not just fly it and use the trim wheel. I have seen lots of posts with people claiming they run out of elevator when they flair, when I ask how the trim is set most don't know or say it was whatever it was on downwind, some even say they use nose down trim. I don't want people to think I have anything against trying to get the CG balanced to reduce stabilizer drag at high speed, better braking, or if you have flare issues. I just think people have to use the trim wheel more. I try to fly literally hands off the stick most of the time so for me trim adjustment is something I do often without thinking about it much. No right or wrong just things to ponder.
DENNY
Due to the angle of attack and the inaccuracy of the airspeed in those conditions it is not possible to determine any difference in speed. Without the ballast there was not enough up elevator to get a clean stall break. With it there was a clean break. Thus during the spin scenario in the first case even though it appeared to be spinning the speed was high with the nose down 100+/- mph. With the ballast the speed was in the 30 +/- mph range with the nose down about the same amount.
I agree with you that the trim should be used first and that it appears that a lot of pilots use the trim very little. When it is not enough to make a clean stall break or land on the tail wheel it needs help in the form of ballast. As always the pilot should know what his approximate loaded CG is. When he operates within the certified limitations all should work as designed and approved. Since we have made all sorts of modifications most of our planes bear no resemblance to the approved version. As long as we maintain the proper CG there should be no issue. Heavy engines, props and forward mounted batteries have a tendency to move the CG forward, sometimes a bit too much.
No issues with anything you said Denny.
Without flaps pulled I do run out of nose up trim with out the weight in it, flying really light. With it in I have nose up trim yet at or near the limit.
Flare is only a wheel landing and almost impossible to 3 pt really slow if at all without the weight in tail.
Also the tail is very light when braking without the weight in it, and when just a few gallons of fuel it is really light on the tail/Nose heavy and if I try to land short and on the brakes it is not a fun ride to drive.
This is the best thing for my set up and the safest when flying light, as in: nothing is the extended baggage, no passenger, low on fuel, basically empty.
When heavy not an issue and I can just fit things in and go, give it 2-3 turns nose down trim and go.
When really light 7-8 gallons of fuel I give it 1 turn nose up and go.
Stall clean MPH 42-43, full flaps 38-39 by GS no wind stall it brakes really gentle.
This is a set up for mine that works and yes I added weight, but the plane flies really nice, next time I'm in Graham TX. Steve Pierce is going to drive it and tell me how he would change things if at all.
Doug
Skywagon/Doug
Thanks for the feedback, having seen Doug fly I can say I have to work hard to keep up with him in his 12. Like I said I am not against adding weight in the tail just want to point out that that is what the stabilizer is for. If I remember Doug has a long mount that would add to the weight issue. I am trying to get up the energy to start a post on the true effects of a 0320 vs 0360 on the nose.
DENNY
Denny
My bird is Short mount Charlie conversion. Yep I get a lot out of this bird, yes it would be very interesting to see what the GW and ECGW numbers are and how does it fly. Big question is-does it fly better with a passenger in, noticeable different?
Flaps used or No flaps used
Certified VS Experimental BIG differences as what we can do legally. RULES!!!
0-320 with a short or long mount, my understanding is that the long mount 0-320 are having the same issues as the Certified 0-360 short mount because of the big paddle prop, the Cato prop helps fix these things on both engine conversions, but not certified.
0-360 I think are all short mount but I'm sure I'm wrong.
Denny
I will have to spend a little time doing some homework to find the sources and references. But the stabilizer and trim do not compensate for CG. They are two different aerodynamic principles. The way I understand your argument is “if an aircraft has a forward center of gravity, if you just put a bigger elevator on it that fixes the problem, or use more trim”.
Elevator size and trim position do not fix center of gravity issues. Two different items. They are related. CG will affect the amount of trim required, but trim does not fix CG.
Bill
Very Blessed.
Denny
I suspect we are basically on the same page, just saying it differently.
An aft CG will lower stall speed. I can't find my notes from my test flights now when I did add lead but it was in the area of 3 to 4 knots. This was also a pretty critical component of the Breeden Cub but he got the aft CG by moving the engine and firewall back rather than adding lead.
"The aerodynamic center of an airfoil is the point along the chord where all changes in lift take place. The location of the aerodynamic center of an airfoil is not affected by Camber, thickness, or angle of attack. Compressible theory will predict the aerodynamic center at 25%." ( from Aerodynamics for Navel Aviators Pg 47).
The aerodynamic center (AC) is important and the most direct application is to the longitudinal stability of an airplane. The CG must be in front of the AC in order for the aircraft to have positive longitudinal stability. The further the CG is in front of the AC the more stable the AC will be. If the CG is behind the AC the aircraft will be unstable and may not be flyable without computer intervention, Like the F-16.
The further the CG is in front of the AC the greater the nose down pitching force from the wing and the greater the force required from the tail to overcome this pitching force.
