Gear Drag Coefficient

[WebMaster]

I have a request from someone.

My name is Patrick Shannon. I am an engineer working on a prototype navigation system for an up and coming UAV made by Aurora Flight Sciences. We will be doing the majority of our electronics testing on a Cessna Skymaster before we load it into the UAV. We currently do not have access to the Skymaster and thus we are in need of some technical data. Specifically we need to get a feel for the increase in drag that occurs when the landing gear is deployed. If you happen to have the coefficients of drag for the landing gear or an idea of what type of cross-sectional area they produce when deployed those numbers would be of great help. I have been poking around your website but have been unable to find that type of information as it appears the site is geared towards pilots and not engineers (which makes perfect sense). If you can not help me but know someone who may please let me know as well. Thanks in advance.

I told him about cycling the gear, with conventional gear doors.
Does anyone have any information on this, it would be most gratefully appreciated if you post it here.

Thank You,
Larry

His contact information is:


Patrick Shannon
Aurora Flight Sciences
Research and Design Center, Cambridge MA

[Roger]

I have no data as needed, but if he doesn't already know, he should be told about the gear door mod, which creates less of a drag during deployment. Perhaps the companies that hold the 337 for the mods have the data he needs.

I would also think from a practical standpoint, and of course he as an engineer will know this, that the drag effect on a smaller and lighter aircraft will be dramaticaly more pronounced if his door system is not proportionatly identical

[Ernie Martin]

Tell Shannon that there are several engineers who participate here (I have an MS from Caltech). I have climb-rate and cruise-speed data that might be adequate for him to calculate/estimate the added drag of the landing gear. The "clean" climb rate is 1100 fpm (270/320 with only front/rear engine) and decreases by 110 fpm with the gear down (240 fpm if the doors stick open). Also, my recollection from what others have written here and from my own experience when I've flown with the gear down because of troubles/concerns with the system, is that the 140 Kts typical cruise speed goes down by about 5 Kts; others are encouraged to add their views if their cruise experience is different.

Ernie

[Skymaster337B]

A note about the gear door deletion. On those aircraft modified with gear door deletion, the wheels "barn-door" (because of the rotoration of the gear arms) during retraction, thus increasing drag while retracting the gear -- even without gear doors. So, buying a gear door removal STC is like buying snake oil -- buyer beware.

As far as flate plate drag of the wheels themselves, I'ved give it about 3sq/ft per tire, for a total of 9 sq/ft.

[Ernie Martin]

I've owned two Skymasters, one with the gear-door-removal STC and one unmodified, with the gear doors. The difference in drag during retraction is significant. I understand that there is a trade-off between cost and performance, and that some may choose not get the mod (my first one had it, when I was a rookie pilot flying out of short runways, my current one does not, in part because I rarely fly out of short runways), but there is a difference in drag and it's quite noticeable. The rotation of the gear arms during retraction is there for both cases, but all you need to do is open the gear doors on the ground (just pump the emergency handle) and look at the enormous sail-like doors that open at the back of the airplane to recognize that removal of these doors would decrease drag significantly.

Ernie

[SteveG]

In trying to understand this thread the following questions come to mind. Although Mr. Shannon uses the verb "deploy" is he actually interested in the change in parasitic drag during the gear cycle which would vary throughout its travel or in the steady state condition of down and locked? As we all know drag abruptly increases with door opening, slowly increases further as the wheels/tires rotate to their maximum wetted area then slowly decreases as they reach their vertical alignment then abruptly decreases as the doors close leaving only the residual drag of tires, gear legs, front strut and strut plate. A five knot loss in this configuration seems reasonable. I suspect it is the "steady state" not the "deployment state" that he is concerned with. The second question involves the cross-sectional area of these components in the down position. By rough guesstimation I can't see a projected area of more than say 60 sq. in. per tire and the strut plate plus say 50 sq. in. for the gear legs and for the strut/fork assembly for a total area of 340 sq. in. or 2.4 sq. ft. The third question concerns how the gear door deletion modification affects short field performance. It would seem that there is no difference in the total drag, in takeoff configuration, with or without doors. If anything, I suspect that total drag would actually increase with doors removed due to the disruption of the boundary layer at this location.

[hharney]

[quote]Originally posted by Skymaster337B
[b]A note about the gear door deletion. On those aircraft modified with gear door deletion, the wheels "barn-door" (because of the rotoration of the gear arms) during retraction, thus increasing drag while retracting the gear -- even without gear doors. So, buying a gear door removal STC is like buying snake oil -- buyer beware.

Do you have the gear door deletion?

[Ernie Martin]

Steve G's message says that he suspects "that total drag would actually increase with doors removed due to the disruption of the boundary layer". In fact, the STC kit includes vortex-generator fins intended to prevent the disruption of the air flow, so that cruise speed (and therefore drag) are unaffected. In my two Skymasters, one with and one without the kit, there was no difference in cruise speed.

Ernie

[SteveG]

Any vortex formed in the airstream produces drag. A vortex generator is, by definition, a disruption of the airflow. I do not doubt, however, that in the case of the door deletion kit that the increase in drag produced using small, controlled vortices is less than that produced by the large, uncontrolled ones which would originate at the sharp wheel well edges and total drag could in fact be less than that produced by the original fuselage form as it wraps around the rear of the aircraft. I do not wish to appear petulantly argumentative but trying to measure such a small change in total drag with an original equipment airspeed indicator would be like trying to measure the diameter of human hair with a yardstick. And on two different airplanes, no less. Such a test has not even begun to control for all of the variables involved. Full scale wind tunnel and super computer might predict an approximate delta drag.

[Dale Campbell]

I have owned a 337H for 8 years and fly out of a short field.
In the first flights I found the large gear door created a lot of drag if I retracted on climb out. I flew my instructors 337 with doors removed and found little drag when retracting on climb out.
I had the doors removed and noticed all the parts that got removed and found the weigh to be considerable and that meant a lighter aircraft. The drag disapeared when I retacted gear. That also meant fewer mechanical parts to fail. I am very happy with gear mod and would do it all over again if I purchased another 337. I now retract gear about 10 feet off the ground and get a faster climb. I did not lose a single knot at cruise either. Dale Campbell