Scrape Points

[Ivan]

Hello All,

I noticed a rather strange thing several days ago. I had a prop strike on a very low high speed pass over the runway. I figured it was a chance to practice a dead-stick landing. After the belly landing, I noticed that the fuselage of the aircraft was well below the ground level. It appeared that the scrape points were set incorrectly. I checked out the locations of the scrape points using DPED and found that they were quite reasonable, but in a belly landing, it appears that the Front scrape point acts as if it is not present.

After a lot of messing around, here is what I observed: The nose of the aircraft (P-47D-23 Thunderbolt) is 102 inches forward of the CoG with the lower side of the cowl 27 inches below the CoG. The eqivalent location on the stock razorback P-47 is only about 55 inches forward. If I the front scrape point back to 55 inches, the aircraft sits correctly on its belly. As I gradually move the scrape point forward, its apparent location appears to move up regardless of the vertical specification in the AIR file.

My method of compensating is to put the scrape point about 15 inches below its actual location.

Anyone have ideas as to how this is REALLY supposed to work?

- Ivan.

[hubbabubba]

:wavey:Ivan,

I have seen your post a while ago but, without seeing the actual model and its scrape points, it is difficult to make any constructive comments.

Nose scrape point height should be calculated from the ground standing attitude, not the in-flight attitude. If you move that point forward, it will go up, at least on a taildragger like the Thunderbolt.

But, again, this is pure conjecture.:typing:

[Ivan]

Hello Hubbabubba,
Your description of ground angle DID occur to me, but the next question would be where is the reference point? I figure it was the offset from the CoG, but if you do some calculations you will probably find that the "moves up as it goes forward" doesn't really make sense. If that were the case, then moving rear scrape point aft would make the scrape point lower, but I can tell you that THAT is not the case. I am guessing this yet another bug in the CFS engine. An easy way to test the ground angle bit would be to make the aircraft a tail dragger, but put in an extra long tail gear strut so the aircraft sits level.

My current working solution is to use the Front scrape point to plant the aircraft correctly for a noseover (harsh braking) and to lower the wingtips by 12-13 inches each. The aircraft STILL sits too low on a belly landing, but at least the angle is more or less level. On the Thunderbolt, it sits as if the intake ducting was all squashed (unlikely).

On another aircraft, I noticed that the plane slides for quite some time before slowing down significantly. I believe that can be adjusted with the ground-effect graph. Need to do some testing first though.

- Ivan.

[Ivan]

Hello again Hubbabubba,

Here is a screenshot that illustrates how the scrape points do not quite coincide with how the aircraft sits.

The front scrape point is about 1.5 feet back from the leading edge of the cowl. Its vertical location is at the bottom of the cowl where one would expect. The rear scrape point is at the trailing edge of the Turbocharger exhaust in the lower rear part of fuselage. Shadows are from Dusk time of day. If things were correct, you should BARELY be able to see a shadow of the turbo's exhaust or the bottom of the cowl. I tried moving the wing scrape points lower but it caused some strange behaviour with the aircraft wobbling quite a bit even after stopping.

The new scrape points leave the aircraft at about the same attitude, but about 1 foot higher. The front scrape point was also moved about 1.5 feet forward which would have put the aircraft MUCH lower than in this screenshot.

I actually did the changes to the P-47D-23 Razorback, but the flight models and visual models WERE the same with regards to this discussion.

- Ivan.

[hubbabubba]

Hi again Ivan:wave:

I think that you're giving too much credit to the shadow's algorithm of CFS1 a/c. I tried basically the same trick with the Taifun after reading the last post and got very similar results with dusk/dawn light angles. I know for a fact that it sits perfectly on its belly.

I will ponder on that while trying to get through a -18F° evening... :snowman:

[Ivan]

Hello Hubbabubba,

It is only in the mid teens (positive though) in our area. I don't want to think of what 18 degrees F would feel like. My transmission didn't shift right until about 5 miles after I left my house this morning.

Regarding shadows, I use them to calculate where the landing gear contact points should be as well as the scrape points. The first illustration is of what I believe the shadows SHOULD look like for the landing gear. The second second shot is what how the plane sits with the the scrape points adjusted closer to the CoG of the plane. The front is at the aft end of the engine cowling (+5.25 Feet forward). The rear is near the trailing edge of the wing (-4 Feet aft). As you can see, the plane sits noticeably higher. I didn't want to put the front scrape point as far forward, but further aft meant the plane flipped over too easily when it first contacted the ground.

I believe that the shadows seem more or less correct in these screenshots.

- Ivan.