"Angle of attack is the primary control of airspeed in steady flight" (Aero for Navel Aviators pg 27) The tail sets the AOA. Trim allows us to take the pressure off the controls (stick) and thus fixes the tail at a given value which offsets the pitching forces from the wing. So the tail sets the wing AOA which corresponds to a given speed. (basically you trim for a speed)
As we change the AOA the amount of pitching force change will be directly proportional to the CG. If the CG is at the AC there would be no corresponding pitch force change thus no required change in tail AOA to compensate. This would create a completely neutral stick which would not have any differing stick pressure regardless of AOA, speed G load etc. and it would be easy to both overload the aircraft, or stall it, with no control feeling at all. It would be most difficult to fly. BUT it would also require the least downforce from the tail and thus be the most efficient aerodynamically. Lindbergh did this intentionally with the Ryan M1 both for efficiency and also to force him to fly the airplane at all times to keep from falling asleep. Sailplanes frequently push the CG back very close to the AC to reduce drag as well. Then a long fuselage (leverage) allows for a small tail (less drag) to still handle the necessary pitch force requirements. High performance sailplanes are notoriously light on the controls.
A supercub tail can only produce so much force at a given speed. The slower we go the less downforce it can produce. Trim will affect that value as you correctly pointed out, but it still has a max value. Now add VG's under there and we get a slightly higher AOA from that airfoil thus producing more downforce. That tail has to overcome the wing pitching moment. The higher the AOA of the wing the greater it wants to pitch down, and the more force the tail has to produce to get it to that AOA and hold it there.
The further the CG is forward of that AC the greater these pitch down forces are. At some point the tail simply can't push the wing to a sufficient AOA to provide sufficient lift for the speed we want to fly. Two possibilities exist here. The wing will stall or the tail will stall. If the wing stalls that is probably not good on short final. If the tail stalls out before the wing then you get a mushing descent. Most of the time, with VG's the tail stalls before the wing and we get mushing descents. If we add power, we increase the airflow over the tail and we can drive the wing to a high enough AOA to get it to break and stall. Or we can add lead to the tail which brings the CG closer to the AC so the wing will not resist the pitching moment so much (less pitch down moment) and the the tail will now have enough force to drive the wing to a higher AOA, perhaps into a stall with a break, or if we have VG's to a higher AOA without a break.
An aft CG reduces the trim required for any given change. Lets say you are motoring along at 100 mph and you slow to 80 as you enter the pattern. With a forward CG the stick will get heavy quickly and it will take 2 turns on the trim wheel to get the stick to neutral again. If your CG is aft in the range the stick would not feel nearly as heavy for that same airspeed change and it might only take a 1/2 turn on the trim wheel.
Hope all this makes sense. Design for Flying by Thurston covers this in Chapter 1. Aerodynamics for Navel Aviators covers it between about page 25 and 51
Hope this helps
Bill
Last edited by Bill Rusk; 12-01-2019 at 01:13 AM.
Very Blessed.
The OP has left the building, but this is interesting. Whether you add tail downforce with mass of steel or catching more molecules of passing air, the effective wing loading as you approach stall is the same.
All the hotrods I have built needed tail weight. One needed an aft battery, one a rear gas tank, one a steel rod right in the tailpost. Once the hangar next door was demillitarizing a TBF and had cast a pile of lead cylinders to replace the tailhook and tail gun. I cut off a piece and fixed it with big rig exhaust clamps. The most fun was a wedge of lead cast in a 3/4" plywood mold. It'll work once. A Surform grater will smooth it up.
Do what you gotta do. My first would have crashed had the motor quit, and I was 3 inches inside spec. The kit manufacturer then changed the old aft limit to the new sweet spot.
What's a go-around?
This is true however, the tail authority is greater and the drag component is less with the ballast.
N1PA
Bill
Thanks for jumping in. I know you are good at documenting what is really happening so appreciate your input. So with your 0360 you did not have enough tail pressure even at full nose up trim and power?
My power off stall speed is about 4 mph faster then power on as expected. I do both at full nose up trim.
I land power on with full nose up trim so for me the more drag the better. Empty, full flaps, at 1600-1800 rpm, my stick pressure will go from pushing forward to neutral as I flare. This will bring my tail lower then the mains and slow me down to +/_ 37 GPS speed in ground effect If I start pulling back it much at all it will stall. As a matter of fact a friend just did it a few weeks ago when he was trying out my ASOS gear. He flys a Dakota slotted wing and was surprised how fast mine dropped. I was in the back and knew we where at the edge but also a foot off the ground so it was a non issue.
It is hard to make the ideal exchange smooth over the internet so I appreciate everyone putting up with my questions. I have the OK from management to build a new cub, if I can get some issues sorted out I would start next year about this time. I just want to cover as many bases as I can before I start to build. I would like a 180 hp so if adding lead to the tail is needed I have no problems doing it. I am not good enough to tell 20 lbs difference anyway. It would also help with hard braking.
I really like the ideal of less drag at higher speed. I know we have cubs but I do some long trips and even a few extra MPH would help. I changed out engine mount bushings and it was like having a Thrustline mod. I changed engine angle about 3 degrees and gained 4 mph in cruse.
DENNY
Posting Permissions
Reply With Quote
Bookmarks