Thread Tools
Oct 25, 2020, 02:08 PM
Registered User
Soren wolff's Avatar
Thread OP
Question

Down and Right Thrust on float plane?


Very new to RC started flying in May, building my first float plane. Refurbished 27-year-old Goldberg cub. Do float planes require down and right thrust?
Sign up now
to remove ads between posts
Oct 26, 2020, 11:19 AM
Registered User
If the airplane required right and down thrust on wheels, than it will require right and down thrust on floats. The only change when going to floats is usually to add a bit of up elevator trim.
Oct 26, 2020, 10:47 PM
Registered User
Soren wolff's Avatar
Thread OP
Cool thanks. Did not know if down thrust would push front of float down into the water more.
Oct 26, 2020, 10:55 PM
Registered User
Quote:
Originally Posted by Soren wolff
Very new to RC started flying in May, building my first float plane. Refurbished 27-year-old Goldberg cub. Do float planes require down and right thrust?
If that is a a Goldberg Anniversary Edition Cub (about a 76 inch wingspan) it has the thrust angles built into the firewall (assuming it is properly built). So mounting the engine to the mounts on the firewall will get you the correct angles (a few degrees right and a degree or two down). The trick with models that have angles built into the firewall is to figure out the offset for the engine mount so that the prop shaft is centered. Otherwise the cowl will fit funny and the nose will look bent. It should fly fine either way.

If you have questions about setting up the floats there are some old RCM articles describing the geometry. Getting it correct makes a difference in how it flies. Look specifically for the float flying articles from Cunningham on RC (he wrote a regular column back in the 80s and 90s). He provides a great explanation for type, proportions, overall size, and location of floats. Other good bits of information like getting the float step positioned correctly relative to the CG position. Also, hist tip to balance a plane before adding the floats is a good one. Then balance it with the floats by adding weight to the floats. That way it will fly well with or without them.

FYI - Those a great flying fun planes. Especially with a bit of extra power (like a .60 2 stroke). I had one for many years and miss it a little now that it's gone.

Have fun and post a picture!
Oct 26, 2020, 11:00 PM
Registered User
Quote:
Originally Posted by Soren wolff
Cool thanks. Did not know if down thrust would push front of float down into the water more.
Your instinct is correct. A float plane will tend to nose over if power is applied too quickly. Just like a tail dragger. They act much like a tail dragger so flying it or another tail dragger may be good practice if you have not done so (you said you are new to the hobby).

Also, don't forget the recovery boat. It's impossible to swim with an airplane
Oct 27, 2020, 12:10 AM
Registered User
Yes, adding throttle will try to pull the nose of the floats into the water, with or without the down thrust. That is a result of the thrust line being so far above the drag of the water. The solution is to accelerate slowly, minimizing that effect. A couple of degrees of down thrust is needed for flight stability but has only a negligible effect on the water.

Be sure to add the ventral (lower) fin at the tail to correct the yaw instability that results from the added side area of the floats ahead of the CG
Last edited by jrf2; Oct 27, 2020 at 11:04 AM.
Oct 27, 2020, 11:58 PM
Registered User
Soren wolff's Avatar
Thread OP
Thank you all for the great info.
Yes, I have been flying as a wheeled plane this summer. Been flying at an empty airport.
Good on the takeoff, sometimes a bit wiggly on the landing.
Had a bit of a crash with it, nothing major, but was enough of a motivator to refit as a float plane. (Something I was planning on doing)
Purchased a set of the hanger9 40 size floats. I pulled the coverings of to see how well they were sealed. They were not sealed at all,the bottom section ahead of step was plyed and glassed and that was it, everything else was raw wood. (Do plan on recovering once sealed) I varnished them with multiple coats and then test floated them, they were not waterproof! Been drying them out and contemplating sealing with a resin coat or glass and resin. Trying to figure out best way to seal and not add to much weight.
When I did the float test I was checking plane position over float. I know to place Step 3/4 to 1 behind CG. Wing incidence relative to float angle looks good. Wing is about +2 incidence to float. Now just trying to see best position of plane over float and how floats sits in water.
It appears that most floats sit a bit more back with back of float a bit lower in water. This looks good to me, It appears when the plane comes up on the step it then levels out and with + wing incidence it should take of relatively gently.
As I mentioned above, may or may not get it in the water before the snow flies.
Oct 28, 2020, 11:30 AM
Registered User
Having the step that far back will make the problem of the floats digging in worse. You will need to accelerate slowly to keep the airplane from darting off to one side or the other. That rule of thumb comes from full scale cubs that are only capable of accelerating very slowly. Floats on a model airplane can be as far as an inch in front of the CG. Another rule of thumb that is far more important for take off performance is to have the front of the floats extend at least half of the prop diameter in front of the prop. Depending on the size and configuration of the floats, it may not be possible to meet both criteria, but the more forward the floats are (within reason) the less drama you will have on takeoff.

Jim
Oct 28, 2020, 02:30 PM
A man with a plan
Balsaworkbench's Avatar
Down thrust requirement is determined by drag in flight (not water drag).

All other things being equal, floats will lower the center of drag by adding drag on the bottom of the plane, and consequently the need for down thrust for proper flight will be reduced.

Elevator management may be necessary to keep the floats from digging in, but that's a different issue.


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Question Right thrust on full size planes scott swanson Modeling Science 12 Aug 16, 2017 05:33 PM
Discussion Down thrust/right thrust troppo Beginner Training Area (Aircraft-Electric) 2 Mar 26, 2017 07:49 PM
Build Log RIGHT THRUST AND DOWN THRUST REQUIRED on easystar brushless..CORRECTION OF NUMBERS leonstevens Foamies (Kits) 0 Jan 28, 2010 01:32 PM
Know right thrust or down thrust on foam planes? Warren Baker Foamies (Kits) 2 Apr 26, 2004 01:37 AM
down/right thrust on the Slow Stick Fly Parkflyers 0 Oct 08, 2002 08:25 AM